[PATCH v3 2/2] Documentation: dmaengine: Add a documentation for the dma controller API
Maxime Ripard
maxime.ripard at free-electrons.com
Tue Oct 28 13:55:50 PDT 2014
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 at free-electrons.com>
---
Documentation/dmaengine/provider.txt | 366 +++++++++++++++++++++++++++++++++++
1 file changed, 366 insertions(+)
create mode 100644 Documentation/dmaengine/provider.txt
diff --git a/Documentation/dmaengine/provider.txt b/Documentation/dmaengine/provider.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-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 transfered 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 theoritical 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)
--
2.1.1
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