[Tinyos-2-commits] CVS: tinyos-2.x/doc/txt tep108.txt, 1.1.2.9,
1.1.2.10
Kevin Klues
klueska at users.sourceforge.net
Thu Sep 7 06:06:48 PDT 2006
Update of /cvsroot/tinyos/tinyos-2.x/doc/txt
In directory sc8-pr-cvs10.sourceforge.net:/tmp/cvs-serv23515
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tep108.txt
Log Message:
Update of tep 108
Index: tep108.txt
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*** 1,540 ****
! ============================
! Resource Arbitration
! ============================
!
! :TEP: 108
! :Group: Core Working Group
! :Type: Documentary
! :Status: Draft
! :TinyOS-Version: 2.x
! :Authors: Kevin Klues, Philip Levis, David Gay, David Culler, Vlado Handziski
!
! :Draft-Created: 28-Mar-2005
! :Draft-Version: $Revision$
! :Draft-Modified: $Date$
! :Draft-Discuss: TinyOS Developer List <tinyos-devel at mail.millennium.berkeley.edu>
!
! .. Note::
!
! This memo documents a part of TinyOS for the TinyOS Community, and
! requests discussion and suggestions for improvements. Distribution
! of this memo is unlimited. This memo is in full compliance with
! TEP 1.
!
! Abstract
! ====================================================================
!
! This memo documents the general resource sharing mechanisms for TinyOS
! 2.x. These mechanisms are used to allow multiple software components to
! arbitrate access to shared abstractions.
!
!
! 1. Introduction
! ====================================================================
!
! TinyOS 1.x has two mechanisms for managing shared resources:
! virtualization and completion events. A virtualized resource appears
! as an independent instance of an abstraction, such as the Timer
! interface is TimerC. A client of a Timer instance can use it
! independently of the others: TimerC virtualizes the underlying
! hardware clock into N separate timers.
!
! Some abstractions, however, are not well suited to virtualization:
! programs need the control provided by a physical abstraction. For
! example, components in 1.x share a single communication stack,
! GenericComm. GenericComm can only handle one outgoing packet at a
! time. If a component tries to send a packet when GenericComm is
! already busy, then the call returns FAIL. Therefore, shared use of
! GenericComm follows a first-come, first-served arbitration policy. If
! a component sends a packet but GenericComm is busy, the component
! needs a way to tell when GenericComm is free so it can retry. TinyOS
! 1.x provides the mechanism of a global completion event which is
! signalled whenever a packet send completes. Interested components can
! handle this event and retry.
!
! The approach to physical (rather than virtualized) abstractions
! has several drawbacks:
!
! - If you need to make several requests, you have to handle the
! possibility of a request returning FAIL at any point. This complicates
! implementations by adding internal states.
!
! - You have no control over the timing of a sequence of operations. One
! example of when this can be a problem is timing-sensitive use of an
! A/D converter.
!
! - If a hardware resource supports reservation, you cannot express this
! via this software interface. For instance, I2C buses have a
! concept of "repeated start" when doing multiple bus transactions,
! but it is not clear how to use this in TinyOS 1.x's I2C abstraction.
!
! - Most TinyOS 1.x services do not provide a very convenient way of
! monitoring an abstractions's availability for the purpose of retries,
! nor very clear documentation of which requests could happen simultaneously.
!
! A single approach to resource sharing is not appropriate for all
! circumstances. For instance, requiring resource reservation allows
! programs to have better timing guarantees for access to an A/D converter.
! But if a program does not need precise timing guarantees (e.g., when measuring
! temperature in a biological monitoring application), this extra resource
! reservation step unnecessarily complicates code.
!
! 2. Resource Classes
! ====================================================================
!
! TinyOS 2.x distinguishes between three kinds of abstractions:
! *dedicated*, *shared*, and *virtualized*.
! Components offer resource sharing mechanisms appropriate to their
! goals and level of abstraction. As discussed in Section 2.1, access
! control to dedicated abstractions is generally handled
! through nesC interfaces. As discussed in Section 2.2, access control
! to virtualized abstractions is handled through software design
! patterns such as the Service Instance [3]_ and/or
! queueing. Section 2.3 addresses with the most complex class of
! abstraction, shared, while Section 3 describes the
! components and interfaces used to arbitrate access to this class.
!
! Hardware Presentation Layer (HPL) components of the HAA [1]_ are not
! virtual, as virtualization inevitably requires state. Depending on their
! expected use, HPL abstractions are either dedicated or
! shared. For example, while hardware timers are rarely
! multiplexed between multiple components, buses almost always are.
! For example, on the MSP430 microcontroller, compare and counter registers are
! dedicated, while the USARTs are shared.
!
! 2.1 Dedicated
! -------------------------------
!
! An abstraction is *dedicated* if it is a resource
! which a subsystem needs exclusive access to at all times. Examples of
! dedicated abstractions include interrupts and counters.
! Generally, a physical and dedicated abstraction is just an interface
! which its user wires to. For example, on the Atmega128, Timer 2 is
! presented by the component HplAtm128Timer2C::
!
! module HplAtm128Timer2C {
! provides {
! interface HplTimer<uint8_t> as Timer2;
! interface HplTimerCtrl8 as Timer2Ctrl;
! interface HplCompare<uint8_t> as Compare2;
! }
! }
!
! Dedicated abstractions MAY be annotated with the nesC attribute
! @atmostonce or @exactlyonce is to provide compile-time checks that
! their usage assumptions are not violated.
!
! 2.2 Virtual
! -------------------------------
!
! *Virtual* abstractions hide multiple clients from each other
! through software virtualization. Every client of the resource thinks it
! has its own independent instance of the resource, but these virtualized
! instances are multiplexed on top of a single underlying resource. Because
! the virtualization is in software, there is no upper bound on the number
! of clients of the abstraction, barring memory or efficiency constraints.
! For example, the TimerMilliC component provides a virtual and shared
! abstraction of millisecond precision timers to application
! components [2]_. As virtualization usually requires keeping state
! that scales with the number of virtualized instances,
! virtualized resources often use the Service Instance pattern [3]_,
! which is based on a parameterized interface. For example, HilTimerMilliC
! provides multiple virtualized timer clients and auto-wires the
! chip timer implementation (HilTimerMilliC [2]_) to the boot initialization
! sequence::
!
! configuration TimerMilliP {
! provides interface Timer<TMilli> as TimerMilli[uint8_t num];
! }
! implementation {
! components HilTimerMilliC, MainC;
! MainC.SoftwareInit -> HilTimerMilliC;
! TimerMilli = HilTimerMilliC;
! }
!
! while TimerMilliC encapsulates this in a generic configuration::
!
! generic configuration TimerMilliC {
! provides interface Timer<TMilli>;
! }
! implementation {
! components HilTimerMilliC;
! Timer = HilTimerMilliC.Timer[unique(UQ_TIMER_MILLI)];
! }
!
! Virtualization generally allows a client to use a very simple interface.
! This simplicity comes at a cost of reduced efficiency and an inability to
! precisely control the underlying resource. For example, TimerMilli32C
! introduces CPU overhead from dispatching and maintaining all of the
! virtual timers as well as jitter from when two timers want to fire at
! the same time.
!
! 2.3 Shared
! -------------------------------
!
! Dedicated abstractions are useful when a resource is
! always controlled by a single component. Virtualized abstractions are
! useful when clients are willing to pay a bit of overhead and sacrifice
! control in order to share a resource in a simple way. There are
! situations, however, when many clients need precise control of a
! resource. Clearly, they can't all have such control at the same time:
! some degree of multiplexing is needed.
!
! In TinyOS 2.x, a resource *arbiter* is responsible for this
! multiplexing. The arbiter determines which client has access to the
! resource. While a client holds a resource, it has complete and
! unfettered control. Arbiters assume that clients are cooperative, only
! acquiring the resource when needed and holding on to it no longer than
! necessary. Clients explicitly release resources: there is no way for
! an arbiter to forcibly reclaim it.
!
! A motivating example of a shared resource is a bus.
! The bus may have multiple peripherals on it, corresponding to
! different subsystems. For example, on the Telos platform the flash
! chip (storage) and the radio (network) share a bus. The storage and
! network stacks need exclusive access to the bus when using it,
! but they also need to share it with the other subsystem. In this
! case, virtualization is problematic, as the radio stack needs to be
! able to perform a series of operations in quick succession without
! having to reacquire the bus in each case. Having the bus be a
! physical but shared resource allows the radio stack to
! send a series of operations atomically across to the radio without
! having to buffer them all up in memory beforehand (which would
! introduce memory pressure).
!
! 3. Resource Arbiters
! ====================================================================
!
! Every shared resource has an arbiter to manage which client
! can use the resource at any given time. Because an arbiter is a
! centralized place that knows whether the resource is in use, it also
! provides information useful for a variety of other services, such as
! power management. An arbiter MUST provide a parameterized Resource
! interface as well as an instance of the ArbiterInfo interface. An
! arbiter SHOULD also provide an instance of ResourceController and
! ResourceConfigure interfaces. An arbiter MAY provide additional
! interfaces or instance of interfaces in order to provide a particular
! arbitration policy.
!
! 3.1 Resource
! -------------------------------
!
! Clients of a shared resource arbiter request access
! with the Resource interface::
!
! interface Resource {
! async command error_t request();
! async command error_t immediateRequest();
! event void granted();
! async command void release();
! }
!
! A client lets an arbiter know it needs access to the resource with
! a call to request(). The arbiter signals the granted() event to a
! client when it gains exclusive access to the resource. A client
! can also acquire the resource with immediateRequest(). The
! return value of this call determines whether the client was able
! to acquire the resource. If immmediateRequest() does not successfully
! acquire the resource (does not return SUCCESS), then it can try to do
! so in the standard,
! split-phase way with request(). If the call to immediateRequest()
! returns SUCCESS, then the arbiter MUST NOT issue a granted() event.
!
! An arbiter MUST provide a parameterized Resource interface,
! where the parameter is a client ID, following the Service
! Instance pattern [3]_. An arbitrated component SomeNameC MUST
! #define SOME_NAME_RESOURCE to a string which can be passed to unique()
! to obtain a client id. For instance, an I2C bus might look like this::
!
! includes I2CPacketC;
! configuration I2CPacketC {
! provides {
! interface Resource[uint8_t id];
! interface I2CPacket[uint8_t busId];
! }
! } ...
!
!
! where I2CPacketC.h contains the #define for the resource::
!
!
! #ifndef I2CPACKETC_H
! #define I2CPACKETC_H
! #define I2CPACKET_RESOURCE "I2CPacket.Resource"
! #endif
!
!
! The #define for the unique string must be placed in a separate file
! because of the way nesC files are preprocessed: referring to I2CPacketC
! isn't enough to ensure that macros #define'd in I2CPacketC are visible
! in the referring component.
!
! For example, clients of the I2C service might use it as follows::
!
! module I2CUserM {
! uses interface Resource as I2CResource;
! uses interface I2CPacket;
! } ...
!
! #include <I2CPacketC.h>
! configuration I2CUserC { }
! implementation {
! components I2CUserM, I2CPacketC;
!
! I2CUserM.I2CResource -> I2CPacketC.Resource[unique(I2C_RESOURCE)];
! I2CUserM.I2CPacket -> I2CPacket.I2CPacket[0x73]; // using I2C device 0x73
! }
!
! 3.2 ResourceController
! -------------------------------
!
! An arbiter SHOULD provide one instance of the ResourceController
! interface and MAY provide more than one. The Resource interface is for
! simple and basic use cases, where clients are peers that share the
! resource in some equal fashion. ResourceController is for clients that
! require additional information due to the policies of the arbiter and
! how they use the resource. The ResourceController interface is based
! on Resource, but introduces two additional events, idle() and
! requested()::
!
! interface ResourceController {
! async command error_t request();
! async command error_t immediateRequest();
! event void granted();
! async command void release();
! async event void requested();
! async event void idle();
! }
!
!
! An arbiter signals the requested event if the client currently has the resource
! and some other client has requested it. It signals the idle() event when
! no client holds the resource.
!
! ResourceController allows an arbiter to provide a much richer set of
! policies than simple sharing. For example, arbiters that want to
! incorporate a power management policy can provide ResourceController
! for a power management component. The power management component can
! detect when nobody is using the resource with idle(), acquire it
! atomically with immediateRequest(), and power it down. When another
! client requests the resource, the power manager will handle the
! requested() event. It can then power up the resource and release it
! when the power up completes. Note that if power up is a split-phase
! operation (takes a while), then calls by clients to immediateRequest()
! when in powered down state will not return SUCCESS. See TEP 115 for
! details. The default arbiters in TinyOS 2.x (see Section 4) all provide a
! single instance of ResourceController, in order to enable power
! management as described above.
!
! ResourceController can also be used for special case clients: the
! algorithm used to determine when its requests are handled in
! comparison to instances of Resource is arbiter specific. Therefore,
! arbiters MAY provide one or more instances of ResourceController. For
! example, the FcfsPriorityArbiter has a single high-priority client who
! is always granted access to the resource before any other client.
! Other clients only obtain the resource if the high-priority client has
! not requested it or when the high-priority client releases it.
!
! 3.3 ArbiterInfo
! -------------------------------
!
! Arbiters MUST provide an instance of the ArbiterInfo interface.
! The ArbiterInfo interface allows a component to query the current
! status of an arbiter::
!
! interface ArbiterInfo {
! async command bool inUse();
! async command uint8_t userId();
! }
!
!
! The ArbiterInfo interface has a variety of uses. For example, the resource
! implementation can use it to refuse requests from clients that do not
! currently have access. In this case, the abstraction would need to provide
! a parameterized interface for its operations so it could distinguish separate
! clients, and the client ID for its operations would need to be the same
! as the client ID for the arbiter.
!
!
! 3.4 ResourceConfigure
! -------------------------------
!
! The ResourceConfigure interface provides a mechanism for clients that need
! to use a resource with different configurations. Rather than forcing a
! client to reconfigure the resource itself, the component representing a
! client can wire to an arbiter's ResourceConfigure interface, which is called
! before the client is granted the resource.
!
! For example, the MSP430 USART0 bus can operate in three modes: SPI, I2C, and
! UART. Using all three concurrently is problematic: only one should be enabled
! at a time. However, different clients of the bus might need different bus
! protocols. For example, Telos sensors use an I2C, while the radio and flash
! chip use SPI.
!
! Arbiters MAY use a parameterized ResourceConfigure interface::
!
! interface ResourceConfigure {
! async command void configure();
! async command void unconfigure();
! }
!
!
! The parameter is the client ID, and corresponds directly to an instance of
! the Resource interface. For example::
!
! generic component RoundRobinArbiterC {
! provides {
! interface Resource[uint8_t id];
! interface ResourceController;
! interface ArbiterInfo;
! }
! uses {
! interface ResourceConfigure[uint8_t id];
! }
! }
!
!
! If an arbiter uses the ResourceConfigure interface, before it signals the
! Resource.granted() event and before it returns SUCCESS from a call to
! Resource.immediateRequest(), it MUST call ResourceConfigure.configure() on
! the granted client ID. Similarly, after a valid call to Resource.release(),
! it MUST call ResourceConfigure.unconfigure() on the releasing client ID.
!
! Using a parameterized interface that calls out rather than a decorator
! on the Resource interface simplifies code reuse. Using a decorator
! could lead to a large number of clients all including redundant
! configuration code, while the call out will only have one instance
! of the code. For example, an SPI client might look like this::
!
! generic component Msp430Spi0ClientC {
! provides {
! interface Resource;
! interface SPIByte;
! interface SPIPacket;
! }
! }
! implementation {
! enum {MSP430_SPI0_CLIENT = unique(MSP430_USART_RESOURCE);
! components Msp430Usart0C, Msp430Spi0Configure as Configure;
!
! Resource = Msp430Usart0C.Resource[MSP430_SPI0_CLIENT];
! Msp430Usart0C.ResourceConfigure[MSP430_SPI0_CLIENT] -> Configure;
! }
!
!
! Arbiters SHOULD provide a parameterized ResourceConfigure interface.
!
! 3.5 Cross-component reservation
! -------------------------------
!
! In some cases, it is desirable to share reservation of resources
! across components. For example, on the TI MSP430, the same pins can
! be used as an I2C bus, a UART, or an SPI connection. Clearly, on this
! chip, a reservation of the I2C bus implicitly reserves the
! corresponding UART and SPI. This can be accomplished in the framework
! described above by:
!
! 1) using the same unique string for all three resources
!
! 2) wiring the three parameterised Resource interfaces to the same
! arbiter
!
!
! The common way to do this is as follows (the UART and SPI components
! are omitted, they are similar to I2CC, low-level I2C component)::
!
! #define I2C_RESOURCE MSP_BUS_RESOURCE
! configuration I2CC {
! provides interface Resource[uint8_t clientId];
! provides interface I2C;
! }
! implementation {
! components MspBusC, I2CM;
!
! Resource = MspBusC.Resource;
! I2C = I2CM.I2C;
! }
!
! MspBusC (the arbiter for the MSP bus)::
!
! #define MSP_BUS_RESOURCE "MspBus.Resource"
! configuration {
! provides interface Resource[uint8_t clientId];
! } ...
!
!
! 4. Implementation
! ====================================================================
!
! Because most components use one of a small number of arbitration
! policies, TinyOS includes a number of default resource arbiters. These
! arbiters can be found in ``tinyos-2.x/tos/system`` and are all
! generic components that include this signature::
!
! generic module ArbiterC {
! provides interface Resource[uint8_t id];
! provides interface ResourceController;
! provides interface ArbiterInfo;
! uses interface ResourceConfigure[uint8_t id];
! }
!
!
! For example, ``RoundRobinArbiterC`` provides round-robin arbitration. This
! arbiter assigns a fixed order to all clients and grants outstanding
! requests in that order, which is based on client ID. ``FcfsArbiterC``
! provides a FIFO order, where requests are serviced in the order they
! are received. ``FcfsPriorityArbiterC`` is similar to FcfsArbiterC, but
! provides an additional ResourceController interface for the
! high-priority client.
!
! 5. Author's Address
! ====================================================================
!
! | Kevin Klues
! | 503 Bryan Hall
! | Washington University
! | St. Louis, MO 63130
! |
! | phone - +1-314-935-6355
! | email - klueska at cs.wustl.edu
! |
! | Philip Levis
! | 358 Gates Hall
! | Stanford University
! | Stanford, CA 94305-9030
! |
! | phone - +1 650 725 9046
! | email - pal at cs.stanford.edu
! |
! | David Gay
! | 2150 Shattuck Ave, Suite 1300
! | Intel Research
! | Berkeley, CA 94704
! |
! | phone - +1 510 495 3055
! | email - david.e.gay at intel.com
! |
! | David Culler
! | 627 Soda Hall
! | UC Berkeley
! | Berkeley, CA 94720
! |
! | phone - +1 510 643 7572
! | email - culler at cs.berkeley.edu
! |
! |
! | Vlado Handziski
! | Sekr FT5
! | Einsteinufer 25
! | 10587 Berlin
! | GERMANY
! |
! | email - handzisk at tkn.tu-berlin.de
!
! 6. Citations
! ====================================================================
!
! .. [1] TEP 2: Hardware Abstraction Architecture.
! .. [2] TEP 102: Timers.
! .. [3] Service Instance Pattern. In *Software Design Patterns for TinyOS.* David Gay, Philip Levis, and David Culler. Published in Proceedings of the ACM SIGPLAN/SIGBED 2005 Conference on Languages, Compilers, and Tools for Embedded Systems (LCTES'05).
!
--- 1 ----
! ============================
Resource Arbitration
============================
:TEP: 108
:Group: Core Working Group
:Type: Documentary
:Status: Draft
:TinyOS-Version: 2.x
:Authors: Kevin Klues, Philip Levis, David Gay, David Culler, Vlado Handziski
:Draft-Created: 28-Mar-2005
:Draft-Version: $Revision$
:Draft-Modified: $Date$
:Draft-Discuss: TinyOS Developer List <tinyos-devel at mail.millennium.berkeley.edu>
.. Note::
This memo documents a part of TinyOS for the TinyOS Community, and
requests discussion and suggestions for improvements. Distribution
of this memo is unlimited. This memo is in full compliance with
TEP 1.
Abstract
====================================================================
This memo documents the general resource sharing mechanisms for TinyOS
2.x. These mechanisms are used to allow multiple software components to
arbitrate access to shared abstractions.
1. Introduction
====================================================================
TinyOS 1.x has two mechanisms for managing shared resources:
virtualization and completion events. A virtualized resource appears
as an independent instance of an abstraction, such as the Timer
interface in TimerC. A client of a Timer instance can use it
independently of the others: TimerC virtualizes the underlying
hardware clock into N separate timers.
Some abstractions, however, are not well suited to virtualization:
programs need the control provided by a physical abstraction. For
example, components in 1.x share a single communication stack,
GenericComm. GenericComm can only handle one outgoing packet at a
time. If a component tries to send a packet when GenericComm is
already busy, then the call returns FAIL. The component needs a way to
tell when GenericComm is free so it can retry. TinyOS
1.x provides the mechanism of a global completion event which is
signaled whenever a packet send completes. Interested components can
handle this event and retry.
This approach to physical (rather than virtualized) abstractions
has several drawbacks:
- If you need to make several requests, you have to handle the
possibility of a request returning FAIL at any point. This complicates
implementations by adding internal states.
- You have no control over the timing of a sequence of operations. One
example of when this can be a problem is timing-sensitive use of an
A/D converter.
- If a hardware resource supports reservation, you cannot express this
via this software interface. For instance, I2C buses have a
concept of "repeated start" when doing multiple bus transactions,
but it is not clear how to use this in TinyOS 1.x's I2C abstraction.
- Most TinyOS 1.x services do not provide a very convenient way of
monitoring an abstraction's availability for the purpose of retries,
nor very clear documentation of which requests could happen simultaneously.
It should be clear that a single approach to resource sharing is not appropriate
for all circumstances. For instance, requiring explicit reservation of a
resource allows programs to have better timing guarantees for access to an A/D
converter. If a program does not need precise timing guarantees, however (e.g.,
when measuring temperature in a biological monitoring application), this extra
resource reservation step unnecessarily complicates code and can be handled
nicely using virtualization. The following section introduces the concept of
resource classes in order to address this issue. The sharing policy used by a
particular resource abstraction is dictated by the resource class it belongs to.
2. Resource Classes
====================================================================
TinyOS 2.x distinguishes between three kinds of abstractions:
*dedicated*, *virtualized*, and *shared*. Components offer resource
sharing mechanisms appropriate to their goals and level of abstraction.
.. Note::
It is important to point out that Hardware Presentation Layer (HPL)
components of the HAA [1]_ can never be virtualized, as virtualization
inevitably requires state. Depending on their
expected use, HPL abstractions can either be dedicated or
shared. For example, while hardware timers are rarely
multiplexed between multiple components, buses almost always are.
This can be seen on the MSP430 microcontroller, where the compare and
counter registers are implemented as dedicated resources, and the USARTs
are shared ones.
2.1 Dedicated
-------------------------------
An abstraction is *dedicated* if it is a resource
which a subsystem needs exclusive access to at all times.
In this class of resources, no sharing policy is needed since only
a single component ever requires use of the resource. Examples of
dedicated abstractions include interrupts and counters.
Dedicated abstractions MAY be annotated with the nesC attribute
@atmostonce or @exactlyonce to provide compile-time checks that
their usage assumptions are not violated.
Please refer to Appendix A for an example of how a dedicated
resource might be represented, including the use of
the nesC @exactlyonce attribute.
2.2 Virtualized
-------------------------------
*Virtual* abstractions hide multiple clients from each other
through software virtualization. Every client of a virtualized resource
interacts with it as if it were a dedicated resource, with all virtualized
instances being multiplexed on top of a single underlying resource. Because
the virtualization is done in software, there is no upper bound on the number
of clients using the abstraction, barring memory or efficiency constraints.
As virtualization usually requires keeping state that scales with the number
of virtualized instances, virtualized resources often use the Service Instance
pattern [3]_, which is based on a parameterized interface.
Virtualization generally provides a very simple interface to its clients.
This simplicity comes at the cost of reduced efficiency and an inability to
precisely control the underlying resource. For example, a virtualized
timer resource introduces CPU overhead from dispatching and maintaining
each individual virtual timer, as well as introducing jitter whenever two
timers happen to fire at the same time. Please refer to Appendix A for an
example of how such a virtualized timer resource might be implemented.
2.3 Shared
-------------------------------
Dedicated abstractions are useful when a resource is
always controlled by a single component. Virtualized abstractions are
useful when clients are willing to pay a bit of overhead and sacrifice
control in order to share a resource in a simple way. There are
situations, however, when many clients need precise control of a
resource. Clearly, they can't all have such control at the same time:
some degree of multiplexing is needed.
A motivating example of a shared resource is a bus.
The bus may have multiple peripherals on it, corresponding to
different subsystems. For example, on the Telos platform the flash
chip (storage) and the radio (network) share a bus. The storage and
network stacks need exclusive access to the bus when using it,
but they also need to share it with the other subsystem. In this
case, virtualization is problematic, as the radio stack needs to be
able to perform a series of operations in quick succession without
having to reacquire the bus in each case. Having the bus be a
shared resource allows the radio stack to send a series of operations
to the radio atomically, without having to buffer them all up
in memory beforehand (introducing memory pressure in the process).
In TinyOS 2.x, a resource *arbiter* is responsible for multiplexing
between the different clients of a shared resource. It determines
which client has access to the resource at which time. While a client
holds a resource, it has complete and unfettered control. Arbiters assume
that clients are cooperative, only acquiring the resource when needed
and holding on to it no longer than necessary. Clients explicitly
release resources: there is no way for an arbiter to forcibly reclaim it.
The following section is dedicated to describing the arbiter and its
interfaces.
3. Resource Arbite
rs
====================================================================
Every shared resource has an arbiter to manage which client
can use the resource at any given time. Because an arbiter is a
centralized place that knows whether the resource is in use, it can also
provide information useful for a variety of other services, such as
power management. An arbiter MUST provide a parameterized Resource
interface as well as an instance of the ArbiterInfo interface. An
arbiter SHOULD also provide a parameterized ResourceRequested interface
and use a parameterized ResourceConfigure interface. It MAY also provide
an instance of the ResourceController interface or any additional
interfaces specific to the particular arbitration policy being
implemented.
3.1 Resource
-------------------------------
Clients of a shared resource arbiter request access
using the Resource interface::
interface Resource {
async command error_t request();
async command error_t immediateRequest();
event void granted();
async command error_t release();
async command bool isOwner();
}
A client lets an arbiter know it needs access to a resource by
making a call to request(). If the resource is free,
SUCCESS is returned, and a granted event is signaled
back to the client. If the resource is busy, SUCCESS will
still be returned, but the request will be queued
according to the queuing policy of the arbiter. Whenever a client is
done with the resource, it calls the release() command, and the next
client in the request queue is given access to the resource and
is signaled its granted() event. If a client ever makes multiple
requests before receiving a granted event, an EBUSY value is returned,
and the request is not queued. Using this policy, clients are not able to
hog the resource queue by making multiple requests, but they may still be
able to hog the resource if they do not release it in a timely manner.
Clients can also request the use of a resource through the
immediateRequest() command. A call to immediateRequest() can either
return SUCCESS or FAIL, with requests made through this command never being
queued. If a call to immediateRequest() returns SUCCESS, the client is granted
access to the resource immediately after the call has returned, and no granted
event is ever signaled. If it returns FAIL, the client is not granted access to
the resource and the request does not get queued. The client will have to try
and gain access to the resource again later.
A client can use the isOwner command of the Resource interface to
check if it is the current owner of the resource. This command is mostly
used to perform runtime checks to make sure that clients not owning a resource
are not able to use it. If a call to isOwner fails, then no call
should be made to commands provided by that resource.
An arbiter MUST provide exactly one parameterized Resource interface,
where the parameter is a client ID, following the Service
Instance pattern[3]_. An arbitrated component SomeNameP MUST
#define SOME_NAME_RESOURCE to a string which can be passed to unique()
to obtain a client id. This #define must be placed in a separate file
because of the way nesC files are preprocessed: including the
SomeNameP component isn't enough to ensure that macros #define'd in
SomeNameP are visible in the referring component.
Please refer to Appendix B for an example of how to wrap this component
inside a generic configuration. Wrapping the component in this way ensures that
each Resource client is given a unique client ID, with the added
benefit of properly coupling multiple components that all need to
refer to the same client ID.
For a complete example of how the I2C resource might be abstracted
according to this pattern, please refer to Appendix B. For further
examples please refer to the various chip implementations in the
tinyos-2.x source tree under tinyos-2.x/chips/
3.2 ArbiterInfo
-------------------------------
Arbiters MUST provide an instance of the ArbiterInfo interface.
The ArbiterInfo interface allows a component to query the current
status of an arbiter::
interface ArbiterInfo {
async command bool inUse();
async command uint8_t userId();
}
The ArbiterInfo interface has a variety of uses. For example, the resource
implementation can use it to refuse requests from clients that do not
currently have access. For an example of how this interface is used
in this fashion please refer to Appendix B.
3.3 ResourceRequested
-------------------------------
Sometimes it is useful for a client to be able to hold onto a resource until
someone else needs it and only at that time decide to release it. Using the
ResourceRequested interface, this information is made available to the current
owner of a resource::
interface ResourceRequested {
async event void requested();
async event void immediateRequested();
}
A requested event is signaled to the current owner of the resource if another
client makes a request for the resource through the request() command of
its Resource interface. If a request is made through the immediateRequest()
command, then the immediateRequested() event is signaled.
An arbiter SHOULD provide a parameterized ResourceRequested interface to its
clients, but is not required to. The client id of the parameterized
ResourceRequested interface should be coupled with the client id of the Resource
interface to ensure that all events are signaled to the proper clients. Please
refer to Appendix B for an example of how this interface might be used.
3.4 ResourceConfigure
-------------------------------
The ResourceConfigure interface provides a mechanism for clients
that need to use a resource with different configurations to do so.
Rather than forcing a client to reconfigure the resource itself, the
component representing a client can wire to an arbiter's
ResourceConfigure interface, which is called just before granting the
client the resource::
interface ResourceConfigure {
async command void configure();
async command void unconfigure();
}
The arbiter SHOULD use a parameterized ResourceConfigure interface, with
its client ID parameter coupled with the client id of its parameterized
Resource interface. If an arbiter uses the ResourceConfigure interface,
it MUST call ResourceConfigure.configure() on the granted client ID
before it signals the Resource.granted() event. Similarly, after a valid
call to Resource.release(), it MUST call ResourceConfigure.unconfigure()
on the releasing client ID. By calling ResourceConfigure.configure() just
before granting a client access to a resource and calling
ResourceConfigure.unconfigure() just before fully releasing it, it is guaranteed
that a resource is always unconfigured before an attempt to configure it can be
made again.
Using the parameterized ResourceConfigure interface that calls out rather than
additional commands being put into the Resource interface
simplifies code reuse. Introducing these new commands into the
Resource interface could lead to a large number of clients all
including redundant configuration code, while using the call out
approach provided by the parameterized interface will only have one
instance of the code.
For an example of how the three different modes of the Msp430 Usart
component can take advantage of this ResourceConfigure interface
please refer to Appendix B as well as section 4 on the use of
cross-component reservation.
3.5 ResourceController
-------------------------------
The normal Resource interface is for use by clients that all share the resource
in an equal fashion. The ResourceController interface is for use by a single
client that needs to be given control of the resource whenever no one else is
using it. An arbiter MAY provide a single instance of the ResourceController
interface.::
interface ResourceController {
async event void granted();
async command error_t release();
async command bool isOwner();
async event void requested();
async event void immediateRequested();
}
The Arbiter MUST guarantee that the user of the ResourceCon
troller interface is
made the owner of the resource before the boot initialization sequence is
completed. When a normal resource client makes a request for the resource, the
ResourceController will receive either a requested() or an immediateRequested()
event depending on how the request was made. It must then decide if and when to
release it. Once released, all clients that have pending requests will be
granted access to the resource in the order determined by the queuing policy of
the arbiter in use. Once all pending requests have been granted (including
those that came in while other clients had access to the resource), the
ResourceController is automatically given control of the resource, receiving its
granted() event in the process. The ResourceController interface also contains
the same isOwner() command as the normal Resource interface, and the semantics
of its use are exactly the same.
Although the ResourceController interface looks similar to a combination of the
normal Resource interface and the ResourceRequested interface, its intended use
is quite different. The ResourceController interface should only be used by
clients that wish to have access to a resource only when no other clients are
using it. They do not actively seek out access to the resource, but rather use
it to perform operations when it would otherwise simply be idle.
Combining the use of the Resource and ResourceRequested interfaces could be made
to operate in a similar manner as the ResourceController interface, except that
there is no way to tell a client with these interfaces that a resource has gone
completely idle. Each client must actively request the use of the resource and
be granted that resource in the order determined by the queuing policy of its
arbiter. With the ResourceController interface, there is no active request of
the resource. The arbiter simply signals the granted event to the
ResourceController whenever there are no more pending requests and the last
client that owned the resource releases it.
The primary motivation behind the definition of the ResourceController
interface was to allow for an easy integration of the power management
policy used by the resource with the arbitration of the resource
itself. Arbiters that want to allow a resource to be controlled by a particular power management policy can provide the ResourceController interface
for use by a component that implements that policy. The power management component will receive the granted() event whenever the resource has gone idle,
and will then proceed in powering it down. When another
client requests the resource, the power manager will be notified through
either the requested() or immediateRequested() events as appropriate. It
can then power up the resource and release it once the power up has completed.
Note that if power up is a split-phase operation (takes a while), then calls
by clients to immediateRequest() when in the powered down state will not return
SUCCESS. Please see the TEP on the Power Management of Non-Virtualized devices
([4]_) for more details.
4. Cross-Component Reservation
====================================================================
In some cases, it is desirable to share the reservation of a
single resource across multiple components. For example, on the TI
MSP430, a single USART component can be used as an I2C bus, a UART,
or an SPI connection. Clearly, on this chip, a reservation of the I2C
bus implicitly restricts the corresponding UART and SPI
services from gaining access to the resource. Enforcing such a policy
can be accomplished in the framework described above by:
1) Creating a set of unique ids for each service using the shared
resource.
2) Mapping these ids onto the ids of the underlying resource
Clients connecting to these services do not know that that this
mapping is taking place. As far as they are concerned, the only
arbitration taking place is between other clients using the same
service. In the MSP430 example, a single client of the I2C bus could
be contending with a single client of the SPI connection, but they
would probably have the same service level client ID. These two
service level client ids would be mapped onto 2 unique resource ids
for use by the shared USART component. The proper way to achieve this
mapping is through the use of generic components. The example given
below shows how to perform this mapping for the SPI component on the
MSP430. It is done similarly for the UART and I2C bus::
#include "Msp430Usart.h"
generic configuration Msp430Spi0C() {
provides interface Resource;
provides interface SpiByte;
provides interface SpiPacket;
}
implementation {
enum { CLIENT_ID = unique(MSP430_SPIO_BUS) };
components Msp430Spi0P as SpiP;
Resource = SpiP.Resource[ CLIENT_ID ];
SpiByte = SpiP.SpiByte;
SpiPacket = SpiP.SpiPacket[ CLIENT_ID ];
components new Msp430Usart0C() as UsartC;
SpiP.UsartResource[ CLIENT_ID ] -> UsartC.Resource;
SpiP.UsartInterrupts -> UsartC.HplMsp430UsartInterrupts;
}
The definition of the MSP430_SPIO_BUS string is defined in
Msp430Usart.h. A unique id is created from this string every time a
new Msp430Spi0C component is instantiated. This id is used as a
parameter to the parameterized Resource interface provided by the
Msp430Spi0P component. This is where the mapping of the two
different ids begins. As well as *providing* a parameterized
Resource interface, the Msp430Spi0P component also *uses* a
parameterized Resource interface. Whenever a client makes a call
through the provided Resource interface with id CLIENT_ID, an
underlying call to the used Resource interface with the same id is
implicitly made. By wiring the used Resource interface with id
CLIENT_ID to the instance of the Resource interface provided by the
instantiation of the Msp430Usart0C component, the mapping is complete.
Any calls to the Resource interface provided by a new instantiation of
the Msp430Spi0C component will now be made through a unique Resource
interface on the underlying Msp430Usart0C component.
This level of indirection is necessary because it may not always be
desirable to directly wire the service level Resource interface to
the underlying shared Resource interface. Sometimes we may want to
perform some operations between a service level command being
called, and calling the underlying command on the shared resource.
With such a mapping, inserting these operations is made possible.
Having such a mapping is also important for services that need to
explicitly keep track of the number of clients they have,
independent from how many total clients the underlying shared
resource has. For example, a sensor implementation that uses an
underlying ADC resource may wish to power down its sensor whenever it
has no clients. It doesn't want to have to wait until the entire ADC
is free to do so. Providing this mapping allows the implicit power
manager components described in TEP 115 to be wired in at both levels
of the abstraction without interfering with one another. In this
way, implementations of these components become much simpler, and code
reuse is encouraged.
Implementations of components similar to this one can be found in the
tinyos-2.x source tree in the tos/chips/msp430/uart directory
5. Implementation
====================================================================
Because most components use one of a small number of arbitration
policies, tinyos-2.x includes a number of default resource arbiters. These
arbiters can be found in ``tinyos-2.x/tos/system`` and are all
generic components that include one of the two signatures seen below::
generic module SimpleArbiter {
provides interface Resource[uint8_t id];
provides interface ResourceRequested[uint8_t id];
provides interface ArbiterInfo;
uses interface ResourceConfigure[uint8_t id];
}
generic module Arbiter {
provides interface Resource[uint8_t id];
provides interface ResourceRequested[uint8_t id];
provides interface ResourceController;
provides interface ArbiterInfo;
uses interface ResourceCon
figure[uint8_t id];
}
The "Simple" arbiters are intended for use by resources that
do not require the additional overhead incurred by providing the
ResourceController interface.
For many situations, changing an arbitration policy requires nothing
more than changing the queuing policy it uses to decide the order in
which incoming requests should be granted. In this way, separating
queuing policy implementations from actual arbitration implementations
encourages code reuse. The introduction of the SimpleArbiterP and
ArbiterP components found under tinyos-2.x/tos/system help in this
separation. They can be wired to components providing
a particular queuing policy through the use of the ResourceQueue
interface.::
interface ResourceQueue {
async command bool isEmpty();
async command bool isEnqueued(resource_client_id_t id);
async command resource_client_id_t dequeue();
async command error_t enqueue(resource_client_id_t id);
}
An example of wiring a First-Come-First-Serve (FCFS) queuing policy to
the SimpleArbiterP component using the ResourceQueue interface
defined above can be seen below::
generic configuration SimpleFcfsArbiterC(char resourceName[]) {
provides {
interface Resource[uint8_t id];
interface ResourceRequested[uint8_t id];
interface ArbiterInfo;
}
uses interface ResourceConfigure[uint8_t id];
}
implementation {
components MainC;
components new FcfsResourceQueueC(uniqueCount(resourceName)) as Queue;
components new SimpleArbiterP() as Arbiter;
MainC.SoftwareInit -> Queue;
Resource = Arbiter;
ResourceRequested = Arbiter;
ArbiterInfo = Arbiter;
ResourceConfigure = Arbiter;
Arbiter.Queue -> Queue;
}
This generic configuration can instantiated by a resource in order
to grant requests made by its clients in an FCFS fashion.
All of the default queuing policies provided in tinyos-2.x along with the
respective arbitration components that have been built using them are
given below:
Queuing Policies:
- FcfsResourceQueueC
- RoundRobinResourceQueueC
Arbiters:
- SimpleFcfsArbiterC
- FcfsArbiterC
- SimpleRoundRobinArbiterC
- RoundRobinArbiterC
6. Author's Address
====================================================================
| Kevin Klues
| 503 Bryan Hall
| Washington University
| St. Louis, MO 63130
|
| phone - +1-314-935-6355
| email - klueska at cs.wustl.edu
|
| Philip Levis
| 358 Gates Hall
| Stanford University
| Stanford, CA 94305-9030
|
| phone - +1 650 725 9046
| email - pal at cs.stanford.edu
|
| David Gay
| 2150 Shattuck Ave, Suite 1300
| Intel Research
| Berkeley, CA 94704
|
| phone - +1 510 495 3055
| email - david.e.gay at intel.com
|
| David Culler
| 627 Soda Hall
| UC Berkeley
| Berkeley, CA 94720
|
| phone - +1 510 643 7572
| email - culler at cs.berkeley.edu
|
| Vlado Handziski
| Sekr FT5
| Einsteinufer 25
| 10587 Berlin
| GERMANY
|
| email - handzisk at tkn.tu-berlin.de
7. Citations
====================================================================
.. [1] TEP 2: Hardware Abstraction Architecture.
.. [2] TEP 102: Timers.
.. [3] Service Instance Pattern. In *Software Design Patterns for TinyOS.* David Gay, Philip Levis, and David Culler. Published in Proceedings of the ACM SIGPLAN/SIGBED 2005 Conference on Languages, Compilers, and Tools for Embedded Systems (LCTES'05).
.. [4] TEP 115: Power Management of Non-Virtualized Devices.
.. [5] TinyOS Programming. http://csl.stanford.edu/~pal/pubs/tinyos-programming-1-0.pdf
Appendix A: Resource Class Examples
====================================================================
Dedicated Resource
--------------------------------------------------------------------
Timer 2 on the Atmega128 microprocessor is a dedicated resource
represented by the HplAtm128Timer2C component::
module HplAtm128Timer2C {
provides {
interface HplTimer<uint8_t> as Timer2 @exactlyonce();
interface HplTimerCtrl8 as Timer2Ctrl @exactlyonce();
interface HplCompare<uint8_t> as Compare2 @exactlyonce();
}
}
Only a single client can wire to any of these interfaces as enforced through
the nesC @exactlyonce attribute. Keep in mind that although the interfaces of
this component are only allowed to be wired to once, nothing prevents the
component wiring to them from virtualizing the services they provide at some
higher level. If you are unfamiliar with how @exactlyonce and other nesC
attributes are used to by the nesC compiler, please refer to section 9.1 of the
TinyOS Programming Manual [5]_.
Virtualized Resource
--------------------------------------------------------------------
The TimerMilliC component provides a virtual abstraction of millisecond
precision timers to application components [2]_. It encapsulates the required
parameterized Timer interface through the use of a generic configuration.
Clients wishing to use a millisecond timer need only instantiate a single
instance of the TimerMilliC generic, leaving the fact that it is virtualized
underneath transparent.::
generic configuration TimerMilliC {
provides interface Timer<TMilli>;
}
implementation {
components TimerMilliP;
Timer = TimerMilliP.TimerMilli[unique(UQ_TIMER_MILLI)];
}
The actual parameterized Timer interface is provided by the chip specific
HilTimerMilliC component. This interface is exposed through
the TimerMilliP component which wires HilTimerMilliC to the boot
initialization sequence::
configuration TimerMilliP {
provides interface Timer<TMilli> as TimerMilli[uint8_t num];
}
implementation {
components HilTimerMilliC, MainC;
MainC.SoftwareInit -> HilTimerMilliC;
TimerMilli = HilTimerMilliC;
}
Appendix B: Arbiter Interface Examples
====================================================================
.. Note:
Most of the examples provided in this section use complex nesC syntax that may
be unfamiliar to the novice nesC programmer. Please refer to the TinyOS
programming Manual [5]_ for clarification as necessary.
Resource
--------------------------------------------------------------------
Examples of how to use the Resource interface for arbitrating
between multiple clients can be found in the tinyos-2.x
source tree under tinyos-2.x/apps/tests/TestArbiter.
A specific example of where the Resource.isOwner() is used
can be seen in the HplTda5250DataP component of the Infineon
Tda5250 radio implementation::
async command error_t HplTda5250Data.tx(uint8_t data) {
if(call UartResource.isOwner() == FALSE)
return FAIL;
call Usart.tx(data);
return SUCCESS;
}
A call to the HplTda5250Data.tx command will fail if the radio does
not currently have control of the underlying Usart resource. If it
does, then the Usart.tx(data) command is called as requested.
A component using the Resource interface to implement an I2C
service might look like this::
#include I2CPacket.h
configuration I2CPacketP {
provides interface Resource[uint8_t client];
provides interface I2CPacket<I2CAddrSize>[uint8_t client];
}
implementation {
components new FcfsArbiterC(I2CPACKET_RESOURCE) as Arbiter;
components I2CPacketImplP() as I2C;
...
Resource = Arbiter;
I2CPacket = I2C;
...
}
where I2CPacketImplP contains the actual implementation of the
I2C service, and I2CPacket.h contains the #define for the
name of the resource required by the arbiter::
#ifndef I2CPACKETC_H
#define I2CPACKETC_H
#define I2CPACKET_RESOURCE "I2CPacket.Resource"
#endif
This service would then be made available to a user through
the generic configuration seen below::
#include I2CPacket.h
generic configuration I2CPacketC {
provides interface Resource;
provides interface I2CPacket<I2CAddrSize>;
}
implementation {
enum { CLIENT_ID = unique(I2CPACKET_RESOURCE) };
components I2CPacketP as I2C;
Resource = I2C.Resource[CLIENT_ID];
I2CPacket = I2C.I2CPacket[CLIENT_ID];
}
In this example, an instance of the I2CPacket interface is coupled with
an instance of the Resource interface on every new instantiatio
n of
the I2CPacketC component. In this way, a single Resource and a
single I2CPacket interface can be exported by this component together
for use by a client.
Clients of the I2C service would use it as follows::
module I2CClientP {
uses interface Resource as I2CResource;
uses interface I2CPacket<I2CAddrSize>;
} ...
configuration I2CClientC { }
implementation {
components I2CClientP, new I2CPacketC();
I2CClientP.I2CResource -> I2CPacketC.Resource;
I2CUserM.I2CPacket -> I2CPacket.I2CPacket;
}
ArbiterInfo
--------------------------------------------------------------------
In the implementation of the ADC component on the Msp430 microcontroller,
a simple arbiter is used to provide a round robin sharing policy
between clients of the ADC::
configuration Msp430Adc12C {
provides interface Resource[uint8_t id];
provides interface Msp430Adc12SingleChannel[uint8_t id];
provides interface Msp430Adc12FastSingleChannel[uint8_t id];
}
implementation {
components Msp430Adc12P,MainC,
new SimpleRoundRobinArbiterC(MSP430ADC12_RESOURCE) as Arbiter,
...
Resource = Arbiter;
SingleChannel = Msp430Adc12P.SingleChannel;
FastSingleChannel = Msp430Adc12P.FastSingleChannel;
Msp430Adc12P.Init <- MainC;
Msp430Adc12P.ADCArbiterInfo -> Arbiter;
...
}
In this configuration we see that the Resource interface provided by
Msp430Adc12C is wired directly to the instance of the SimpleRoundRobinArbiterC
component that is created. The ArbiterInfo interface provided by
SimpleRoundRobinArbiterC is then wired to Msp430Adc12P. The Msp430Adc12P
component then uses this interface to perform run time checks to ensure that
only the client that currently has access to the ADC resource is able to
use it::
async command error_t Msp430Adc12SingleChannel.getSingleData[uint8_t id]() {
if (call ADCArbiterInfo.userId() == id){
configureChannel()
// Start getting data
}
else return ERESERVE;
}
async command error_t Msp430Adc12FastSingleChannel.configure[uint8_t id]() {
if (call ADCArbiterInfo.userId() == id){
clientID = id
configureChannel()
}
else return ERESERVE;
}
async command error_t Msp430Adc12FastSingleChannel.getSingleData[uint8_t id]()
{
if (clientID = id)
// Start getting data
else return ERESERVE;
}
In order for these runtime checks to succeed, users of the
Msp430Adc12SingleChannel and Msp430Adc12FastSingleChannel interfaces will have
to match their client id's with the client id of a corresponding Resource
interface. This can be done in the following way::
generic configuration Msp430Adc12ClientC() {
provides interface Resource;
provides interface Msp430Adc12SingleChannel;
}
implementation {
components Msp430Adc12C;
enum { ID = unique(MSP430ADC12_RESOURCE) };
Resource = Msp430Adc12C.Resource[ID];
Msp430Adc12SingleChannel = Msp430Adc12C.SingleChannel[ID];
}
::
generic configuration Msp430Adc12FastClientC() {
provides interface Resource;
provides interface Msp430Adc12FastSingleChannel;
}
implementation {
components Msp430Adc12C;
enum { ID = unique(MSP430ADC12_RESOURCE) };
Resource = Msp430Adc12C.Resource[ID];
Msp430Adc12FastSingleChannel = Msp430Adc12C.SingleChannel[ID];
}
Since these are generic components, users simply need to instantiate them
in order to get access to a single Resource interface that is already
properly coupled with a Msp430Adc12SingleChannel or Msp430Adc12FastSingleChannel
interface.
Take a look in the tinyos-2.x source tree under tinyos-2.x/tos/chips/adc12
to see the full implementation of these components in their original context.
ResourceRequested
--------------------------------------------------------------------
On the eyesIFXv2 platform, both the radio and the flash need access to the bus
provided by the Usart0 component on the Msp430 microcontroller. Using
a simple cooperative arbitration policy, these two components should able to
share the Usart resource by only holding on to it as long as they need it and
then releasing it for use by the other component. In the case of the MAC
implementation of the Tda5250 radio component, however, the Msp430 Usart
resource needs be held onto indefinitely. It only ever considers releasing the
resource if a request from the flash component comes in through its
ResourceRequested interface. If it cannot release it right away (i.e. it is in
the middle of receiving a packet), it defers the release until some later point
in time. Once it is ready to release the resource, it releases it, but then
immediately requests it again. The flash is then able to do what it wants with
the Usart, with the radio regaining control soon thereafter.
In the CsmaMacP implementation of the Tda5250 radio we see the ResourceRequested
event being implemented::
async event void ResourceRequested.requested() {
atomic {
/* This gives other devices the chance to get the Resource
because RxMode implies a new arbitration round. */
if (macState == RX) call Tda5250Control.RxMode()();
}
}
Through several levels of wiring, the Tda5250 interface is provided to this
component by the Tda5250RadioP component::
module Tda5250RadioP {
provides interface Tda5250Control;
uses interface Resource as UsartResource;
...
}
implementation {
async command error_t Tda5250Control.RxMode() {
if(radioBusy() == FALSE)
call UsartResource.release();
call UsartResource.request();
}
event void UsartResource.granted() {
// Finish setting to RX Mode
}
...
}
Although the full implementation of these components is not provided, the
functionality they exhibit should be clear. The CsmaMacP component receives the
ResourceRequested.requested() event when the flash requests the use of the
Msp430 Usart0 resource. If it is already in the receive state, it tries to
reset the receive state through a call to a lower level component. This
component checks to see if the radio is in the middle of doing anything (i.e.
the radioBusy() == FALSE check), and if not, releases the resource and then
requests it again.
To see the full implementations of these components and their wirings to one
another, please refer to the tinyos-2.x source tree under
tinyos-2.x/tos/chips/tda5250, tinyos-2.x/tos/chips/tda5250/mac,
tinyos-2.x/tos/platforms/eyesIFX/chips/tda5250, and
tinyos-2.x/tos/platforms/eyesIFX/chips/msp430.
Resource Configure
--------------------------------------------------------------------
The Msp430 Usart0 bus can operate in three modes: SPI, I2C,
and UART. Using all three concurrently is problematic: only one should
be enabled at any given time. However, different clients of the bus might
require the bus to be configured for different protocols. On Telos, for example
many of the available sensors use an I2C bus, while the radio and flash chip use
SPI.
A component providing the SPI service on top of the shared Usart component looks
like this::
generic configuration Msp430Spi0C() {
provides interface Resource;
provides interface SpiByte;
provides interface SpiPacket;
...
}
implementation {
enum { CLIENT_ID = unique( MSP430_SPIO_BUS ) };
components Msp430SpiNoDma0P as SpiP;
components new Msp430Usart0C() as UsartC;
SpiP.ResourceConfigure[ CLIENT_ID ] <- UsartC.ResourceConfigure;
SpiP.UsartResource[ CLIENT_ID ] -> UsartC.Resource;
SpiP.UsartInterrupts -> UsartC.HplMsp430UsartInterrupts;
...
}
And one providing the I2C service looks like this::
generic configuration Msp430I2CC() {
provides interface Resource;
provides interface I2CPacket<TI2CBasicAddr> as I2CBasicAddr;
...
}
implementation {
enum { CLIENT_ID = unique( MSP430_I2CO_BUS ) };
components Msp430I2C0P as I2CP;
components new Msp430Usart0C() as UsartC;
I2CP.ResourceConfigur
e[ CLIENT_ID ] <- UsartC.ResourceConfigure;
I2CP.UsartResource[ CLIENT_ID ] -> UsartC.Resource;
I2CP.I2CInterrupts -> UsartC.HplMsp430UsartInterrupts;
...
}
The implementation of the ResourceConfigure interface is
provided by both the Msp430SpiNoDma0P and the Msp430I2C0P. In the
two different components, the same Msp430Usart0C component is used,
but wired to the proper implementation of the ResourceConfigure
interface. In this way, different instances of the Msp430Usart0C
can each have different configurations associated with them, but
still provide the same functionality.
Take a look in the tinyos-2.x source tree under
tinyos-2.x/tos/chips/msp430/usart to see the full implementation of
these components along with the corresponding Uart implementation.
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