On Tue, 2011-07-26 at 08:57 +0100, Russell King - ARM Linux wrote: > Here's an updated patch. > > 8<---------- > From: Russell King <rmk+kernel@xxxxxxxxxxxxxxxx> > DMAEngine: Improve slave/cyclic documentation > > Improve the documentation for the slave and cyclic DMA engine support > reformatting it for easier reading, adding further APIs, splitting it > into five steps, and including references to the documentation in > dmaengine.h. > > Signed-off-by: Russell King <rmk+kernel@xxxxxxxxxxxxxxxx> > --- > Documentation/dmaengine.txt | 211 ++++++++++++++++++++++++++++++------------- > 1 files changed, 146 insertions(+), 65 deletions(-) > > diff --git a/Documentation/dmaengine.txt b/Documentation/dmaengine.txt > index 5a0cb1e..8c2e888 100644 > --- a/Documentation/dmaengine.txt > +++ b/Documentation/dmaengine.txt > @@ -10,87 +10,168 @@ > 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 > +5. Issue pending requests Thanks Russell, Applied with change to 4 above. Moved "and wait for callback notification" to 5. > > 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)( > + 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_dma_sg)( > struct dma_chan *chan, > struct scatterlist *dst_sg, unsigned int dst_nents, > struct scatterlist *src_sg, unsigned int src_nents, > 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 > + Once a descriptor has been obtained, the callback information can be > + added and 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. > > _______________________________________________ > linux-arm-kernel mailing list > linux-arm-kernel@xxxxxxxxxxxxxxxxxxx > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel -- ~Vinod Koul Intel Corp. -- To unsubscribe from this list: send the line "unsubscribe linux-samsung-soc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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