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<concept id="GUID-0D2F811C-81C3-526F-8EA4-98E50261BF4B" xml:lang="en"><title>DMA

Framework Technology</title><shortdesc>Describes the classes that the DMA Framework provides to transfer

data using DMA. </shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

<p>The following diagram shows the general relationship between the various

classes and structs that form the DMA Framework. The individual items are

described in more detail below. </p>

<fig id="GUID-C6B032F0-7E05-5E29-8024-B713C42A2434">

<image href="GUID-D43AB2F5-32AE-540C-80D8-DE8B2072F1E6_d0e389226_href.png" placement="inline"/>

</fig>

<section id="GUID-D9855D5F-3D4D-5A65-A1BB-B7CB94E60316"><title>The DMA Software

Controller</title> <p>This is the <xref href="GUID-25398075-927B-36E4-B953-16785EC4086C.dita"><apiname>TDmac</apiname></xref> object. It has

two purposes: </p> <ul>

<li id="GUID-E4C8A5C5-192B-5E3E-A2F1-D884BF7FE641"><p>It defines the main

interface between the platform independent and platform specific layers. </p> </li>

<li id="GUID-98888CE1-5AA6-5D91-A5C5-E60F1200465A"><p>it is a container for

channels, descriptors and descriptor headers </p> </li>

</ul> </section>

<section id="GUID-37DF82BE-5F8E-5497-90E8-C48F44C2A7F1"><title>The Channel

Manager</title> <p>The channel manager is a <xref href="GUID-176B8E0D-0422-341B-A134-7C85432E1303.dita"><apiname>DmaChannelMgr</apiname></xref> object. </p> <p> <codeph> DmaChannelMgr</codeph> is

a static class defined by the platform independent layer but implemented in

the platform specific layer. The functionality is used by the platform independent

layer. It contains: </p> <ul>

<li id="GUID-B5626DA4-1465-5469-BE3D-36F31996C75A"><p>a function to open a

DMA channel: <xref href="GUID-176B8E0D-0422-341B-A134-7C85432E1303.dita#GUID-176B8E0D-0422-341B-A134-7C85432E1303/GUID-934F46B3-1DF9-3870-87EE-A8E2DEF82810"><apiname>DmaChannelMgr::Open()</apiname></xref>  </p> </li>

<li id="GUID-67C7DE2F-196E-5919-BA80-99898DF6837D"><p>a function that is called

when a channel is closed: <xref href="GUID-176B8E0D-0422-341B-A134-7C85432E1303.dita#GUID-176B8E0D-0422-341B-A134-7C85432E1303/GUID-909D795A-7303-3A76-9C8E-3B07A97DD716"><apiname>DmaChannelMgr::Close()</apiname></xref>  </p> </li>

<li id="GUID-F044E05A-A307-51B0-BDD5-8A29A950BE5F"><p>a function that can

be used to extend the framework with platform specific functionality on a

channel independent basis: <xref href="GUID-176B8E0D-0422-341B-A134-7C85432E1303.dita#GUID-176B8E0D-0422-341B-A134-7C85432E1303/GUID-C733B302-4269-3391-8ADE-617CFF198B56"><apiname>DmaChannelMgr::StaticExtension()</apiname></xref>  </p> </li>

</ul> </section>

<section id="GUID-34B8E965-9C5C-533E-BFE6-EB9B486A81BB"><title>Descriptors</title> <p>DMA

controllers operating in scatter/gather mode are configured via a linked list

of small data structures describing the data to be transferred. These data

structures are called descriptors. (Note that the use of the term descriptor

in the context of DMA should not be confused with the same term widely used

in Symbian platform to refer to the family of <xref href="GUID-52D07F46-2162-380C-A775-C3BB335C42F5.dita"><apiname>TDesC</apiname></xref> derived

classes). </p> <p>The Symbian platform DMA Framework always uses descriptor

data structures to store transfer-configuration-information, even if the underlying

DMA controller does not support scatter/gather mode. </p> <p>The following

example illustrates the idea: assume that a device driver needs to transfer

two disjoint blocks of memory, A and B, into another block C. Block A starts

at address 1000 and is 300 bytes long. Block B starts at address 2000 and

is 700 bytes long. The destination buffer C starts at address 5000 and is

1000 bytes long. Assume that the DMA descriptors are allocated in a pool starting

at address 600. The following diagram shows the scatter/gather list that the

device driver might create: </p> <fig id="GUID-966DC875-FA9E-59E5-A1CA-842F4A82592D">

<image href="GUID-EA0C5715-7CE8-5415-A915-D5701E3C957A_d0e389323_href.png" placement="inline"/>

</fig> <p>If the DMA controller supports the scatter/gather arrangement, then

the framework uses a structure that will be specific to the hardware. This

structure is defined in the platform specific layer. </p> <p>If the DMA controller

does not support scatter/gather, then the framework uses the generic structure <xref href="GUID-D7934AD9-38F6-325A-A734-F867D886D7C2.dita"><apiname>SDmaPseudoDes</apiname></xref> containing

the following information: </p> <ul>

<li id="GUID-D5EA7DC2-FC49-5FF3-95FE-6E5426D25BFF"><p>a set of generic flags

that characterise the transfer. For example, is the source of the data memory

or a peripheral; is the destination memory or peripheral; what addressing

mode is to be used? </p> </li>

<li id="GUID-4D54D3E6-0252-5F15-9966-E7F09A10020E"><p>the source and destination

location. This is in the form of the base virtual address for a memory buffer,

and a 32-bit value (or cookie) for peripherals. The meaning of the 32-bit

value is interpreted by the platform specific layer. </p> </li>

<li id="GUID-416CB113-5973-50AD-9FF3-F3AF78B47D0C"><p>The number of bytes

to be transferred. </p> </li>

<li id="GUID-D76CE2CE-5005-5C2F-B17D-F882D6BD7EB8"><p>A word that only has

meaning for the platform specific layer, and passed by the client at request

fragmentation time. </p> </li>

</ul> </section>

<section id="GUID-BC1C248D-170D-51AE-B2E5-61FFEC779D61"><title>Descriptor

headers</title> <p>These are objects of type <xref href="GUID-710B3501-FF22-307D-AC88-D142E9A07CB8.dita"><apiname>SDmaDesHdr</apiname></xref>. </p> <p>A

descriptor header allows additional information about a descriptor to be stored.

The header is a separate object from the descriptor because it is difficult

to embed additional data in a structure whose layout may be hardware-imposed. </p> <p>Descriptors

and descriptor headers are stored in two parallel arrays allocated at boot-time,

and each descriptor is always associated with the header of same index, so

that there is always a one-to-one relationship between the header and the

descriptor. </p> <fig id="GUID-A635E3EA-883C-5857-8690-E0AE6F43CAC4">

<image href="GUID-99F7E70F-2733-57B2-94F5-A0C0FF9219FE_d0e389379_href.png" placement="inline"/>

</fig> <p>In the current design, the only information in the descriptor header

is a pointer, <codeph>SDmaDesHdr::iNext</codeph>, that is used to chain headers

together on various lists. So, although the pool of headers is allocated as

an array, they are almost always accessed by following the chain of pointers

linking one header to the next. </p> <p>Descriptors are <i>always</i> accessed

through their associated header. </p> <p>The platform independent layer never

directly accesses hardware-specific descriptors. It just passes descriptor

headers to the platform specific layer. However, the platform independent

layer does directly manipulate the generic descriptors, <xref href="GUID-D7934AD9-38F6-325A-A734-F867D886D7C2.dita"><apiname>SDmaPseudoDes</apiname></xref>. </p> </section>

<section id="GUID-F4685621-1FAA-544C-B4C0-B4DD566B856F"><title>Transfer Requests</title> <p>A

transfer request is the way in which a device driver sets up and initiates

a DMA transfer, and is represented by a <xref href="GUID-780F4D53-5546-3B69-B328-0226C70EBDE2.dita"><apiname>DDmaRequest</apiname></xref> object. </p> <p>A

transfer request has a number of characteristics: </p> <ul>

<li id="GUID-D5B73919-B6C8-54F9-8C1E-87467E32EAA0"><p>it is always associated

with exactly one channel. </p> </li>

<li id="GUID-FE3326FA-B0CF-50A2-BD4D-72F868226EFB"><p>it stores a client-provided

callback function, which is invoked when the whole request completes, whether

successfully or not. </p> </li>

<li id="GUID-627B7D15-0119-5D72-A517-84832D90C3D1"><p>it can be in one of

four states: </p> <ul>

<li id="GUID-1D600F30-F06C-58FB-B258-8167164FE1FC"><p>not configured </p> </li>

<li id="GUID-194A4B9A-12E6-50B6-816A-73717CD100F9"><p>idle </p> </li>

<li id="GUID-0B7A0249-9F5D-5D5D-AB3C-4DDE8AFC58E6"><p>being transferred </p> </li>

<li id="GUID-AF57DDF1-406D-55D4-A001-F62921165189"><p>pending; this state

only occurs in streaming mode. </p> </li>

</ul> </li>

</ul> <p>Internally, a transfer request is represented as a singly linked

list of descriptor headers. Each header in the list is associated with a descriptor

that specifies how to transfer one fragment of the whole request. Transfer

requests have pointers to the first and last headers in the list. </p> <p>When

the request is idle, the header list ends with a NULL pointer. This is not

always true when the request is queued (i.e. when the request is being transferred

or is still pending). The following diagram shows an idle request with three

fragments. </p> <fig id="GUID-0A5594BD-05E4-5A97-A3F7-459E3F8638A2">

<image href="GUID-CF252B09-335E-5831-94A6-0B16B64C5030_d0e389465_href.png" placement="inline"/>

</fig> <p>Splitting a request into fragments is useful because: </p> <ul>

<li id="GUID-BEA0F4C5-4308-54E9-82AB-91570ACC9ADC"><p>it insulates device

drivers from the maximum transfer size supported by the underlying DMA controller. </p> </li>

<li id="GUID-A768A76B-DD9E-5A25-92D3-23A69299C096"><p>the source and destination

DMA buffers may not be physically contiguous and thus require fragmenting. </p> </li>

</ul> <p>Both of these situations can be handled by using the generic fragmentation

algorithm <xref href="GUID-780F4D53-5546-3B69-B328-0226C70EBDE2.dita#GUID-780F4D53-5546-3B69-B328-0226C70EBDE2/GUID-B14B0478-80D6-3F5E-88B6-1676411D9CF2"><apiname>DDmaRequest::Fragment()</apiname></xref>. </p> <p>Some device

drivers may have to create custom descriptors lists. For example, the USB

section of the PXA250 manual describes how to build custom lists where a descriptor

containing data transfer information is followed by another one poking an

I/O port to issue a command to the USB controller. </p> <p>To cover that case, <xref href="GUID-780F4D53-5546-3B69-B328-0226C70EBDE2.dita"><apiname>DDmaRequest</apiname></xref> provides

the <xref href="GUID-780F4D53-5546-3B69-B328-0226C70EBDE2.dita#GUID-780F4D53-5546-3B69-B328-0226C70EBDE2/GUID-AAB7C209-7D30-3282-B084-D80E12BB5152"><apiname>DDmaRequest::ExpandDesList()</apiname></xref> member function to allocate

a list of headers associated with blank descriptors whose content is then

set by device drivers. Blank descriptors thus created can be accessed in the

following way (this example was taken and simplified from the PXA250 USB controller

driver): </p> <codeblock id="GUID-EFA935A1-0D7E-5153-9071-2D0C58E8E920" xml:space="preserve">TDmaChannel* channel;

TDmaChannel::SCreateInfo info;

info.x = y;

if (TDmaChannel::Open(info, channel) != KErrNone)

    ... return;

DDmaRequest* req = new DDmaRequest(*channel);

if (!req)

    ... return;

const TDmac* dmac = req->iChannel.Controller();

if (req->ExpandDesList(2) != KErrNone)

    ... return;

TDmaDesc* desc1 = static_cast<TDmaDesc*>(dmac->HdrToHwDes(*req->iFirstHdr));

TDmaDesc* desc2 = static_cast<TDmaDesc*>(dmac->HdrToHwDes(*req->iFirstHdr->iNext));

desc1->iSrcAddr  = ...;

desc1->iDestAddr = ...;

desc1->iDescAddr = ...;

desc1->iCmd      = ...;

desc2...

...

</codeblock> </section>

<section id="GUID-4BF7605E-55D4-5B3A-BCD7-1F976765D738"><title>Channels</title> <p>A

channel is a <xref href="GUID-83882548-FAC5-3EFF-92ED-14D1D9A85D37.dita"><apiname>TDmaChannel</apiname></xref> object, and there is one of these

for each hardware DMA channel. </p> <p>A channel can be in one of 4 states: </p> <ul>

<li id="GUID-09581FA5-D01A-502A-9841-52ABDF0BB16E"><p>closed </p> </li>

<li id="GUID-D03F51BA-1493-5B23-B029-4FF5FCC4DE5E"><p>open and idle </p> </li>

<li id="GUID-606FA290-FB8D-5CBE-88C2-BCD03062EE25"><p>open and transferring

data </p> </li>

<li id="GUID-2E845C88-292D-54B9-9F99-AAC3775768E4"><p>suspended following

an error. </p> </li>

</ul> <p>On opening a channel, the client device driver specifies: </p> <ul>

<li id="GUID-6F373D39-4078-5C76-BF5E-69294297D8CE"><p>A 32-bit value (cookie)

that is used by the platform specific layer to select which channel is to

be opened </p> </li>

<li id="GUID-71E4D026-4BA5-592B-BB3E-5F5354640E50"><p>The number of descriptors

that must be reserved for this channel. </p> </li>

<li id="GUID-4C34A82A-AF7E-5683-986B-A65EA6C8F5B2"><p>A DFC to be used by

the framework to service DMA interrupts. </p> </li>

</ul> <p>A channel maintains a queue of transfer requests. If the channel

is being used for one-shot transfers, the queue will always contain an idle

or a transferring request. In streaming mode, the queue may contain several

requests, the first one being transferred and the remaining ones pending.

When a request is completely transferred, it is removed from the queue. The

first request is always the one being transferred. </p> <p>A transferring

channel has a pointer to the header associated with the descriptor being transferred.

The headers of all queued requests are linked together on one linked list. </p> <p>The

following diagram shows a DMA channel with a three-fragment request being

transferred and a two-fragment one pending. The fragment currently being transferred

is the second one of the first request. </p> <fig id="GUID-36E11A64-4626-53F9-925C-3DC323DEA895">

<image href="GUID-86C21C9B-9F08-579F-84E9-CBE46F756373_d0e389578_href.png" placement="inline"/>

</fig> <p>The <xref href="GUID-83882548-FAC5-3EFF-92ED-14D1D9A85D37.dita"><apiname>TDmaChannel</apiname></xref> class contains the code and data

that is common to all of the channel types. The code and data for the specific

channel types: single buffer, double buffer, and scatter/gather channels,

are implemented in the derived classes: <xref href="GUID-A8B4AD1B-770C-363E-A0DE-C78A9786CBDC.dita"><apiname>TDmaSbChannel</apiname></xref>, <xref href="GUID-FD76AF08-6250-3054-8A06-16343E385B23.dita"><apiname>TDmaDbChannel</apiname></xref>,

and <xref href="GUID-2D4CFBB1-8D64-3CF5-B6F0-B24D16D5BAD4.dita"><apiname>TDmaSgChannel</apiname></xref> respectively. </p> <p><b>TDmaSbChannel

State Machine</b> </p> <p>For reference purposes, the following diagram shows

the state machine that <xref href="GUID-A8B4AD1B-770C-363E-A0DE-C78A9786CBDC.dita"><apiname>TDmaSbChannel</apiname></xref> implements to deal

with a single buffer channel. </p> <fig id="GUID-82A8742D-CE5E-5EA2-A19E-6DCA30D9AE61">

<image href="GUID-85332468-292D-589B-891B-0E7ACBADC7BA_d0e389614_href.png" placement="inline"/>

</fig> <p><b>TDmaDbChannel

State Machine</b> </p> <p>For reference purposes, the following diagram shows

the state machine that <xref href="GUID-FD76AF08-6250-3054-8A06-16343E385B23.dita"><apiname>TDmaDbChannel</apiname></xref> implements to deal

with a double buffer channel. </p> <fig id="GUID-1CF4D292-310C-5D75-9E3B-2DB65217FEEA">

<image href="GUID-91958EA5-9444-5895-B4B8-F2C670B81CD7_d0e389631_href.png" placement="inline"/>

</fig> <p><b>TDmaSgChannel

State Machine</b> </p> <p>For reference purposes, the following diagram shows

the state machine that <xref href="GUID-2D4CFBB1-8D64-3CF5-B6F0-B24D16D5BAD4.dita"><apiname>TDmaSgChannel</apiname></xref> implements to deal

with a scatter/gather channel. </p> <fig id="GUID-1206C662-9856-57A1-A0C5-FDF74F2A3BD3">

<image href="GUID-09EE01E2-BF5E-5302-BA25-46C440ADECF5_d0e389648_href.png" placement="inline"/>

</fig> </section>

<section id="GUID-2F8979D3-A4DC-5919-B027-F8C17739A05D"><title>Streaming and

Scatter/Gather DMA Controllers</title> <p>When a transfer request is queued

onto a channel that is already transferring data, the header descriptor for

the new list of descriptors is appended to the header of the previous request

on the queue. If the underlying DMA controller supports hardware descriptors,

they must also be linked together. </p> <p>The following diagram shows how

headers and descriptors for a new request are appended to the existing ones

for a DMA controller. The dashed arrows represent the new links. </p> <fig id="GUID-7E06E92E-C8D1-53FF-AF03-5B1A3EBC1EA5">

<image href="GUID-28F3F720-A2E0-59C9-8BB4-B6124CFC6C89_d0e389664_href.png" placement="inline"/>

</fig> <p> <i>Note that hardware descriptors are linked together using physical

addresses, not virtual ones.</i>  </p> <p>Linking hardware descriptors together

is implemented in the platform specific layer. There are two issues to consider: </p> <ul>

<li id="GUID-4236DD82-FFAD-56D3-B46F-9D2198E0349A"><p>The channel may go idle

before the linking is complete, which means that the data for the new request

would not be transferred. </p> </li>

<li id="GUID-48AAA44F-1EEF-5899-9D62-587AA0EC8303"><p>If the channel is transferring

the last descriptor in the list, then updating the “next” field in the descriptor

will have no effect because the content of the descriptor will already have

been loaded in the controller registers. Again the channel would go idle without

transferring the data associated with the new request. </p> </li>

</ul> <p>The solutions available when porting the DMA Framework are: </p> <ul>

<li id="GUID-DC8EBCE6-E355-5D93-A265-0D591F06841C"><p>If the DMA controller

has hardware support for appending descriptors while transferring data, then

there is no problem. </p> </li>

<li id="GUID-77DE96E7-9D9A-543F-8CD9-A36055F38E7A"><p>If the peripheral attached

to the channel can withstand a short disruption in the transfer flow, then

the channel can be suspended while the appending takes place. This should

be done with interrupts disabled to minimise disruption. </p> </li>

<li id="GUID-962DAD2E-2684-5F51-8F80-A1E3C86BC8F7"><p>The alternative technique

is to append the new list with interrupts disabled and set a flag. When the

next interrupt occurs, if the flag is set and the channel idle, then the interrupt

service routine must restart the transfer. </p> </li>

</ul> <p>See <xref href="GUID-0D2F811C-81C3-526F-8EA4-98E50261BF4B.dita#GUID-0D2F811C-81C3-526F-8EA4-98E50261BF4B/GUID-4BF7605E-55D4-5B3A-BCD7-1F976765D738">Channels</xref>. </p> </section>

<section id="GUID-106BBBE0-C18B-5321-96AE-F1A65FAB4FF7"><title>Interrupt Service

Routine (ISR) and DFC Issues</title> <p>The platform specific layer must notify

the platform independent layer when the following events occur: </p> <ul>

<li id="GUID-13589C1E-5146-5044-9587-092C4347D4EC"><p>when an error occurs. </p> </li>

<li id="GUID-47C15152-8C06-5864-9292-4EF6A9A458E6"><p>when each fragment completes,

for single-buffer and double-buffer controllers. </p> </li>

<li id="GUID-BCDD4ECD-4396-5F2C-9A3A-B0C3B20DCC90"><p>when the last fragment

of a request completes, for scatter/gather controllers. </p> </li>

</ul> <p>The ISR, as implemented in the platform specific layer, must: </p> <ul>

<li id="GUID-037B8F96-3C27-5D2F-8E70-35C66C601D63"><p>determine which channel

the interrupt is for. If the DMA controller uses dedicated interrupt lines

per channel, the ASSP/variant interrupt dispatcher will do this. See <xref href="GUID-3C34724F-B476-5329-B0B1-6D5A34294979.dita">Interrupt Dispatcher Tutorial</xref>. </p> </li>

<li id="GUID-58A2D3AE-BF42-56E6-9EB1-B77DC9B2B934"><p>determine whether the

interrupt was asserted following a successful transfer completion or a failure.

If the DMA controller uses different interrupt lines for completion and failure,

the ASSP/variant interrupt dispatcher will do this. </p> </li>

<li id="GUID-F9D50FBF-171A-5D26-851E-3FDDB4AE38BB"><p>Call <xref href="GUID-25398075-927B-36E4-B953-16785EC4086C.dita#GUID-25398075-927B-36E4-B953-16785EC4086C/GUID-187EBCEB-4488-3606-BB71-8813111D8AF4"><apiname>TDmac::HandleIsr()</apiname></xref> in

the platform specific layer. This function queues the DFC associated with

the relevant channel. It takes care of the case where several interrupts occur

before the DFC gets a chance to run. </p> </li>

</ul> <p>The DFC updates the state of the channel and, for single and double

buffer DMA controllers, configures the next fragment to transfer, if any.

When all fragments making up a request have been transferred, the DFC calls

the client-supplied callback associated with the completed request. The callback

is also called if an error occurs during transfer. </p> </section>

<section id="GUID-2CB7E268-0535-5AD0-8B08-3FEDBA39016D"><title>Locking Strategy</title> <p>For

a given device driver, <xref href="GUID-83882548-FAC5-3EFF-92ED-14D1D9A85D37.dita"><apiname>TDmaChannel</apiname></xref> and <xref href="GUID-780F4D53-5546-3B69-B328-0226C70EBDE2.dita"><apiname>DDmaRequest</apiname></xref> instances

can be accessed concurrently from the device driver thread and from a DFC

queue thread. To avoid race conditions, each <xref href="GUID-83882548-FAC5-3EFF-92ED-14D1D9A85D37.dita"><apiname>TDmaChannel</apiname></xref> instance

has a fast mutex, the protected data member <codeph>TDmaChannel::iLock</codeph>.

This lock is acquired when queuing a new request, cancelling requests and

executing the DFC. </p> <p> <xref href="GUID-83882548-FAC5-3EFF-92ED-14D1D9A85D37.dita"><apiname>TDmaChannel</apiname></xref> and <xref href="GUID-780F4D53-5546-3B69-B328-0226C70EBDE2.dita"><apiname>DDmaRequest</apiname></xref> instances

are not protected against concurrent access from several client threads because

this is assumed to be an exceptional case. A device driver with such need

would have to implement its own locking scheme around calls to <xref href="GUID-83882548-FAC5-3EFF-92ED-14D1D9A85D37.dita"><apiname>TDmaChannel</apiname></xref> and <xref href="GUID-780F4D53-5546-3B69-B328-0226C70EBDE2.dita"><apiname>DDmaRequest</apiname></xref> member

functions. </p> <p>Each <xref href="GUID-25398075-927B-36E4-B953-16785EC4086C.dita"><apiname>TDmac</apiname></xref> instance includes a fast

mutex, the private member <codeph>TDmac::iLock</codeph> to protect descriptor

reservation and allocation, because several device drivers may try to reserve

or allocate descriptors concurrently. </p> <p>The <xref href="GUID-176B8E0D-0422-341B-A134-7C85432E1303.dita"><apiname>DmaChannelMgr</apiname></xref> static

class includes a fast mutex which is used to protect channel opening as several

device drivers may compete for the same channel. This lock is also used at

channel closing time. </p> </section>