Initial import

4 files changed, 678 insertions, 0 deletions

diff --git a/Documentation/dmaengine/00-INDEX b/Documentation/dmaengine/00-INDEX
new file mode 100644
index 000000000..07de6573d
--- /dev/null
+++ b/Documentation/dmaengine/00-INDEX
@@ -0,0 +1,8 @@
+00-INDEX
+	- this file.
+client.txt
+	-the DMA Engine API Guide.
+dmatest.txt
+	- how to compile, configure and use the dmatest system.
+provider.txt
+	- the DMA controller API.
\ No newline at end of file
diff --git a/Documentation/dmaengine/client.txt b/Documentation/dmaengine/client.txt
new file mode 100644
index 000000000..11fb87ff6
--- /dev/null
+++ b/Documentation/dmaengine/client.txt
@@ -0,0 +1,199 @@
+			DMA Engine API Guide
+			====================
+
+		 Vinod Koul <vinod dot koul at intel.com>
+
+NOTE: For DMA Engine usage in async_tx please see:
+	Documentation/crypto/async-tx-api.txt
+
+
+Below is a guide to device driver writers on how to use the Slave-DMA API of the
+DMA Engine. This is applicable only for slave DMA usage only.
+
+The slave DMA usage consists of following steps:
+1. Allocate a DMA slave channel
+2. Set slave and controller specific parameters
+3. Get a descriptor for transaction
+4. Submit the transaction
+5. Issue pending requests and wait for callback notification
+
+1. Allocate a DMA slave channel
+
+   Channel allocation is slightly different in the slave DMA context,
+   client drivers typically need a channel from a particular DMA
+   controller only and even in some cases a specific channel is desired.
+   To request a channel dma_request_channel() API is used.
+
+   Interface:
+	struct dma_chan *dma_request_channel(dma_cap_mask_t mask,
+			dma_filter_fn filter_fn,
+			void *filter_param);
+   where dma_filter_fn is defined as:
+	typedef bool (*dma_filter_fn)(struct dma_chan *chan, void *filter_param);
+
+   The 'filter_fn' parameter is optional, but highly recommended for
+   slave and cyclic channels as they typically need to obtain a specific
+   DMA channel.
+
+   When the optional 'filter_fn' parameter is NULL, dma_request_channel()
+   simply returns the first channel that satisfies the capability mask.
+
+   Otherwise, the 'filter_fn' routine will be called once for each free
+   channel which has a capability in 'mask'.  'filter_fn' is expected to
+   return 'true' when the desired DMA channel is found.
+
+   A channel allocated via this interface is exclusive to the caller,
+   until dma_release_channel() is called.
+
+2. Set slave and controller specific parameters
+
+   Next step is always to pass some specific information to the DMA
+   driver.  Most of the generic information which a slave DMA can use
+   is in struct dma_slave_config.  This allows the clients to specify
+   DMA direction, DMA addresses, bus widths, DMA burst lengths etc
+   for the peripheral.
+
+   If some DMA controllers have more parameters to be sent then they
+   should try to embed struct dma_slave_config in their controller
+   specific structure. That gives flexibility to client to pass more
+   parameters, if required.
+
+   Interface:
+	int dmaengine_slave_config(struct dma_chan *chan,
+				  struct dma_slave_config *config)
+
+   Please see the dma_slave_config structure definition in dmaengine.h
+   for a detailed explanation of the struct members.  Please note
+   that the 'direction' member will be going away as it duplicates the
+   direction given in the prepare call.
+
+3. Get a descriptor for transaction
+
+   For slave usage the various modes of slave transfers supported by the
+   DMA-engine are:
+
+   slave_sg	- DMA a list of scatter gather buffers from/to a peripheral
+   dma_cyclic	- Perform a cyclic DMA operation from/to a peripheral till the
+		  operation is explicitly stopped.
+   interleaved_dma - This is common to Slave as well as M2M clients. For slave
+		 address of devices' fifo could be already known to the driver.
+		 Various types of operations could be expressed by setting
+		 appropriate values to the 'dma_interleaved_template' members.
+
+   A non-NULL return of this transfer API represents a "descriptor" for
+   the given transaction.
+
+   Interface:
+	struct dma_async_tx_descriptor *dmaengine_prep_slave_sg(
+		struct dma_chan *chan, struct scatterlist *sgl,
+		unsigned int sg_len, enum dma_data_direction direction,
+		unsigned long flags);
+
+	struct dma_async_tx_descriptor *dmaengine_prep_dma_cyclic(
+		struct dma_chan *chan, dma_addr_t buf_addr, size_t buf_len,
+		size_t period_len, enum dma_data_direction direction);
+
+	struct dma_async_tx_descriptor *dmaengine_prep_interleaved_dma(
+		struct dma_chan *chan, struct dma_interleaved_template *xt,
+		unsigned long flags);
+
+   The peripheral driver is expected to have mapped the scatterlist for
+   the DMA operation prior to calling dmaengine_prep_slave_sg(), and must
+   keep the scatterlist mapped until the DMA operation has completed.
+   The scatterlist must be mapped using the DMA struct device.
+   If a mapping needs to be synchronized later, dma_sync_*_for_*() must be
+   called using the DMA struct device, too.
+   So, normal setup should look like this:
+
+	nr_sg = dma_map_sg(chan->device->dev, sgl, sg_len);
+	if (nr_sg == 0)
+		/* error */
+
+	desc = dmaengine_prep_slave_sg(chan, sgl, nr_sg, direction, flags);
+
+   Once a descriptor has been obtained, the callback information can be
+   added and the descriptor must then be submitted.  Some DMA engine
+   drivers may hold a spinlock between a successful preparation and
+   submission so it is important that these two operations are closely
+   paired.
+
+   Note:
+	Although the async_tx API specifies that completion callback
+	routines cannot submit any new operations, this is not the
+	case for slave/cyclic DMA.
+
+	For slave DMA, the subsequent transaction may not be available
+	for submission prior to callback function being invoked, so
+	slave DMA callbacks are permitted to prepare and submit a new
+	transaction.
+
+	For cyclic DMA, a callback function may wish to terminate the
+	DMA via dmaengine_terminate_all().
+
+	Therefore, it is important that DMA engine drivers drop any
+	locks before calling the callback function which may cause a
+	deadlock.
+
+	Note that callbacks will always be invoked from the DMA
+	engines tasklet, never from interrupt context.
+
+4. Submit the transaction
+
+   Once the descriptor has been prepared and the callback information
+   added, it must be placed on the DMA engine drivers pending queue.
+
+   Interface:
+	dma_cookie_t dmaengine_submit(struct dma_async_tx_descriptor *desc)
+
+   This returns a cookie can be used to check the progress of DMA engine
+   activity via other DMA engine calls not covered in this document.
+
+   dmaengine_submit() will not start the DMA operation, it merely adds
+   it to the pending queue.  For this, see step 5, dma_async_issue_pending.
+
+5. Issue pending DMA requests and wait for callback notification
+
+   The transactions in the pending queue can be activated by calling the
+   issue_pending API. If channel is idle then the first transaction in
+   queue is started and subsequent ones queued up.
+
+   On completion of each DMA operation, the next in queue is started and
+   a tasklet triggered. The tasklet will then call the client driver
+   completion callback routine for notification, if set.
+
+   Interface:
+	void dma_async_issue_pending(struct dma_chan *chan);
+
+Further APIs:
+
+1. int dmaengine_terminate_all(struct dma_chan *chan)
+
+   This causes all activity for the DMA channel to be stopped, and may
+   discard data in the DMA FIFO which hasn't been fully transferred.
+   No callback functions will be called for any incomplete transfers.
+
+2. int dmaengine_pause(struct dma_chan *chan)
+
+   This pauses activity on the DMA channel without data loss.
+
+3. int dmaengine_resume(struct dma_chan *chan)
+
+   Resume a previously paused DMA channel.  It is invalid to resume a
+   channel which is not currently paused.
+
+4. enum dma_status dma_async_is_tx_complete(struct dma_chan *chan,
+        dma_cookie_t cookie, dma_cookie_t *last, dma_cookie_t *used)
+
+   This can be used to check the status of the channel.  Please see
+   the documentation in include/linux/dmaengine.h for a more complete
+   description of this API.
+
+   This can be used in conjunction with dma_async_is_complete() and
+   the cookie returned from dmaengine_submit() to check for
+   completion of a specific DMA transaction.
+
+   Note:
+	Not all DMA engine drivers can return reliable information for
+	a running DMA channel.  It is recommended that DMA engine users
+	pause or stop (via dmaengine_terminate_all()) the channel before
+	using this API.
diff --git a/Documentation/dmaengine/dmatest.txt b/Documentation/dmaengine/dmatest.txt
new file mode 100644
index 000000000..dd77a81bd
--- /dev/null
+++ b/Documentation/dmaengine/dmatest.txt
@@ -0,0 +1,92 @@
+				DMA Test Guide
+				==============
+
+		Andy Shevchenko <andriy.shevchenko@linux.intel.com>
+
+This small document introduces how to test DMA drivers using dmatest module.
+
+	Part 1 - How to build the test module
+
+The menuconfig contains an option that could be found by following path:
+	Device Drivers -> DMA Engine support -> DMA Test client
+
+In the configuration file the option called CONFIG_DMATEST. The dmatest could
+be built as module or inside kernel. Let's consider those cases.
+
+	Part 2 - When dmatest is built as a module...
+
+Example of usage:
+	% modprobe dmatest channel=dma0chan0 timeout=2000 iterations=1 run=1
+
+...or:
+	% modprobe dmatest
+	% echo dma0chan0 > /sys/module/dmatest/parameters/channel
+	% echo 2000 > /sys/module/dmatest/parameters/timeout
+	% echo 1 > /sys/module/dmatest/parameters/iterations
+	% echo 1 > /sys/module/dmatest/parameters/run
+
+...or on the kernel command line:
+
+	dmatest.channel=dma0chan0 dmatest.timeout=2000 dmatest.iterations=1 dmatest.run=1
+
+Hint: available channel list could be extracted by running the following
+command:
+	% ls -1 /sys/class/dma/
+
+Once started a message like "dmatest: Started 1 threads using dma0chan0" is
+emitted.  After that only test failure messages are reported until the test
+stops.
+
+Note that running a new test will not stop any in progress test.
+
+The following command returns the state of the test.
+	% cat /sys/module/dmatest/parameters/run
+
+To wait for test completion userpace can poll 'run' until it is false, or use
+the wait parameter.  Specifying 'wait=1' when loading the module causes module
+initialization to pause until a test run has completed, while reading
+/sys/module/dmatest/parameters/wait waits for any running test to complete
+before returning.  For example, the following scripts wait for 42 tests
+to complete before exiting.  Note that if 'iterations' is set to 'infinite' then
+waiting is disabled.
+
+Example:
+	% modprobe dmatest run=1 iterations=42 wait=1
+	% modprobe -r dmatest
+...or:
+	% modprobe dmatest run=1 iterations=42
+	% cat /sys/module/dmatest/parameters/wait
+	% modprobe -r dmatest
+
+	Part 3 - When built-in in the kernel...
+
+The module parameters that is supplied to the kernel command line will be used
+for the first performed test. After user gets a control, the test could be
+re-run with the same or different parameters. For the details see the above
+section "Part 2 - When dmatest is built as a module..."
+
+In both cases the module parameters are used as the actual values for the test
+case. You always could check them at run-time by running
+	% grep -H . /sys/module/dmatest/parameters/*
+
+	Part 4 - Gathering the test results
+
+Test results are printed to the kernel log buffer with the format:
+
+"dmatest: result <channel>: <test id>: '<error msg>' with src_off=<val> dst_off=<val> len=<val> (<err code>)"
+
+Example of output:
+	% dmesg | tail -n 1
+	dmatest: result dma0chan0-copy0: #1: No errors with src_off=0x7bf dst_off=0x8ad len=0x3fea (0)
+
+The message format is unified across the different types of errors. A number in
+the parens represents additional information, e.g. error code, error counter,
+or status.  A test thread also emits a summary line at completion listing the
+number of tests executed, number that failed, and a result code.
+
+Example:
+	% dmesg | tail -n 1
+	dmatest: dma0chan0-copy0: summary 1 test, 0 failures 1000 iops 100000 KB/s (0)
+
+The details of a data miscompare error are also emitted, but do not follow the
+above format.
diff --git a/Documentation/dmaengine/provider.txt b/Documentation/dmaengine/provider.txt
new file mode 100644
index 000000000..05d228019
--- /dev/null
+++ b/Documentation/dmaengine/provider.txt
@@ -0,0 +1,379 @@
+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_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_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,
+       pausing right away the work of the given channel
+
+   * device_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
+
+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)