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

Maxime Ripard
2014-07-30T16:03:13Z

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.

Signed-off-by: Maxime Ripard <maxime.ripard@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
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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-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