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)