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 ==================================
 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.rst.
 
 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
 
   - No matter what the overall size of the combined chunks for source and
     destination is, only as many bytes as the smallest of the two will be
     transmitted. That means the number and size of the scatter-gather buffers in
     both lists need not be the same, and that the operation functionally is
     equivalent to a ``strncpy`` where the ``count`` argument equals the smallest
     total size of the two scatter-gather list buffers.
 
   - It's usually used for copying pixel data between host memory and
     memory-mapped GPU device memory, such as found on modern PCI video graphics
     cards. The most immediate example is the OpenGL API function
     ``glReadPielx()``, which might require a verbatim copy of a huge framebuffer
     from local device memory onto host memory.
 
 - 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_MEMSET
 
   - The device is able to fill memory with the provided pattern
 
   - The pattern is treated as a single byte signed value.
 
 - 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
 
 - DMA_COMPLETION_NO_ORDER
 
   - The device does not support in order completion.
 
   - The driver should return DMA_OUT_OF_ORDER for device_tx_status if
     the device is setting this capability.
 
   - All cookie tracking and checking API should be treated as invalid if
     the device exports this capability.
 
   - At this point, this is incompatible with polling option for dmatest.
 
   - If this cap is set, the user is recommended to provide an unique
     identifier for each descriptor sent to the DMA device in order to
     properly track the completion.
 
 - DMA_REPEAT
 
   - The device supports repeated transfers. A repeated transfer, indicated by
     the DMA_PREP_REPEAT transfer flag, is similar to a cyclic transfer in that
     it gets automatically repeated when it ends, but can additionally be
     replaced by the client.
 
   - This feature is limited to interleaved transfers, this flag should thus not
     be set if the DMA_INTERLEAVE flag isn't set. This limitation is based on
     the current needs of DMA clients, support for additional transfer types
     should be added in the future if and when the need arises.
 
 - DMA_LOAD_EOT
 
   - The device supports replacing repeated transfers at end of transfer (EOT)
     by queuing a new transfer with the DMA_PREP_LOAD_EOT flag set.
 
   - Support for replacing a currently running transfer at another point (such
     as end of burst instead of end of transfer) will be added in the future
     based on DMA clients needs, if and when the need arises.
 
 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.
 
 Per descriptor metadata support
 -------------------------------
 Some data movement architecture (DMA controller and peripherals) uses metadata
 associated with a transaction. The DMA controller role is to transfer the
 payload and the metadata alongside.
 The metadata itself is not used by the DMA engine itself, but it contains
 parameters, keys, vectors, etc for peripheral or from the peripheral.
 
 The DMAengine framework provides a generic ways to facilitate the metadata for
 descriptors. Depending on the architecture the DMA driver can implement either
 or both of the methods and it is up to the client driver to choose which one
 to use.
 
 - DESC_METADATA_CLIENT
 
   The metadata buffer is allocated/provided by the client driver and it is
   attached (via the dmaengine_desc_attach_metadata() helper to the descriptor.
 
   From the DMA driver the following is expected for this mode:
 
   - DMA_MEM_TO_DEV / DEV_MEM_TO_MEM
 
     The data from the provided metadata buffer should be prepared for the DMA
     controller to be sent alongside of the payload data. Either by copying to a
     hardware descriptor, or highly coupled packet.
 
   - DMA_DEV_TO_MEM
 
     On transfer completion the DMA driver must copy the metadata to the client
     provided metadata buffer before notifying the client about the completion.
     After the transfer completion, DMA drivers must not touch the metadata
     buffer provided by the client.
 
 - DESC_METADATA_ENGINE
 
   The metadata buffer is allocated/managed by the DMA driver. The client driver
   can ask for the pointer, maximum size and the currently used size of the
   metadata and can directly update or read it. dmaengine_desc_get_metadata_ptr()
   and dmaengine_desc_set_metadata_len() is provided as helper functions.
 
   From the DMA driver the following is expected for this mode:
 
   - get_metadata_ptr()
 
     Should return a pointer for the metadata buffer, the maximum size of the
     metadata buffer and the currently used / valid (if any) bytes in the buffer.
 
   - set_metadata_len()
 
     It is called by the clients after it have placed the metadata to the buffer
     to let the DMA driver know the number of valid bytes provided.
 
   Note: since the client will ask for the metadata pointer in the completion
   callback (in DMA_DEV_TO_MEM case) the DMA driver must ensure that the
   descriptor is not freed up prior the callback is called.
 
 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 total size of the cyclic buffer.
 
   - Should return DMA_OUT_OF_ORDER if the device does not support in order
     completion and is completing the operation out of order.
 
   - 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 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.
 
 - DMA_PREP_REPEAT
 
   - If set, the transfer will be automatically repeated when it ends until a
     new transfer is queued on the same channel with the DMA_PREP_LOAD_EOT flag.
     If the next transfer to be queued on the channel does not have the
     DMA_PREP_LOAD_EOT flag set, the current transfer will be repeated until the
     client terminates all transfers.
 
   - This flag is only supported if the channel reports the DMA_REPEAT
     capability.
 
 - DMA_PREP_LOAD_EOT
 
   - If set, the transfer will replace the transfer currently being executed at
     the end of the transfer.
 
   - This is the default behaviour for non-repeated transfers, specifying
     DMA_PREP_LOAD_EOT for non-repeated transfers will thus make no difference.
 
   - When using repeated transfers, DMA clients will usually need to set the
     DMA_PREP_LOAD_EOT flag on all transfers, otherwise the channel will keep
     repeating the last repeated transfer and ignore the new transfers being
     queued. Failure to set DMA_PREP_LOAD_EOT will appear as if the channel was
     stuck on the previous transfer.
 
   - This flag is only supported if the channel reports the DMA_LOAD_EOT
     capability.
 
 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)

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