1DMAengine controller documentation
2==================================
3
4Hardware Introduction
5+++++++++++++++++++++
6
7Most of the Slave DMA controllers have the same general principles of
8operations.
9
10They have a given number of channels to use for the DMA transfers, and
11a given number of requests lines.
12
13Requests and channels are pretty much orthogonal. Channels can be used
14to serve several to any requests. To simplify, channels are the
15entities that will be doing the copy, and requests what endpoints are
16involved.
17
18The request lines actually correspond to physical lines going from the
19DMA-eligible devices to the controller itself. Whenever the device
20will want to start a transfer, it will assert a DMA request (DRQ) by
21asserting that request line.
22
23A very simple DMA controller would only take into account a single
24parameter: the transfer size. At each clock cycle, it would transfer a
25byte of data from one buffer to another, until the transfer size has
26been reached.
27
28That wouldn't work well in the real world, since slave devices might
29require a specific number of bits to be transferred in a single
30cycle. For example, we may want to transfer as much data as the
31physical bus allows to maximize performances when doing a simple
32memory copy operation, but our audio device could have a narrower FIFO
33that requires data to be written exactly 16 or 24 bits at a time. This
34is why most if not all of the DMA controllers can adjust this, using a
35parameter called the transfer width.
36
37Moreover, some DMA controllers, whenever the RAM is used as a source
38or destination, can group the reads or writes in memory into a buffer,
39so instead of having a lot of small memory accesses, which is not
40really efficient, you'll get several bigger transfers. This is done
41using a parameter called the burst size, that defines how many single
42reads/writes it's allowed to do without the controller splitting the
43transfer into smaller sub-transfers.
44
45Our theoretical DMA controller would then only be able to do transfers
46that involve a single contiguous block of data. However, some of the
47transfers we usually have are not, and want to copy data from
48non-contiguous buffers to a contiguous buffer, which is called
49scatter-gather.
50
51DMAEngine, at least for mem2dev transfers, require support for
52scatter-gather. So we're left with two cases here: either we have a
53quite simple DMA controller that doesn't support it, and we'll have to
54implement it in software, or we have a more advanced DMA controller,
55that implements in hardware scatter-gather.
56
57The latter are usually programmed using a collection of chunks to
58transfer, and whenever the transfer is started, the controller will go
59over that collection, doing whatever we programmed there.
60
61This collection is usually either a table or a linked list. You will
62then push either the address of the table and its number of elements,
63or the first item of the list to one channel of the DMA controller,
64and whenever a DRQ will be asserted, it will go through the collection
65to know where to fetch the data from.
66
67Either way, the format of this collection is completely dependent on
68your hardware. Each DMA controller will require a different structure,
69but all of them will require, for every chunk, at least the source and
70destination addresses, whether it should increment these addresses or
71not and the three parameters we saw earlier: the burst size, the
72transfer width and the transfer size.
73
74The one last thing is that usually, slave devices won't issue DRQ by
75default, and you have to enable this in your slave device driver first
76whenever you're willing to use DMA.
77
78These were just the general memory-to-memory (also called mem2mem) or
79memory-to-device (mem2dev) kind of transfers. Most devices often
80support other kind of transfers or memory operations that dmaengine
81support and will be detailed later in this document.
82
83DMA Support in Linux
84++++++++++++++++++++
85
86Historically, DMA controller drivers have been implemented using the
87async TX API, to offload operations such as memory copy, XOR,
88cryptography, etc., basically any memory to memory operation.
89
90Over time, the need for memory to device transfers arose, and
91dmaengine was extended. Nowadays, the async TX API is written as a
92layer on top of dmaengine, and acts as a client. Still, dmaengine
93accommodates that API in some cases, and made some design choices to
94ensure that it stayed compatible.
95
96For more information on the Async TX API, please look the relevant
97documentation file in Documentation/crypto/async-tx-api.txt.
98
99DMAEngine Registration
100++++++++++++++++++++++
101
102struct dma_device Initialization
103--------------------------------
104
105Just like any other kernel framework, the whole DMAEngine registration
106relies on the driver filling a structure and registering against the
107framework. In our case, that structure is dma_device.
108
109The first thing you need to do in your driver is to allocate this
110structure. Any of the usual memory allocators will do, but you'll also
111need to initialize a few fields in there:
112
113  * channels:	should be initialized as a list using the
114		INIT_LIST_HEAD macro for example
115
116  * src_addr_widths:
117    - should contain a bitmask of the supported source transfer width
118
119  * dst_addr_widths:
120    - should contain a bitmask of the supported destination transfer
121      width
122
123  * directions:
124    - should contain a bitmask of the supported slave directions
125      (i.e. excluding mem2mem transfers)
126
127  * residue_granularity:
128    - Granularity of the transfer residue reported to dma_set_residue.
129    - This can be either:
130      + Descriptor
131        -> Your device doesn't support any kind of residue
132           reporting. The framework will only know that a particular
133           transaction descriptor is done.
134      + Segment
135        -> Your device is able to report which chunks have been
136           transferred
137      + Burst
138        -> Your device is able to report which burst have been
139           transferred
140
141  * dev: 	should hold the pointer to the struct device associated
142		to your current driver instance.
143
144Supported transaction types
145---------------------------
146
147The next thing you need is to set which transaction types your device
148(and driver) supports.
149
150Our dma_device structure has a field called cap_mask that holds the
151various types of transaction supported, and you need to modify this
152mask using the dma_cap_set function, with various flags depending on
153transaction types you support as an argument.
154
155All those capabilities are defined in the dma_transaction_type enum,
156in include/linux/dmaengine.h
157
158Currently, the types available are:
159  * DMA_MEMCPY
160    - The device is able to do memory to memory copies
161
162  * DMA_XOR
163    - The device is able to perform XOR operations on memory areas
164    - Used to accelerate XOR intensive tasks, such as RAID5
165
166  * DMA_XOR_VAL
167    - The device is able to perform parity check using the XOR
168      algorithm against a memory buffer.
169
170  * DMA_PQ
171    - The device is able to perform RAID6 P+Q computations, P being a
172      simple XOR, and Q being a Reed-Solomon algorithm.
173
174  * DMA_PQ_VAL
175    - The device is able to perform parity check using RAID6 P+Q
176      algorithm against a memory buffer.
177
178  * DMA_INTERRUPT
179    - The device is able to trigger a dummy transfer that will
180      generate periodic interrupts
181    - Used by the client drivers to register a callback that will be
182      called on a regular basis through the DMA controller interrupt
183
184  * DMA_SG
185    - The device supports memory to memory scatter-gather
186      transfers.
187    - Even though a plain memcpy can look like a particular case of a
188      scatter-gather transfer, with a single chunk to transfer, it's a
189      distinct transaction type in the mem2mem transfers case
190
191  * DMA_PRIVATE
192    - The devices only supports slave transfers, and as such isn't
193      available for async transfers.
194
195  * DMA_ASYNC_TX
196    - Must not be set by the device, and will be set by the framework
197      if needed
198    - /* TODO: What is it about? */
199
200  * DMA_SLAVE
201    - The device can handle device to memory transfers, including
202      scatter-gather transfers.
203    - While in the mem2mem case we were having two distinct types to
204      deal with a single chunk to copy or a collection of them, here,
205      we just have a single transaction type that is supposed to
206      handle both.
207    - If you want to transfer a single contiguous memory buffer,
208      simply build a scatter list with only one item.
209
210  * DMA_CYCLIC
211    - The device can handle cyclic transfers.
212    - A cyclic transfer is a transfer where the chunk collection will
213      loop over itself, with the last item pointing to the first.
214    - It's usually used for audio transfers, where you want to operate
215      on a single ring buffer that you will fill with your audio data.
216
217  * DMA_INTERLEAVE
218    - The device supports interleaved transfer.
219    - These transfers can transfer data from a non-contiguous buffer
220      to a non-contiguous buffer, opposed to DMA_SLAVE that can
221      transfer data from a non-contiguous data set to a continuous
222      destination buffer.
223    - It's usually used for 2d content transfers, in which case you
224      want to transfer a portion of uncompressed data directly to the
225      display to print it
226
227These various types will also affect how the source and destination
228addresses change over time.
229
230Addresses pointing to RAM are typically incremented (or decremented)
231after each transfer. In case of a ring buffer, they may loop
232(DMA_CYCLIC). Addresses pointing to a device's register (e.g. a FIFO)
233are typically fixed.
234
235Device operations
236-----------------
237
238Our dma_device structure also requires a few function pointers in
239order to implement the actual logic, now that we described what
240operations we were able to perform.
241
242The functions that we have to fill in there, and hence have to
243implement, obviously depend on the transaction types you reported as
244supported.
245
246   * device_alloc_chan_resources
247   * device_free_chan_resources
248     - These functions will be called whenever a driver will call
249       dma_request_channel or dma_release_channel for the first/last
250       time on the channel associated to that driver.
251     - They are in charge of allocating/freeing all the needed
252       resources in order for that channel to be useful for your
253       driver.
254     - These functions can sleep.
255
256   * device_prep_dma_*
257     - These functions are matching the capabilities you registered
258       previously.
259     - These functions all take the buffer or the scatterlist relevant
260       for the transfer being prepared, and should create a hardware
261       descriptor or a list of hardware descriptors from it
262     - These functions can be called from an interrupt context
263     - Any allocation you might do should be using the GFP_NOWAIT
264       flag, in order not to potentially sleep, but without depleting
265       the emergency pool either.
266     - Drivers should try to pre-allocate any memory they might need
267       during the transfer setup at probe time to avoid putting to
268       much pressure on the nowait allocator.
269
270     - It should return a unique instance of the
271       dma_async_tx_descriptor structure, that further represents this
272       particular transfer.
273
274     - This structure can be initialized using the function
275       dma_async_tx_descriptor_init.
276     - You'll also need to set two fields in this structure:
277       + flags:
278		TODO: Can it be modified by the driver itself, or
279		should it be always the flags passed in the arguments
280
281       + tx_submit:	A pointer to a function you have to implement,
282			that is supposed to push the current
283			transaction descriptor to a pending queue, waiting
284			for issue_pending to be called.
285
286   * device_issue_pending
287     - Takes the first transaction descriptor in the pending queue,
288       and starts the transfer. Whenever that transfer is done, it
289       should move to the next transaction in the list.
290     - This function can be called in an interrupt context
291
292   * device_tx_status
293     - Should report the bytes left to go over on the given channel
294     - Should only care about the transaction descriptor passed as
295       argument, not the currently active one on a given channel
296     - The tx_state argument might be NULL
297     - Should use dma_set_residue to report it
298     - In the case of a cyclic transfer, it should only take into
299       account the current period.
300     - This function can be called in an interrupt context.
301
302   * device_config
303     - Reconfigures the channel with the configuration given as
304       argument
305     - This command should NOT perform synchronously, or on any
306       currently queued transfers, but only on subsequent ones
307     - In this case, the function will receive a dma_slave_config
308       structure pointer as an argument, that will detail which
309       configuration to use.
310     - Even though that structure contains a direction field, this
311       field is deprecated in favor of the direction argument given to
312       the prep_* functions
313     - This call is mandatory for slave operations only. This should NOT be
314       set or expected to be set for memcpy operations.
315       If a driver support both, it should use this call for slave
316       operations only and not for memcpy ones.
317
318   * device_pause
319     - Pauses a transfer on the channel
320     - This command should operate synchronously on the channel,
321       pausing right away the work of the given channel
322
323   * device_resume
324     - Resumes a transfer on the channel
325     - This command should operate synchronously on the channel,
326       pausing right away the work of the given channel
327
328   * device_terminate_all
329     - Aborts all the pending and ongoing transfers on the channel
330     - This command should operate synchronously on the channel,
331       terminating right away all the channels
332
333Misc notes (stuff that should be documented, but don't really know
334where to put them)
335------------------------------------------------------------------
336  * dma_run_dependencies
337    - Should be called at the end of an async TX transfer, and can be
338      ignored in the slave transfers case.
339    - Makes sure that dependent operations are run before marking it
340      as complete.
341
342  * dma_cookie_t
343    - it's a DMA transaction ID that will increment over time.
344    - Not really relevant any more since the introduction of virt-dma
345      that abstracts it away.
346
347  * DMA_CTRL_ACK
348    - Undocumented feature
349    - No one really has an idea of what it's about, besides being
350      related to reusing the DMA transaction descriptors or having
351      additional transactions added to it in the async-tx API
352    - Useless in the case of the slave API
353
354General Design Notes
355--------------------
356
357Most of the DMAEngine drivers you'll see are based on a similar design
358that handles the end of transfer interrupts in the handler, but defer
359most work to a tasklet, including the start of a new transfer whenever
360the previous transfer ended.
361
362This is a rather inefficient design though, because the inter-transfer
363latency will be not only the interrupt latency, but also the
364scheduling latency of the tasklet, which will leave the channel idle
365in between, which will slow down the global transfer rate.
366
367You should avoid this kind of practice, and instead of electing a new
368transfer in your tasklet, move that part to the interrupt handler in
369order to have a shorter idle window (that we can't really avoid
370anyway).
371
372Glossary
373--------
374
375Burst: 		A number of consecutive read or write operations
376		that can be queued to buffers before being flushed to
377		memory.
378Chunk:		A contiguous collection of bursts
379Transfer:	A collection of chunks (be it contiguous or not)
380