| Message ID | 20110725113926.GH9653@n2100.arm.linux.org.uk (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Mon, 2011-07-25 at 12:39 +0100, Russell King - ARM Linux wrote: > On Mon, Jul 25, 2011 at 04:31:57PM +0530, Vinod Koul wrote: > > > I know that very well thank you. Please look at my previous post in the > > > previous round of patches, and the response from Boojin Kim to see why > > > I used this as an *EXAMPLE* to get this fixed. > > Russell, > > Sorry that was not intended for you but for the author of patch Boojin > > Kim... Agree on your EXAMPLE, just wanted to ensure authors have read it > > as going thru this patch made it clear that they haven't. > > That wasn't obvious because of the To: and the use of 'you' in your reply. Laziness on a rainy monday evening caused me to miss that > > In any case, let's fix up the documentation to be a little more complete > and expansive about these conditions, and improve its formatting so it's > easier to read. > > A few points: > > 1. This really should describe the dma_slave_config structure in detail > (or refer to the documentation in dmaengine.h) especially describing > what is mandatory there. I think you missed mandatory fields here, Pls note I am dropping the direction field in this, as we have discussed it makes no sense > > 2. Note the filter function thing - I think this should be mandatory > for slave and cyclic channels. These typically have a DMA request > signal associated with each channel, and so 'any random channel' isn't > really acceptable like it is for the async_tx APIs. > > 3. I think this should specify that dmaengine_slave_config() should be > specified as mandatory after obtaining the DMA engine channel. > > Any comments on this before I prepare this as a proper patch? > > diff --git a/Documentation/dmaengine.txt b/Documentation/dmaengine.txt > index 5a0cb1e..8ba8d3c 100644 > --- a/Documentation/dmaengine.txt > +++ b/Documentation/dmaengine.txt > @@ -10,87 +10,163 @@ > 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 > > 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) > > 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); > > + 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 and wait for callback notification As you are adding 5 for issue pending then this needs change > -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 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 getting 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. >
diff --git a/Documentation/dmaengine.txt b/Documentation/dmaengine.txt index 5a0cb1e..8ba8d3c 100644 --- a/Documentation/dmaengine.txt +++ b/Documentation/dmaengine.txt @@ -10,87 +10,163 @@ 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 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) 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); + 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 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 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 + + 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.