dmaengine: doc: ReSTize provider doc

This moves and converts provider file with some format changes
for RST style

Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

3 files changed, 519 insertions, 424 deletions

diff --git a/Documentation/dmaengine/provider.txt b/Documentation/dmaengine/provider.txt
deleted file mode 100644
index 5dbe054a40ad..000000000000
--- a/Documentation/dmaengine/provider.txt
+++ /dev/null
@@ -1,424 +0,0 @@
-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)
diff --git a/Documentation/driver-api/dmaengine/index.rst b/Documentation/driver-api/dmaengine/index.rst
index 8c90a6443810..eee0377e0d7b 100644
--- a/Documentation/driver-api/dmaengine/index.rst
+++ b/Documentation/driver-api/dmaengine/index.rst
@@ -5,6 +5,17 @@ DMAEngine documentation
 DMAEngine documentation provides documents for various aspects of DMAEngine
 framework.
 
+DMAEngine documentation
+-----------------------
+
+This book helps with DMAengine internal APIs and guide for DMAEngine device
+driver writers.
+
+.. toctree::
+   :maxdepth: 1
+
+   provider
+
 .. only::  subproject
 
    Indices
diff --git a/Documentation/driver-api/dmaengine/provider.rst b/Documentation/driver-api/dmaengine/provider.rst
new file mode 100644
index 000000000000..814acb4d2294
--- /dev/null
+++ b/Documentation/driver-api/dmaengine/provider.rst
@@ -0,0 +1,508 @@
+==================================
+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 APIs
+==============
+
+``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)