[PATCH] Documentation: dmaengine: Add a documentation for the dma controller API

[Ludovic Desroches]

On Wed, Jul 30, 2014 at 06:03:13PM +0200, Maxime Ripard wrote:
> The dmaengine is neither trivial nor properly documented at the moment, which
> means a lot of trial and error development, which is not that good for such a
> central piece of the system.
> 
> Attempt at making such a documentation.

Good idea, many questions are asked when writing a new dmaengine driver.

For instance I didn't find how to use the DMA_CTRL_ACK flags.

- How this flag has to be managed? For instance, async_tx_ack is used in
dmaengine driver but also in some devices.
- Is it mandatory to deal with this flag? It seems some dmaengine
drivers don't care about it.

> 
> Signed-off-by: Maxime Ripard <maxime.ripard at free-electrons.com>
> ---
>  Documentation/dmaengine-driver.txt | 293 +++++++++++++++++++++++++++++++++++++
>  1 file changed, 293 insertions(+)
>  create mode 100644 Documentation/dmaengine-driver.txt
> 
> diff --git a/Documentation/dmaengine-driver.txt b/Documentation/dmaengine-driver.txt
> new file mode 100644
> index 000000000000..4828b50038c0
> --- /dev/null
> +++ b/Documentation/dmaengine-driver.txt
> @@ -0,0 +1,293 @@
> +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-elligible 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 to have to retrieve various number of bits from memory at a
> +time. For example, we probably want to transfer 32 bits at a time when
> +doing a simple memory copy operation, but our audio device will
> +require to have 16 or 24 bits written to its FIFO. This is why most if
> +not all of the DMA controllers can adjust this, using a parameter
> +called the width.
> +
> +Moreover, some DMA controllers, whenever the RAM is involved, 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 in a single clock cycle.
> +
> +Our theorical 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 of
> +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, wether it should increment these addresses or
> +not and the three parameters we saw earlier: the burst size, the bus
> +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) transfers. Other kind of transfers might be
> +offered by your DMA controller, and are probably already supported by
> +dmaengine.
> +
> +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 allocator will do, but you'll also
> +need to initialize a few fields in there:
> +
> +  * chancnt:	should be the number of channels your driver is exposing
> +		to the system.
> +		This doesn't have to be the number of physical
> +		channels: some DMA controllers also expose virtual
> +		channels to the system to overcome the case where you
> +		have more consumers than physical channels available.
> +
> +  * channels:	should be initialized as a list using the
> +		INIT_LIST_HEAD macro for example
> +
> +  * 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 actually set which transaction type your
> +device (and driver) supports.
> +
> +Our dma_device structure has a field called caps_mask that holds the
> +various type 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
> +    - Particularly useful 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
> +    /* TODO: Is it that the device has one interrupt per channel? */
> +
> +  * DMA_SG
> +    - The device supports memory to memory scatter-gather
> +      transfers.
> +    - Even though a plain memcpy can look like a particular case of a
> +      scatter-gather transfer, with a single chunk to transfer, it's a
> +      distinct transaction type in the mem2mem transfers case
> +
> +  * DMA_PRIVATE
> +    - The device can have several client at a time, most likely
> +      because it has several parallel channels.
> +
> +  * 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 type 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.
> +
> +  * 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 big buffer that you will fill with your audio data.
> +
> +  * DMA_INTERLEAVE
> +    - The device support interleaved transfer. Those transfers usually
> +      involve an interleaved set of data, with chunks a few bytes
> +      wide, where a scatter-gather transfer would be quite
> +      inefficient.
> +
> +These various types will also affect how the source and destination
> +addresses change over time, as DMA_SLAVE transfers will usually have
> +one of the addresses that will increment, while the other will not,
> +DMA_CYCLIC will have one address that will loop, while the other, will
> +not change, etc.
> +
> +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 type 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 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.
> +
> +     - It should return a unique instance of the
> +       dma_async_tx_descriptor structure, that further represents this
> +       particular transfer.
> +
> +     - This structure can be allocated 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 descriptor
> +			to a pending queue, waiting for issue_pending to
> +			be called.
> +
> +   * device_issue_pending
> +     - Takes the first descriptor in the pending queue, and start the
> +       transfer. Whenever that transfer is done, it should move to the
> +       next transaction in the list.
> +     - It should call the registered callback if any each time a
> +       transaction is done.
> +     - This function can be called in an interrupt context
> +
> +   * device_tx_status
> +     - Should report the bytes left to go over in the current transfer
> +       for the given channel
> +     - 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_control
> +     - Used by client drivers to control and configure the channel it
> +       has a handle on.
> +     - Called with a command and an argument
> +       + The command is one of the values listed by the enum
> +         dma_ctrl_cmd. To this date, the valid commands are:
> +         + DMA_RESUME
> +           + Restarts a transfer on the channel
> +         + DMA_PAUSE
> +           + Pauses a transfer on the channel
> +         + DMA_TERMINATE_ALL
> +           + Aborts all the pending and ongoing transfers on the
> +             channel
> +         + DMA_SLAVE_CONFIG
> +           + Reconfigures the channel with passed configuration
> +         + FSLDMA_EXTERNAL_START
> +           + TODO: Why does that even exist?
> +       + The argument is an opaque unsigned long. This actually is a
> +         pointer to a struct dma_slave_config that should be used only
> +         in the DMA_SLAVE_CONFIG.
> +
> +Misc notes (stuff that should be documented, but don't really know
> +what to say about it)
> +------------------------------------------------------------------
> +  * dma_run_dependencies
> +    - What is it supposed to do/when should it be called?
> +    - Some drivers seems to implement it at the end of a transfer, but
> +      not all of them do, so it seems we can get away without it
> +
> +  * device_slave_caps
> +    - Isn't that redundant with the cap_mask already?
> +    - Only a few drivers seem to implement it
> +
> +  * dma cookies?
> +
> +Glossary
> +--------
> +
> +Burst:		Usually a few contiguous bytes that will be transfered
> +		at once by the DMA controller
> +Chunk:		A contiguous collection of bursts
> +Transfer:	A collection of chunks (be it contiguous or not)
> -- 
> 2.0.2
>