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Diffstat (limited to 'Documentation/dmaengine.txt')
| -rw-r--r-- | Documentation/dmaengine.txt | 234 |
1 files changed, 164 insertions, 70 deletions
diff --git a/Documentation/dmaengine.txt b/Documentation/dmaengine.txt index 5a0cb1e..94b7e0f 100644 --- a/Documentation/dmaengine.txt +++ b/Documentation/dmaengine.txt @@ -10,87 +10,181 @@ NOTE: For DMA Engine usage in async_tx please see: 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 +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 and wait for callback notification +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); - -When the optional 'filter_fn' parameter is set to NULL dma_request_channel -simply returns the first channel that satisfies the capability mask. Otherwise, -when the mask parameter is insufficient for specifying the necessary channel, -the filter_fn routine can be used to disposition the available channels in the -system. The filter_fn routine is called once for each free channel in the -system. Upon seeing a suitable channel filter_fn returns DMA_ACK which flags -that channel to be the return value from dma_request_channel. A channel -allocated via this interface is exclusive to the caller, until -dma_release_channel() is called. + + 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. -It allows the clients to specify DMA direction, DMA addresses, bus widths, DMA -burst lengths etc. 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) + + 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 explaination 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 + + 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. -The non NULL return of this transfer API represents a "descriptor" for the given -transaction. - -Interface: -struct dma_async_tx_descriptor *(*chan->device->device_prep_dma_sg)( - struct dma_chan *chan, - struct scatterlist *dst_sg, unsigned int dst_nents, - struct scatterlist *src_sg, unsigned int src_nents, + + A non-NULL return of this transfer API represents a "descriptor" for + the given transaction. + + Interface: + struct dma_async_tx_descriptor *(*chan->device->device_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 *(*chan->device->device_prep_dma_cyclic)( + + struct dma_async_tx_descriptor *(*chan->device->device_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); -4. Submit the transaction and wait for callback notification -To schedule the transaction to be scheduled by dma device, the "descriptor" -returned in above (3) needs to be submitted. -To tell the dma driver that a transaction is ready to be serviced, the -descriptor->submit() callback needs to be invoked. This chains the descriptor to -the pending queue. -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 the DMA operation the next in queue is submitted and a tasklet -triggered. The tasklet would then call the client driver completion callback -routine for notification, if set. -Interface: -void dma_async_issue_pending(struct dma_chan *chan); - -============================================================================== - -Additional usage notes for dma driver writers -1/ Although DMA engine specifies that completion callback routines cannot submit -any new operations, but typically for slave DMA subsequent transaction may not -be available for submit prior to callback routine being called. This requirement -is not a requirement for DMA-slave devices. But they should take care to drop -the spin-lock they might be holding before calling the callback routine + The peripheral driver is expected to have mapped the scatterlist for + the DMA operation prior to calling device_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. So, + normal setup should look like this: + + nr_sg = dma_map_sg(chan->device->dev, sgl, sg_len); + if (nr_sg == 0) + /* error */ + + desc = chan->device->device_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 'descriptor->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. |
