From patchwork Fri Nov 3 04:49:38 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Vinod Koul X-Patchwork-Id: 10039523 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork.web.codeaurora.org (Postfix) with ESMTP id BCAE3600C5 for ; Fri, 3 Nov 2017 04:46:53 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id A8CCD294B1 for ; Fri, 3 Nov 2017 04:46:53 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 9B298294B5; Fri, 3 Nov 2017 04:46:53 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.9 required=2.0 tests=BAYES_00,RCVD_IN_DNSWL_HI autolearn=ham version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 7E732294B1 for ; Fri, 3 Nov 2017 04:46:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752119AbdKCEqu (ORCPT ); Fri, 3 Nov 2017 00:46:50 -0400 Received: from mga07.intel.com ([134.134.136.100]:2369 "EHLO mga07.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750794AbdKCEqt (ORCPT ); Fri, 3 Nov 2017 00:46:49 -0400 Received: from fmsmga002.fm.intel.com ([10.253.24.26]) by orsmga105.jf.intel.com with ESMTP; 02 Nov 2017 21:46:49 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.44,336,1505804400"; d="scan'208";a="1239135568" Received: from vkoul-udesk7.iind.intel.com ([10.223.84.143]) by fmsmga002.fm.intel.com with ESMTP; 02 Nov 2017 21:46:47 -0700 From: Vinod Koul To: dmaengine@vger.kernel.org Cc: linux-doc@vger.kernel.org, Jonathan Corbet , Vinod Koul Subject: [PATCH v2 2/6] dmaengine: doc: ReSTize provider doc Date: Fri, 3 Nov 2017 10:19:38 +0530 Message-Id: <1509684582-11930-3-git-send-email-vinod.koul@intel.com> X-Mailer: git-send-email 2.7.4 In-Reply-To: <1509684582-11930-1-git-send-email-vinod.koul@intel.com> References: <1509684582-11930-1-git-send-email-vinod.koul@intel.com> Sender: dmaengine-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: dmaengine@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP This moves and converts provider file with some format changes for RST style Signed-off-by: Vinod Koul --- Documentation/dmaengine/provider.txt | 424 -------------------- Documentation/driver-api/dmaengine/index.rst | 11 + Documentation/driver-api/dmaengine/provider.rst | 508 ++++++++++++++++++++++++ 3 files changed, 519 insertions(+), 424 deletions(-) delete mode 100644 Documentation/dmaengine/provider.txt create mode 100644 Documentation/driver-api/dmaengine/provider.rst diff --git a/Documentation/dmaengine/provider.txt b/Documentation/dmaengine/provider.txt deleted file mode 100644 index 5dbe054a40ad..000000000000 --- a/Documentation/dmaengine/provider.txt +++ /dev/null @@ -1,424 +0,0 @@ -DMAengine controller documentation -================================== - -Hardware Introduction -+++++++++++++++++++++ - -Most of the Slave DMA controllers have the same general principles of -operations. - -They have a given number of channels to use for the DMA transfers, and -a given number of requests lines. - -Requests and channels are pretty much orthogonal. Channels can be used -to serve several to any requests. To simplify, channels are the -entities that will be doing the copy, and requests what endpoints are -involved. - -The request lines actually correspond to physical lines going from the -DMA-eligible devices to the controller itself. Whenever the device -will want to start a transfer, it will assert a DMA request (DRQ) by -asserting that request line. - -A very simple DMA controller would only take into account a single -parameter: the transfer size. At each clock cycle, it would transfer a -byte of data from one buffer to another, until the transfer size has -been reached. - -That wouldn't work well in the real world, since slave devices might -require a specific number of bits to be transferred in a single -cycle. For example, we may want to transfer as much data as the -physical bus allows to maximize performances when doing a simple -memory copy operation, but our audio device could have a narrower FIFO -that requires data to be written exactly 16 or 24 bits at a time. This -is why most if not all of the DMA controllers can adjust this, using a -parameter called the transfer width. - -Moreover, some DMA controllers, whenever the RAM is used as a source -or destination, can group the reads or writes in memory into a buffer, -so instead of having a lot of small memory accesses, which is not -really efficient, you'll get several bigger transfers. This is done -using a parameter called the burst size, that defines how many single -reads/writes it's allowed to do without the controller splitting the -transfer into smaller sub-transfers. - -Our theoretical DMA controller would then only be able to do transfers -that involve a single contiguous block of data. However, some of the -transfers we usually have are not, and want to copy data from -non-contiguous buffers to a contiguous buffer, which is called -scatter-gather. - -DMAEngine, at least for mem2dev transfers, require support for -scatter-gather. So we're left with two cases here: either we have a -quite simple DMA controller that doesn't support it, and we'll have to -implement it in software, or we have a more advanced DMA controller, -that implements in hardware scatter-gather. - -The latter are usually programmed using a collection of chunks to -transfer, and whenever the transfer is started, the controller will go -over that collection, doing whatever we programmed there. - -This collection is usually either a table or a linked list. You will -then push either the address of the table and its number of elements, -or the first item of the list to one channel of the DMA controller, -and whenever a DRQ will be asserted, it will go through the collection -to know where to fetch the data from. - -Either way, the format of this collection is completely dependent on -your hardware. Each DMA controller will require a different structure, -but all of them will require, for every chunk, at least the source and -destination addresses, whether it should increment these addresses or -not and the three parameters we saw earlier: the burst size, the -transfer width and the transfer size. - -The one last thing is that usually, slave devices won't issue DRQ by -default, and you have to enable this in your slave device driver first -whenever you're willing to use DMA. - -These were just the general memory-to-memory (also called mem2mem) or -memory-to-device (mem2dev) kind of transfers. Most devices often -support other kind of transfers or memory operations that dmaengine -support and will be detailed later in this document. - -DMA Support in Linux -++++++++++++++++++++ - -Historically, DMA controller drivers have been implemented using the -async TX API, to offload operations such as memory copy, XOR, -cryptography, etc., basically any memory to memory operation. - -Over time, the need for memory to device transfers arose, and -dmaengine was extended. Nowadays, the async TX API is written as a -layer on top of dmaengine, and acts as a client. Still, dmaengine -accommodates that API in some cases, and made some design choices to -ensure that it stayed compatible. - -For more information on the Async TX API, please look the relevant -documentation file in Documentation/crypto/async-tx-api.txt. - -DMAEngine Registration -++++++++++++++++++++++ - -struct dma_device Initialization --------------------------------- - -Just like any other kernel framework, the whole DMAEngine registration -relies on the driver filling a structure and registering against the -framework. In our case, that structure is dma_device. - -The first thing you need to do in your driver is to allocate this -structure. Any of the usual memory allocators will do, but you'll also -need to initialize a few fields in there: - - * channels: should be initialized as a list using the - INIT_LIST_HEAD macro for example - - * src_addr_widths: - - should contain a bitmask of the supported source transfer width - - * dst_addr_widths: - - should contain a bitmask of the supported destination transfer - width - - * directions: - - should contain a bitmask of the supported slave directions - (i.e. excluding mem2mem transfers) - - * residue_granularity: - - Granularity of the transfer residue reported to dma_set_residue. - - This can be either: - + Descriptor - -> Your device doesn't support any kind of residue - reporting. The framework will only know that a particular - transaction descriptor is done. - + Segment - -> Your device is able to report which chunks have been - transferred - + Burst - -> Your device is able to report which burst have been - transferred - - * dev: should hold the pointer to the struct device associated - to your current driver instance. - -Supported transaction types ---------------------------- - -The next thing you need is to set which transaction types your device -(and driver) supports. - -Our dma_device structure has a field called cap_mask that holds the -various types of transaction supported, and you need to modify this -mask using the dma_cap_set function, with various flags depending on -transaction types you support as an argument. - -All those capabilities are defined in the dma_transaction_type enum, -in include/linux/dmaengine.h - -Currently, the types available are: - * DMA_MEMCPY - - The device is able to do memory to memory copies - - * DMA_XOR - - The device is able to perform XOR operations on memory areas - - Used to accelerate XOR intensive tasks, such as RAID5 - - * DMA_XOR_VAL - - The device is able to perform parity check using the XOR - algorithm against a memory buffer. - - * DMA_PQ - - The device is able to perform RAID6 P+Q computations, P being a - simple XOR, and Q being a Reed-Solomon algorithm. - - * DMA_PQ_VAL - - The device is able to perform parity check using RAID6 P+Q - algorithm against a memory buffer. - - * DMA_INTERRUPT - - The device is able to trigger a dummy transfer that will - generate periodic interrupts - - Used by the client drivers to register a callback that will be - called on a regular basis through the DMA controller interrupt - - * DMA_PRIVATE - - The devices only supports slave transfers, and as such isn't - available for async transfers. - - * DMA_ASYNC_TX - - Must not be set by the device, and will be set by the framework - if needed - - /* TODO: What is it about? */ - - * DMA_SLAVE - - The device can handle device to memory transfers, including - scatter-gather transfers. - - While in the mem2mem case we were having two distinct types to - deal with a single chunk to copy or a collection of them, here, - we just have a single transaction type that is supposed to - handle both. - - If you want to transfer a single contiguous memory buffer, - simply build a scatter list with only one item. - - * DMA_CYCLIC - - The device can handle cyclic transfers. - - A cyclic transfer is a transfer where the chunk collection will - loop over itself, with the last item pointing to the first. - - It's usually used for audio transfers, where you want to operate - on a single ring buffer that you will fill with your audio data. - - * DMA_INTERLEAVE - - The device supports interleaved transfer. - - These transfers can transfer data from a non-contiguous buffer - to a non-contiguous buffer, opposed to DMA_SLAVE that can - transfer data from a non-contiguous data set to a continuous - destination buffer. - - It's usually used for 2d content transfers, in which case you - want to transfer a portion of uncompressed data directly to the - display to print it - -These various types will also affect how the source and destination -addresses change over time. - -Addresses pointing to RAM are typically incremented (or decremented) -after each transfer. In case of a ring buffer, they may loop -(DMA_CYCLIC). Addresses pointing to a device's register (e.g. a FIFO) -are typically fixed. - -Device operations ------------------ - -Our dma_device structure also requires a few function pointers in -order to implement the actual logic, now that we described what -operations we were able to perform. - -The functions that we have to fill in there, and hence have to -implement, obviously depend on the transaction types you reported as -supported. - - * device_alloc_chan_resources - * device_free_chan_resources - - These functions will be called whenever a driver will call - dma_request_channel or dma_release_channel for the first/last - time on the channel associated to that driver. - - They are in charge of allocating/freeing all the needed - resources in order for that channel to be useful for your - driver. - - These functions can sleep. - - * device_prep_dma_* - - These functions are matching the capabilities you registered - previously. - - These functions all take the buffer or the scatterlist relevant - for the transfer being prepared, and should create a hardware - descriptor or a list of hardware descriptors from it - - These functions can be called from an interrupt context - - Any allocation you might do should be using the GFP_NOWAIT - flag, in order not to potentially sleep, but without depleting - the emergency pool either. - - Drivers should try to pre-allocate any memory they might need - during the transfer setup at probe time to avoid putting to - much pressure on the nowait allocator. - - - It should return a unique instance of the - dma_async_tx_descriptor structure, that further represents this - particular transfer. - - - This structure can be initialized using the function - dma_async_tx_descriptor_init. - - You'll also need to set two fields in this structure: - + flags: - TODO: Can it be modified by the driver itself, or - should it be always the flags passed in the arguments - - + tx_submit: A pointer to a function you have to implement, - that is supposed to push the current - transaction descriptor to a pending queue, waiting - for issue_pending to be called. - - In this structure the function pointer callback_result can be - initialized in order for the submitter to be notified that a - transaction has completed. In the earlier code the function pointer - callback has been used. However it does not provide any status to the - transaction and will be deprecated. The result structure defined as - dmaengine_result that is passed in to callback_result has two fields: - + result: This provides the transfer result defined by - dmaengine_tx_result. Either success or some error - condition. - + residue: Provides the residue bytes of the transfer for those that - support residue. - - * device_issue_pending - - Takes the first transaction descriptor in the pending queue, - and starts the transfer. Whenever that transfer is done, it - should move to the next transaction in the list. - - This function can be called in an interrupt context - - * device_tx_status - - Should report the bytes left to go over on the given channel - - Should only care about the transaction descriptor passed as - argument, not the currently active one on a given channel - - The tx_state argument might be NULL - - Should use dma_set_residue to report it - - In the case of a cyclic transfer, it should only take into - account the current period. - - This function can be called in an interrupt context. - - * device_config - - Reconfigures the channel with the configuration given as - argument - - This command should NOT perform synchronously, or on any - currently queued transfers, but only on subsequent ones - - In this case, the function will receive a dma_slave_config - structure pointer as an argument, that will detail which - configuration to use. - - Even though that structure contains a direction field, this - field is deprecated in favor of the direction argument given to - the prep_* functions - - This call is mandatory for slave operations only. This should NOT be - set or expected to be set for memcpy operations. - If a driver support both, it should use this call for slave - operations only and not for memcpy ones. - - * device_pause - - Pauses a transfer on the channel - - This command should operate synchronously on the channel, - pausing right away the work of the given channel - - * device_resume - - Resumes a transfer on the channel - - This command should operate synchronously on the channel, - resuming right away the work of the given channel - - * device_terminate_all - - Aborts all the pending and ongoing transfers on the channel - - For aborted transfers the complete callback should not be called - - Can be called from atomic context or from within a complete - callback of a descriptor. Must not sleep. Drivers must be able - to handle this correctly. - - Termination may be asynchronous. The driver does not have to - wait until the currently active transfer has completely stopped. - See device_synchronize. - - * device_synchronize - - Must synchronize the termination of a channel to the current - context. - - Must make sure that memory for previously submitted - descriptors is no longer accessed by the DMA controller. - - Must make sure that all complete callbacks for previously - submitted descriptors have finished running and none are - scheduled to run. - - May sleep. - - -Misc notes (stuff that should be documented, but don't really know -where to put them) ------------------------------------------------------------------- - * dma_run_dependencies - - Should be called at the end of an async TX transfer, and can be - ignored in the slave transfers case. - - Makes sure that dependent operations are run before marking it - as complete. - - * dma_cookie_t - - it's a DMA transaction ID that will increment over time. - - Not really relevant any more since the introduction of virt-dma - that abstracts it away. - - * DMA_CTRL_ACK - - If clear, the descriptor cannot be reused by provider until the - client acknowledges receipt, i.e. has has a chance to establish any - dependency chains - - This can be acked by invoking async_tx_ack() - - If set, does not mean descriptor can be reused - - * DMA_CTRL_REUSE - - If set, the descriptor can be reused after being completed. It should - not be freed by provider if this flag is set. - - The descriptor should be prepared for reuse by invoking - dmaengine_desc_set_reuse() which will set DMA_CTRL_REUSE. - - dmaengine_desc_set_reuse() will succeed only when channel support - reusable descriptor as exhibited by capabilities - - As a consequence, if a device driver wants to skip the dma_map_sg() and - dma_unmap_sg() in between 2 transfers, because the DMA'd data wasn't used, - it can resubmit the transfer right after its completion. - - Descriptor can be freed in few ways - - Clearing DMA_CTRL_REUSE by invoking dmaengine_desc_clear_reuse() - and submitting for last txn - - Explicitly invoking dmaengine_desc_free(), this can succeed only - when DMA_CTRL_REUSE is already set - - Terminating the channel - - * DMA_PREP_CMD - - If set, the client driver tells DMA controller that passed data in DMA - API is command data. - - Interpretation of command data is DMA controller specific. It can be - used for issuing commands to other peripherals/register reads/register - writes for which the descriptor should be in different format from - normal data descriptors. - -General Design Notes --------------------- - -Most of the DMAEngine drivers you'll see are based on a similar design -that handles the end of transfer interrupts in the handler, but defer -most work to a tasklet, including the start of a new transfer whenever -the previous transfer ended. - -This is a rather inefficient design though, because the inter-transfer -latency will be not only the interrupt latency, but also the -scheduling latency of the tasklet, which will leave the channel idle -in between, which will slow down the global transfer rate. - -You should avoid this kind of practice, and instead of electing a new -transfer in your tasklet, move that part to the interrupt handler in -order to have a shorter idle window (that we can't really avoid -anyway). - -Glossary --------- - -Burst: A number of consecutive read or write operations - that can be queued to buffers before being flushed to - memory. -Chunk: A contiguous collection of bursts -Transfer: A collection of chunks (be it contiguous or not) diff --git a/Documentation/driver-api/dmaengine/index.rst b/Documentation/driver-api/dmaengine/index.rst index 8c90a6443810..eee0377e0d7b 100644 --- a/Documentation/driver-api/dmaengine/index.rst +++ b/Documentation/driver-api/dmaengine/index.rst @@ -5,6 +5,17 @@ DMAEngine documentation DMAEngine documentation provides documents for various aspects of DMAEngine framework. +DMAEngine documentation +----------------------- + +This book helps with DMAengine internal APIs and guide for DMAEngine device +driver writers. + +.. toctree:: + :maxdepth: 1 + + provider + .. only:: subproject Indices diff --git a/Documentation/driver-api/dmaengine/provider.rst b/Documentation/driver-api/dmaengine/provider.rst new file mode 100644 index 000000000000..814acb4d2294 --- /dev/null +++ b/Documentation/driver-api/dmaengine/provider.rst @@ -0,0 +1,508 @@ +================================== +DMAengine controller documentation +================================== + +Hardware Introduction +===================== + +Most of the Slave DMA controllers have the same general principles of +operations. + +They have a given number of channels to use for the DMA transfers, and +a given number of requests lines. + +Requests and channels are pretty much orthogonal. Channels can be used +to serve several to any requests. To simplify, channels are the +entities that will be doing the copy, and requests what endpoints are +involved. + +The request lines actually correspond to physical lines going from the +DMA-eligible devices to the controller itself. Whenever the device +will want to start a transfer, it will assert a DMA request (DRQ) by +asserting that request line. + +A very simple DMA controller would only take into account a single +parameter: the transfer size. At each clock cycle, it would transfer a +byte of data from one buffer to another, until the transfer size has +been reached. + +That wouldn't work well in the real world, since slave devices might +require a specific number of bits to be transferred in a single +cycle. For example, we may want to transfer as much data as the +physical bus allows to maximize performances when doing a simple +memory copy operation, but our audio device could have a narrower FIFO +that requires data to be written exactly 16 or 24 bits at a time. This +is why most if not all of the DMA controllers can adjust this, using a +parameter called the transfer width. + +Moreover, some DMA controllers, whenever the RAM is used as a source +or destination, can group the reads or writes in memory into a buffer, +so instead of having a lot of small memory accesses, which is not +really efficient, you'll get several bigger transfers. This is done +using a parameter called the burst size, that defines how many single +reads/writes it's allowed to do without the controller splitting the +transfer into smaller sub-transfers. + +Our theoretical DMA controller would then only be able to do transfers +that involve a single contiguous block of data. However, some of the +transfers we usually have are not, and want to copy data from +non-contiguous buffers to a contiguous buffer, which is called +scatter-gather. + +DMAEngine, at least for mem2dev transfers, require support for +scatter-gather. So we're left with two cases here: either we have a +quite simple DMA controller that doesn't support it, and we'll have to +implement it in software, or we have a more advanced DMA controller, +that implements in hardware scatter-gather. + +The latter are usually programmed using a collection of chunks to +transfer, and whenever the transfer is started, the controller will go +over that collection, doing whatever we programmed there. + +This collection is usually either a table or a linked list. You will +then push either the address of the table and its number of elements, +or the first item of the list to one channel of the DMA controller, +and whenever a DRQ will be asserted, it will go through the collection +to know where to fetch the data from. + +Either way, the format of this collection is completely dependent on +your hardware. Each DMA controller will require a different structure, +but all of them will require, for every chunk, at least the source and +destination addresses, whether it should increment these addresses or +not and the three parameters we saw earlier: the burst size, the +transfer width and the transfer size. + +The one last thing is that usually, slave devices won't issue DRQ by +default, and you have to enable this in your slave device driver first +whenever you're willing to use DMA. + +These were just the general memory-to-memory (also called mem2mem) or +memory-to-device (mem2dev) kind of transfers. Most devices often +support other kind of transfers or memory operations that dmaengine +support and will be detailed later in this document. + +DMA Support in Linux +==================== + +Historically, DMA controller drivers have been implemented using the +async TX API, to offload operations such as memory copy, XOR, +cryptography, etc., basically any memory to memory operation. + +Over time, the need for memory to device transfers arose, and +dmaengine was extended. Nowadays, the async TX API is written as a +layer on top of dmaengine, and acts as a client. Still, dmaengine +accommodates that API in some cases, and made some design choices to +ensure that it stayed compatible. + +For more information on the Async TX API, please look the relevant +documentation file in Documentation/crypto/async-tx-api.txt. + +DMAEngine APIs +============== + +``struct dma_device`` Initialization +------------------------------------ + +Just like any other kernel framework, the whole DMAEngine registration +relies on the driver filling a structure and registering against the +framework. In our case, that structure is dma_device. + +The first thing you need to do in your driver is to allocate this +structure. Any of the usual memory allocators will do, but you'll also +need to initialize a few fields in there: + +- channels: should be initialized as a list using the + INIT_LIST_HEAD macro for example + +- src_addr_widths: + should contain a bitmask of the supported source transfer width + +- dst_addr_widths: + should contain a bitmask of the supported destination transfer width + +- directions: + should contain a bitmask of the supported slave directions + (i.e. excluding mem2mem transfers) + +- residue_granularity: + + - Granularity of the transfer residue reported to dma_set_residue. + This can be either: + + - Descriptor + + - Your device doesn't support any kind of residue + reporting. The framework will only know that a particular + transaction descriptor is done. + + - Segment + + - Your device is able to report which chunks have been transferred + + - Burst + + - Your device is able to report which burst have been transferred + + - dev: should hold the pointer to the ``struct device`` associated + to your current driver instance. + +Supported transaction types +--------------------------- + +The next thing you need is to set which transaction types your device +(and driver) supports. + +Our ``dma_device structure`` has a field called cap_mask that holds the +various types of transaction supported, and you need to modify this +mask using the dma_cap_set function, with various flags depending on +transaction types you support as an argument. + +All those capabilities are defined in the ``dma_transaction_type enum``, +in ``include/linux/dmaengine.h`` + +Currently, the types available are: + +- DMA_MEMCPY + + - The device is able to do memory to memory copies + +- DMA_XOR + + - The device is able to perform XOR operations on memory areas + + - Used to accelerate XOR intensive tasks, such as RAID5 + +- DMA_XOR_VAL + + - The device is able to perform parity check using the XOR + algorithm against a memory buffer. + +- DMA_PQ + + - The device is able to perform RAID6 P+Q computations, P being a + simple XOR, and Q being a Reed-Solomon algorithm. + +- DMA_PQ_VAL + + - The device is able to perform parity check using RAID6 P+Q + algorithm against a memory buffer. + +- DMA_INTERRUPT + + - The device is able to trigger a dummy transfer that will + generate periodic interrupts + + - Used by the client drivers to register a callback that will be + called on a regular basis through the DMA controller interrupt + +- DMA_PRIVATE + + - The devices only supports slave transfers, and as such isn't + available for async transfers. + +- DMA_ASYNC_TX + + - Must not be set by the device, and will be set by the framework + if needed + + - TODO: What is it about? + +- DMA_SLAVE + + - The device can handle device to memory transfers, including + scatter-gather transfers. + + - While in the mem2mem case we were having two distinct types to + deal with a single chunk to copy or a collection of them, here, + we just have a single transaction type that is supposed to + handle both. + + - If you want to transfer a single contiguous memory buffer, + simply build a scatter list with only one item. + +- DMA_CYCLIC + + - The device can handle cyclic transfers. + + - A cyclic transfer is a transfer where the chunk collection will + loop over itself, with the last item pointing to the first. + + - It's usually used for audio transfers, where you want to operate + on a single ring buffer that you will fill with your audio data. + +- DMA_INTERLEAVE + + - The device supports interleaved transfer. + + - These transfers can transfer data from a non-contiguous buffer + to a non-contiguous buffer, opposed to DMA_SLAVE that can + transfer data from a non-contiguous data set to a continuous + destination buffer. + + - It's usually used for 2d content transfers, in which case you + want to transfer a portion of uncompressed data directly to the + display to print it + +These various types will also affect how the source and destination +addresses change over time. + +Addresses pointing to RAM are typically incremented (or decremented) +after each transfer. In case of a ring buffer, they may loop +(DMA_CYCLIC). Addresses pointing to a device's register (e.g. a FIFO) +are typically fixed. + +Device operations +----------------- + +Our dma_device structure also requires a few function pointers in +order to implement the actual logic, now that we described what +operations we were able to perform. + +The functions that we have to fill in there, and hence have to +implement, obviously depend on the transaction types you reported as +supported. + +- ``device_alloc_chan_resources`` + +- ``device_free_chan_resources`` + + - These functions will be called whenever a driver will call + ``dma_request_channel`` or ``dma_release_channel`` for the first/last + time on the channel associated to that driver. + + - They are in charge of allocating/freeing all the needed + resources in order for that channel to be useful for your driver. + + - These functions can sleep. + +- ``device_prep_dma_*`` + + - These functions are matching the capabilities you registered + previously. + + - These functions all take the buffer or the scatterlist relevant + for the transfer being prepared, and should create a hardware + descriptor or a list of hardware descriptors from it + + - These functions can be called from an interrupt context + + - Any allocation you might do should be using the GFP_NOWAIT + flag, in order not to potentially sleep, but without depleting + the emergency pool either. + + - Drivers should try to pre-allocate any memory they might need + during the transfer setup at probe time to avoid putting to + much pressure on the nowait allocator. + + - It should return a unique instance of the + ``dma_async_tx_descriptor structure``, that further represents this + particular transfer. + + - This structure can be initialized using the function + ``dma_async_tx_descriptor_init``. + + - You'll also need to set two fields in this structure: + + - flags: + TODO: Can it be modified by the driver itself, or + should it be always the flags passed in the arguments + + - tx_submit: A pointer to a function you have to implement, + that is supposed to push the current transaction descriptor to a + pending queue, waiting for issue_pending to be called. + + - In this structure the function pointer callback_result can be + initialized in order for the submitter to be notified that a + transaction has completed. In the earlier code the function pointer + callback has been used. However it does not provide any status to the + transaction and will be deprecated. The result structure defined as + ``dmaengine_result`` that is passed in to callback_result + has two fields: + + - result: This provides the transfer result defined by + ``dmaengine_tx_result``. Either success or some error condition. + + - residue: Provides the residue bytes of the transfer for those that + support residue. + +- ``device_issue_pending`` + + - Takes the first transaction descriptor in the pending queue, + and starts the transfer. Whenever that transfer is done, it + should move to the next transaction in the list. + + - This function can be called in an interrupt context + +- ``device_tx_status`` + + - Should report the bytes left to go over on the given channel + + - Should only care about the transaction descriptor passed as + argument, not the currently active one on a given channel + + - The tx_state argument might be NULL + + - Should use dma_set_residue to report it + + - In the case of a cyclic transfer, it should only take into + account the current period. + + - This function can be called in an interrupt context. + +- device_config + + - Reconfigures the channel with the configuration given as argument + + - This command should NOT perform synchronously, or on any + currently queued transfers, but only on subsequent ones + + - In this case, the function will receive a ``dma_slave_config`` + structure pointer as an argument, that will detail which + configuration to use. + + - Even though that structure contains a direction field, this + field is deprecated in favor of the direction argument given to + the prep_* functions + + - This call is mandatory for slave operations only. This should NOT be + set or expected to be set for memcpy operations. + If a driver support both, it should use this call for slave + operations only and not for memcpy ones. + +- device_pause + + - Pauses a transfer on the channel + + - This command should operate synchronously on the channel, + pausing right away the work of the given channel + +- device_resume + + - Resumes a transfer on the channel + + - This command should operate synchronously on the channel, + resuming right away the work of the given channel + +- device_terminate_all + + - Aborts all the pending and ongoing transfers on the channel + + - For aborted transfers the complete callback should not be called + + - Can be called from atomic context or from within a complete + callback of a descriptor. Must not sleep. Drivers must be able + to handle this correctly. + + - Termination may be asynchronous. The driver does not have to + wait until the currently active transfer has completely stopped. + See device_synchronize. + +- device_synchronize + + - Must synchronize the termination of a channel to the current + context. + + - Must make sure that memory for previously submitted + descriptors is no longer accessed by the DMA controller. + + - Must make sure that all complete callbacks for previously + submitted descriptors have finished running and none are + scheduled to run. + + - May sleep. + + +Misc notes +========== + +(stuff that should be documented, but don't really know +where to put them) + +``dma_run_dependencies`` + +- Should be called at the end of an async TX transfer, and can be + ignored in the slave transfers case. + +- Makes sure that dependent operations are run before marking it + as complete. + +dma_cookie_t + +- it's a DMA transaction ID that will increment over time. + +- Not really relevant any more since the introduction of ``virt-dma`` + that abstracts it away. + +DMA_CTRL_ACK + +- If clear, the descriptor cannot be reused by provider until the + client acknowledges receipt, i.e. has has a chance to establish any + dependency chains + +- This can be acked by invoking async_tx_ack() + +- If set, does not mean descriptor can be reused + +DMA_CTRL_REUSE + +- If set, the descriptor can be reused after being completed. It should + not be freed by provider if this flag is set. + +- The descriptor should be prepared for reuse by invoking + ``dmaengine_desc_set_reuse()`` which will set DMA_CTRL_REUSE. + +- ``dmaengine_desc_set_reuse()`` will succeed only when channel support + reusable descriptor as exhibited by capabilities + +- As a consequence, if a device driver wants to skip the + ``dma_map_sg()`` and ``dma_unmap_sg()`` in between 2 transfers, + because the DMA'd data wasn't used, it can resubmit the transfer right after + its completion. + +- Descriptor can be freed in few ways + + - Clearing DMA_CTRL_REUSE by invoking + ``dmaengine_desc_clear_reuse()`` and submitting for last txn + + - Explicitly invoking ``dmaengine_desc_free()``, this can succeed only + when DMA_CTRL_REUSE is already set + + - Terminating the channel + +- DMA_PREP_CMD + + - If set, the client driver tells DMA controller that passed data in DMA + API is command data. + + - Interpretation of command data is DMA controller specific. It can be + used for issuing commands to other peripherals/register reads/register + writes for which the descriptor should be in different format from + normal data descriptors. + +General Design Notes +==================== + +Most of the DMAEngine drivers you'll see are based on a similar design +that handles the end of transfer interrupts in the handler, but defer +most work to a tasklet, including the start of a new transfer whenever +the previous transfer ended. + +This is a rather inefficient design though, because the inter-transfer +latency will be not only the interrupt latency, but also the +scheduling latency of the tasklet, which will leave the channel idle +in between, which will slow down the global transfer rate. + +You should avoid this kind of practice, and instead of electing a new +transfer in your tasklet, move that part to the interrupt handler in +order to have a shorter idle window (that we can't really avoid +anyway). + +Glossary +======== + +- Burst: A number of consecutive read or write operations that + can be queued to buffers before being flushed to memory. + +- Chunk: A contiguous collection of bursts + +- Transfer: A collection of chunks (be it contiguous or not)