[mirror_ubuntu-zesty-kernel.git] / Documentation / dmaengine / client.txt

			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_chan() API is used.

   Interface:
	struct dma_chan *dma_request_chan(struct device *dev, const char *name);

   Which will find and return the 'name' DMA channel associated with the 'dev'
   device. The association is done via DT, ACPI or board file based
   dma_slave_map matching table.

   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_async().

	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_sync(struct dma_chan *chan)
   int dmaengine_terminate_async(struct dma_chan *chan)
   int dmaengine_terminate_all(struct dma_chan *chan) /* DEPRECATED */

   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.

   Two variants of this function are available.

   dmaengine_terminate_async() might not wait until the DMA has been fully
   stopped or until any running complete callbacks have finished. But it is
   possible to call dmaengine_terminate_async() from atomic context or from
   within a complete callback. dmaengine_synchronize() must be called before it
   is safe to free the memory accessed by the DMA transfer or free resources
   accessed from within the complete callback.

   dmaengine_terminate_sync() will wait for the transfer and any running
   complete callbacks to finish before it returns. But the function must not be
   called from atomic context or from within a complete callback.

   dmaengine_terminate_all() is deprecated and should not be used in new code.

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.

5. void dmaengine_synchronize(struct dma_chan *chan)

  Synchronize the termination of the DMA channel to the current context.

  This function should be used after dmaengine_terminate_async() to synchronize
  the termination of the DMA channel to the current context. The function will
  wait for the transfer and any running complete callbacks to finish before it
  returns.

  If dmaengine_terminate_async() is used to stop the DMA channel this function
  must be called before it is safe to free memory accessed by previously
  submitted descriptors or to free any resources accessed within the complete
  callback of previously submitted descriptors.

  The behavior of this function is undefined if dma_async_issue_pending() has
  been called between dmaengine_terminate_async() and this function.
Commit	Line	Data
46b2903c VK	1	DMA Engine API Guide
	2	====================
	3
	4	Vinod Koul <vinod dot koul at intel.com>
	5
	6	NOTE: For DMA Engine usage in async_tx please see:
	7	Documentation/crypto/async-tx-api.txt
	8
	9
	10	Below is a guide to device driver writers on how to use the Slave-DMA API of the
	11	DMA Engine. This is applicable only for slave DMA usage only.
	12
5a42fb93	13	The slave DMA usage consists of following steps:
46b2903c VK	14	1. Allocate a DMA slave channel
	15	2. Set slave and controller specific parameters
	16	3. Get a descriptor for transaction
5a42fb93 RKAL	17	4. Submit the transaction
5a42fb93 RKAL	18	5. Issue pending requests and wait for callback notification
46b2903c VK	19
46b2903c VK	20	1. Allocate a DMA slave channel
5a42fb93 RKAL	21
	22	Channel allocation is slightly different in the slave DMA context,
	23	client drivers typically need a channel from a particular DMA
	24	controller only and even in some cases a specific channel is desired.
a8135d0d	25	To request a channel dma_request_chan() API is used.
5a42fb93 RKAL	26
5a42fb93 RKAL	27	Interface:
a8135d0d PU	28	struct dma_chan dma_request_chan(struct device dev, const char *name);
	29
	30	Which will find and return the 'name' DMA channel associated with the 'dev'
	31	device. The association is done via DT, ACPI or board file based
	32	dma_slave_map matching table.
5a42fb93 RKAL	33
	34	A channel allocated via this interface is exclusive to the caller,
	35	until dma_release_channel() is called.
46b2903c VK	36
46b2903c VK	37	2. Set slave and controller specific parameters
5a42fb93 RKAL	38
5a42fb93 RKAL	39	Next step is always to pass some specific information to the DMA
4dc7b436 VK	40	driver. Most of the generic information which a slave DMA can use
4dc7b436 VK	41	is in struct dma_slave_config. This allows the clients to specify
5a42fb93 RKAL	42	DMA direction, DMA addresses, bus widths, DMA burst lengths etc
	43	for the peripheral.
	44
	45	If some DMA controllers have more parameters to be sent then they
	46	should try to embed struct dma_slave_config in their controller
	47	specific structure. That gives flexibility to client to pass more
	48	parameters, if required.
	49
	50	Interface:
	51	int dmaengine_slave_config(struct dma_chan *chan,
	52	struct dma_slave_config *config)
	53
	54	Please see the dma_slave_config structure definition in dmaengine.h
4dc7b436	55	for a detailed explanation of the struct members. Please note
5a42fb93 RKAL	56	that the 'direction' member will be going away as it duplicates the
5a42fb93 RKAL	57	direction given in the prepare call.
46b2903c VK	58
46b2903c VK	59	3. Get a descriptor for transaction
5a42fb93 RKAL	60
	61	For slave usage the various modes of slave transfers supported by the
	62	DMA-engine are:
	63
	64	slave_sg - DMA a list of scatter gather buffers from/to a peripheral
	65	dma_cyclic - Perform a cyclic DMA operation from/to a peripheral till the
46b2903c	66	operation is explicitly stopped.
b14dab79 JB	67	interleaved_dma - This is common to Slave as well as M2M clients. For slave
	68	address of devices' fifo could be already known to the driver.
	69	Various types of operations could be expressed by setting
	70	appropriate values to the 'dma_interleaved_template' members.
5a42fb93 RKAL	71
	72	A non-NULL return of this transfer API represents a "descriptor" for
	73	the given transaction.
	74
	75	Interface:
96cb9898	76	struct dma_async_tx_descriptor *dmaengine_prep_slave_sg(
5a42fb93 RKAL	77	struct dma_chan chan, struct scatterlist sgl,
5a42fb93 RKAL	78	unsigned int sg_len, enum dma_data_direction direction,
46b2903c	79	unsigned long flags);
5a42fb93	80
96cb9898	81	struct dma_async_tx_descriptor *dmaengine_prep_dma_cyclic(
46b2903c VK	82	struct dma_chan *chan, dma_addr_t buf_addr, size_t buf_len,
	83	size_t period_len, enum dma_data_direction direction);
	84
96cb9898	85	struct dma_async_tx_descriptor *dmaengine_prep_interleaved_dma(
b14dab79 JB	86	struct dma_chan chan, struct dma_interleaved_template xt,
	87	unsigned long flags);
	88
5a42fb93	89	The peripheral driver is expected to have mapped the scatterlist for
58d06e98	90	the DMA operation prior to calling dmaengine_prep_slave_sg(), and must
5a42fb93	91	keep the scatterlist mapped until the DMA operation has completed.
70c9b8bd GU	92	The scatterlist must be mapped using the DMA struct device.
	93	If a mapping needs to be synchronized later, dma_sync__for_() must be
	94	called using the DMA struct device, too.
	95	So, normal setup should look like this:
5a42fb93 RKAL	96
	97	nr_sg = dma_map_sg(chan->device->dev, sgl, sg_len);
	98	if (nr_sg == 0)
	99	/* error */
	100
96cb9898	101	desc = dmaengine_prep_slave_sg(chan, sgl, nr_sg, direction, flags);
5a42fb93 RKAL	102
5a42fb93 RKAL	103	Once a descriptor has been obtained, the callback information can be
4dc7b436	104	added and the descriptor must then be submitted. Some DMA engine
5a42fb93 RKAL	105	drivers may hold a spinlock between a successful preparation and
	106	submission so it is important that these two operations are closely
	107	paired.
	108
	109	Note:
	110	Although the async_tx API specifies that completion callback
	111	routines cannot submit any new operations, this is not the
	112	case for slave/cyclic DMA.
	113
	114	For slave DMA, the subsequent transaction may not be available
	115	for submission prior to callback function being invoked, so
	116	slave DMA callbacks are permitted to prepare and submit a new
	117	transaction.
	118
	119	For cyclic DMA, a callback function may wish to terminate the
b36f09c3	120	DMA via dmaengine_terminate_async().
5a42fb93 RKAL	121
	122	Therefore, it is important that DMA engine drivers drop any
	123	locks before calling the callback function which may cause a
	124	deadlock.
	125
	126	Note that callbacks will always be invoked from the DMA
	127	engines tasklet, never from interrupt context.
	128
	129	4. Submit the transaction
	130
	131	Once the descriptor has been prepared and the callback information
	132	added, it must be placed on the DMA engine drivers pending queue.
	133
	134	Interface:
	135	dma_cookie_t dmaengine_submit(struct dma_async_tx_descriptor *desc)
	136
	137	This returns a cookie can be used to check the progress of DMA engine
	138	activity via other DMA engine calls not covered in this document.
	139
	140	dmaengine_submit() will not start the DMA operation, it merely adds
4dc7b436	141	it to the pending queue. For this, see step 5, dma_async_issue_pending.
5a42fb93 RKAL	142
	143	5. Issue pending DMA requests and wait for callback notification
	144
	145	The transactions in the pending queue can be activated by calling the
	146	issue_pending API. If channel is idle then the first transaction in
	147	queue is started and subsequent ones queued up.
	148
	149	On completion of each DMA operation, the next in queue is started and
	150	a tasklet triggered. The tasklet will then call the client driver
	151	completion callback routine for notification, if set.
	152
	153	Interface:
	154	void dma_async_issue_pending(struct dma_chan *chan);
	155
	156	Further APIs:
	157
b36f09c3 LPC	158	1. int dmaengine_terminate_sync(struct dma_chan *chan)
	159	int dmaengine_terminate_async(struct dma_chan *chan)
	160	int dmaengine_terminate_all(struct dma_chan chan) / DEPRECATED */
5a42fb93 RKAL	161
	162	This causes all activity for the DMA channel to be stopped, and may
	163	discard data in the DMA FIFO which hasn't been fully transferred.
	164	No callback functions will be called for any incomplete transfers.
	165
b36f09c3 LPC	166	Two variants of this function are available.
	167
	168	dmaengine_terminate_async() might not wait until the DMA has been fully
	169	stopped or until any running complete callbacks have finished. But it is
	170	possible to call dmaengine_terminate_async() from atomic context or from
	171	within a complete callback. dmaengine_synchronize() must be called before it
	172	is safe to free the memory accessed by the DMA transfer or free resources
	173	accessed from within the complete callback.
	174
	175	dmaengine_terminate_sync() will wait for the transfer and any running
	176	complete callbacks to finish before it returns. But the function must not be
	177	called from atomic context or from within a complete callback.
	178
	179	dmaengine_terminate_all() is deprecated and should not be used in new code.
	180
5a42fb93 RKAL	181	2. int dmaengine_pause(struct dma_chan *chan)
	182
	183	This pauses activity on the DMA channel without data loss.
	184
	185	3. int dmaengine_resume(struct dma_chan *chan)
	186
4dc7b436	187	Resume a previously paused DMA channel. It is invalid to resume a
5a42fb93 RKAL	188	channel which is not currently paused.
	189
	190	4. enum dma_status dma_async_is_tx_complete(struct dma_chan *chan,
	191	dma_cookie_t cookie, dma_cookie_t last, dma_cookie_t used)
	192
4dc7b436	193	This can be used to check the status of the channel. Please see
5a42fb93 RKAL	194	the documentation in include/linux/dmaengine.h for a more complete
	195	description of this API.
	196
	197	This can be used in conjunction with dma_async_is_complete() and
96cb9898	198	the cookie returned from dmaengine_submit() to check for
5a42fb93 RKAL	199	completion of a specific DMA transaction.
	200
	201	Note:
	202	Not all DMA engine drivers can return reliable information for
4dc7b436	203	a running DMA channel. It is recommended that DMA engine users
58d06e98	204	pause or stop (via dmaengine_terminate_all()) the channel before
5a42fb93	205	using this API.
b36f09c3 LPC	206
	207	5. void dmaengine_synchronize(struct dma_chan *chan)
	208
	209	Synchronize the termination of the DMA channel to the current context.
	210
	211	This function should be used after dmaengine_terminate_async() to synchronize
	212	the termination of the DMA channel to the current context. The function will
	213	wait for the transfer and any running complete callbacks to finish before it
	214	returns.
	215
	216	If dmaengine_terminate_async() is used to stop the DMA channel this function
	217	must be called before it is safe to free memory accessed by previously
	218	submitted descriptors or to free any resources accessed within the complete
	219	callback of previously submitted descriptors.
	220
	221	The behavior of this function is undefined if dma_async_issue_pending() has
	222	been called between dmaengine_terminate_async() and this function.