Documentation: dmaengine: Add a documentation for the dma controller API

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>
[fixed some minor typos]
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

Documentation/dmaengine/provider.txt

+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
+
+  * 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_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 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.
+
+   * 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_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. The valid commands are:
+         + DMA_PAUSE
+           + Pauses a transfer on the channel
+           + This command should operate synchronously on the channel,
+             pausing right away the work of the given channel
+         + DMA_RESUME
+           + Restarts a transfer on the channel
+           + This command should operate synchronously on the channel,
+             resuming right away the work of the given channel
+         + DMA_TERMINATE_ALL
+           + Aborts all the pending and ongoing transfers on the
+             channel
+           + This command should operate synchronously on the channel,
+             terminating right away all the channels
+         + DMA_SLAVE_CONFIG
+           + Reconfigures the channel with passed configuration
+           + 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
+         + 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.
+
+  * device_slave_caps
+    - Called through the framework by client drivers in order to have
+      an idea of what are the properties of the channel allocated to
+      them.
+    - Such properties are the buswidth, available directions, etc.
+    - Required for every generic layer doing DMA transfers, such as
+      ASoC.
+
+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
+    - Undocumented feature
+    - No one really has an idea of what it's about, besides being
+      related to reusing the DMA transaction descriptors or having
+      additional transactions added to it in the async-tx API
+    - Useless in the case of the slave API
+
+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)