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dmaengine: doc: Add sections for per descriptor metadata support
Update the provider and client documentation with details about the metadata support. Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com> Reviewed-by: Tero Kristo <t-kristo@ti.com> Tested-by: Keerthy <j-keerthy@ti.com> Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com> Link: https://lore.kernel.org/r/20191223110458.30766-4-peter.ujfalusi@ti.com Signed-off-by: Vinod Koul <vkoul@kernel.org>
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Peter Ujfalusi
Vinod Koul
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2 changed files with 135 additions and 0 deletions
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@ -151,6 +151,93 @@ The details of these operations are:
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Note that callbacks will always be invoked from the DMA
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engines tasklet, never from interrupt context.
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Optional: per descriptor metadata
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---------------------------------
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DMAengine provides two ways for metadata support.
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DESC_METADATA_CLIENT
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The metadata buffer is allocated/provided by the client driver and it is
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attached to the descriptor.
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.. code-block:: c
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int dmaengine_desc_attach_metadata(struct dma_async_tx_descriptor *desc,
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void *data, size_t len);
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DESC_METADATA_ENGINE
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The metadata buffer is allocated/managed by the DMA driver. The client
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driver can ask for the pointer, maximum size and the currently used size of
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the metadata and can directly update or read it.
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Becasue the DMA driver manages the memory area containing the metadata,
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clients must make sure that they do not try to access or get the pointer
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after their transfer completion callback has run for the descriptor.
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If no completion callback has been defined for the transfer, then the
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metadata must not be accessed after issue_pending.
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In other words: if the aim is to read back metadata after the transfer is
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completed, then the client must use completion callback.
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.. code-block:: c
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void *dmaengine_desc_get_metadata_ptr(struct dma_async_tx_descriptor *desc,
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size_t *payload_len, size_t *max_len);
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int dmaengine_desc_set_metadata_len(struct dma_async_tx_descriptor *desc,
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size_t payload_len);
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Client drivers can query if a given mode is supported with:
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.. code-block:: c
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bool dmaengine_is_metadata_mode_supported(struct dma_chan *chan,
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enum dma_desc_metadata_mode mode);
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Depending on the used mode client drivers must follow different flow.
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DESC_METADATA_CLIENT
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- DMA_MEM_TO_DEV / DEV_MEM_TO_MEM:
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1. prepare the descriptor (dmaengine_prep_*)
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construct the metadata in the client's buffer
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2. use dmaengine_desc_attach_metadata() to attach the buffer to the
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descriptor
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3. submit the transfer
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- DMA_DEV_TO_MEM:
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1. prepare the descriptor (dmaengine_prep_*)
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2. use dmaengine_desc_attach_metadata() to attach the buffer to the
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descriptor
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3. submit the transfer
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4. when the transfer is completed, the metadata should be available in the
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attached buffer
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DESC_METADATA_ENGINE
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- DMA_MEM_TO_DEV / DEV_MEM_TO_MEM:
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1. prepare the descriptor (dmaengine_prep_*)
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2. use dmaengine_desc_get_metadata_ptr() to get the pointer to the
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engine's metadata area
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3. update the metadata at the pointer
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4. use dmaengine_desc_set_metadata_len() to tell the DMA engine the
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amount of data the client has placed into the metadata buffer
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5. submit the transfer
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- DMA_DEV_TO_MEM:
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1. prepare the descriptor (dmaengine_prep_*)
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2. submit the transfer
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3. on transfer completion, use dmaengine_desc_get_metadata_ptr() to get
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the pointer to the engine's metadata area
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4. read out the metadata from the pointer
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.. note::
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When DESC_METADATA_ENGINE mode is used the metadata area for the descriptor
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is no longer valid after the transfer has been completed (valid up to the
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point when the completion callback returns if used).
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Mixed use of DESC_METADATA_CLIENT / DESC_METADATA_ENGINE is not allowed,
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client drivers must use either of the modes per descriptor.
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4. Submit the transaction
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Once the descriptor has been prepared and the callback information
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@ -247,6 +247,54 @@ after each transfer. In case of a ring buffer, they may loop
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(DMA_CYCLIC). Addresses pointing to a device's register (e.g. a FIFO)
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are typically fixed.
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Per descriptor metadata support
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-------------------------------
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Some data movement architecture (DMA controller and peripherals) uses metadata
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associated with a transaction. The DMA controller role is to transfer the
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payload and the metadata alongside.
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The metadata itself is not used by the DMA engine itself, but it contains
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parameters, keys, vectors, etc for peripheral or from the peripheral.
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The DMAengine framework provides a generic ways to facilitate the metadata for
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descriptors. Depending on the architecture the DMA driver can implement either
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or both of the methods and it is up to the client driver to choose which one
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to use.
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- DESC_METADATA_CLIENT
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The metadata buffer is allocated/provided by the client driver and it is
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attached (via the dmaengine_desc_attach_metadata() helper to the descriptor.
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From the DMA driver the following is expected for this mode:
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- DMA_MEM_TO_DEV / DEV_MEM_TO_MEM
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The data from the provided metadata buffer should be prepared for the DMA
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controller to be sent alongside of the payload data. Either by copying to a
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hardware descriptor, or highly coupled packet.
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- DMA_DEV_TO_MEM
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On transfer completion the DMA driver must copy the metadata to the client
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provided metadata buffer before notifying the client about the completion.
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After the transfer completion, DMA drivers must not touch the metadata
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buffer provided by the client.
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- DESC_METADATA_ENGINE
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The metadata buffer is allocated/managed by the DMA driver. The client driver
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can ask for the pointer, maximum size and the currently used size of the
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metadata and can directly update or read it. dmaengine_desc_get_metadata_ptr()
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and dmaengine_desc_set_metadata_len() is provided as helper functions.
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From the DMA driver the following is expected for this mode:
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- get_metadata_ptr
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Should return a pointer for the metadata buffer, the maximum size of the
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metadata buffer and the currently used / valid (if any) bytes in the buffer.
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- set_metadata_len
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It is called by the clients after it have placed the metadata to the buffer
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to let the DMA driver know the number of valid bytes provided.
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Note: since the client will ask for the metadata pointer in the completion
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callback (in DMA_DEV_TO_MEM case) the DMA driver must ensure that the
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descriptor is not freed up prior the callback is called.
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Device operations
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-----------------
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