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diff --git a/Documentation/driver-api/dmaengine/client.rst b/Documentation/driver-api/dmaengine/client.rst new file mode 100644 index 000000000..09a3f66dc --- /dev/null +++ b/Documentation/driver-api/dmaengine/client.rst @@ -0,0 +1,377 @@ +==================== +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.rst`` + + +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. + +DMA usage +========= + +The slave DMA usage consists of following steps: + +- Allocate a DMA slave channel + +- Set slave and controller specific parameters + +- Get a descriptor for transaction + +- Submit the transaction + +- Issue pending requests and wait for callback notification + +The details of these operations are: + +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: + + .. code-block:: c + + 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: + + .. code-block:: c + + 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. Cyclic + interleaved DMA transfers are also possible if supported by the channel by + setting the DMA_PREP_REPEAT transfer flag. + + A non-NULL return of this transfer API represents a "descriptor" for + the given transaction. + + Interface: + + .. code-block:: c + + 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: + + .. code-block:: c + + 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. + + **Optional: per descriptor metadata** + + DMAengine provides two ways for metadata support. + + DESC_METADATA_CLIENT + + The metadata buffer is allocated/provided by the client driver and it is + attached to the descriptor. + + .. code-block:: c + + int dmaengine_desc_attach_metadata(struct dma_async_tx_descriptor *desc, + void *data, size_t len); + + DESC_METADATA_ENGINE + + The metadata buffer is allocated/managed by the DMA driver. The client + driver can ask for the pointer, maximum size and the currently used size of + the metadata and can directly update or read it. + + Becasue the DMA driver manages the memory area containing the metadata, + clients must make sure that they do not try to access or get the pointer + after their transfer completion callback has run for the descriptor. + If no completion callback has been defined for the transfer, then the + metadata must not be accessed after issue_pending. + In other words: if the aim is to read back metadata after the transfer is + completed, then the client must use completion callback. + + .. code-block:: c + + void *dmaengine_desc_get_metadata_ptr(struct dma_async_tx_descriptor *desc, + size_t *payload_len, size_t *max_len); + + int dmaengine_desc_set_metadata_len(struct dma_async_tx_descriptor *desc, + size_t payload_len); + + Client drivers can query if a given mode is supported with: + + .. code-block:: c + + bool dmaengine_is_metadata_mode_supported(struct dma_chan *chan, + enum dma_desc_metadata_mode mode); + + Depending on the used mode client drivers must follow different flow. + + DESC_METADATA_CLIENT + + - DMA_MEM_TO_DEV / DEV_MEM_TO_MEM: + + 1. prepare the descriptor (dmaengine_prep_*) + construct the metadata in the client's buffer + 2. use dmaengine_desc_attach_metadata() to attach the buffer to the + descriptor + 3. submit the transfer + + - DMA_DEV_TO_MEM: + + 1. prepare the descriptor (dmaengine_prep_*) + 2. use dmaengine_desc_attach_metadata() to attach the buffer to the + descriptor + 3. submit the transfer + 4. when the transfer is completed, the metadata should be available in the + attached buffer + + DESC_METADATA_ENGINE + + - DMA_MEM_TO_DEV / DEV_MEM_TO_MEM: + + 1. prepare the descriptor (dmaengine_prep_*) + 2. use dmaengine_desc_get_metadata_ptr() to get the pointer to the + engine's metadata area + 3. update the metadata at the pointer + 4. use dmaengine_desc_set_metadata_len() to tell the DMA engine the + amount of data the client has placed into the metadata buffer + 5. submit the transfer + + - DMA_DEV_TO_MEM: + + 1. prepare the descriptor (dmaengine_prep_*) + 2. submit the transfer + 3. on transfer completion, use dmaengine_desc_get_metadata_ptr() to get + the pointer to the engine's metadata area + 4. read out the metadata from the pointer + + .. note:: + + When DESC_METADATA_ENGINE mode is used the metadata area for the descriptor + is no longer valid after the transfer has been completed (valid up to the + point when the completion callback returns if used). + + Mixed use of DESC_METADATA_CLIENT / DESC_METADATA_ENGINE is not allowed, + client drivers must use either of the modes per descriptor. + +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: + + .. code-block:: c + + 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. + + .. note:: + + After calling ``dmaengine_submit()`` the submitted transfer descriptor + (``struct dma_async_tx_descriptor``) belongs to the DMA engine. + Consequently, the client must consider invalid the pointer to that + descriptor. + +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: + + .. code-block:: c + + void dma_async_issue_pending(struct dma_chan *chan); + +Further APIs +------------ + +1. Terminate APIs + + .. code-block:: c + + 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. Pause API + + .. code-block:: c + + int dmaengine_pause(struct dma_chan *chan) + + This pauses activity on the DMA channel without data loss. + +3. Resume API + + .. code-block:: c + + 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. Check Txn complete + + .. code-block:: c + + 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. Synchronize termination API + + .. code-block:: c + + 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. |