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Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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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.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)