Re: [PATCH v3 03/14] dmaengine: doc: Add sections for per descriptor metadata support

[Tero Kristo]

On 01/10/2019 09:16, Peter Ujfalusi wrote:
> Update the provider and client documentation with details about the
> metadata support.
>
> Signed-off-by: Peter Ujfalusi <peter.ujfalusi@xxxxxx>

Couple of typos below, but they don't really change the readability of 
the document so:

Reviewed-by: Tero Kristo <t-kristo@xxxxxx>

> ---
>   Documentation/driver-api/dmaengine/client.rst | 75 +++++++++++++++++++
>   .../driver-api/dmaengine/provider.rst         | 46 ++++++++++++
>   2 files changed, 121 insertions(+)
>
> diff --git a/Documentation/driver-api/dmaengine/client.rst b/Documentation/driver-api/dmaengine/client.rst
> index 45953f171500..d708e46b88a2 100644
> --- a/Documentation/driver-api/dmaengine/client.rst
> +++ b/Documentation/driver-api/dmaengine/client.rst
> @@ -151,6 +151,81 @@ The details of these operations are:
>        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.
> +
> +  .. 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 are

are = area?

> +      4. Read out the metadate from the pointer

metadate = metadata?

> +
> +  .. note::
> +
> +     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
> diff --git a/Documentation/driver-api/dmaengine/provider.rst b/Documentation/driver-api/dmaengine/provider.rst
> index dfc4486b5743..9e6d87b3c477 100644
> --- a/Documentation/driver-api/dmaengine/provider.rst
> +++ b/Documentation/driver-api/dmaengine/provider.rst
> @@ -247,6 +247,52 @@ after each transfer. In case of a ring buffer, they may loop
>   (DMA_CYCLIC). Addresses pointing to a device's register (e.g. a FIFO)
>   are typically fixed.
>   
> +Per descriptor metadata support
> +-------------------------------
> +Some data movement architecure (DMA controller and peripherals) uses metadata

architecure = architecture?

> +associated with a transaction. The DMA controller role is to transfer the
> +payload and the metadata alongside.
> +The metadata itself is not used by the DMA engine itself, but it contains
> +parameters, keys, vectors, etc for peripheral or from the peripheral.
> +
> +The DMAengine framework provides a generic ways to facilitate the metadata for
> +descriptors. Depending on the architecture the DMA driver can implement either
> +or both of the methods and it is up to the client driver to choose which one
> +to use.
> +
> +- DESC_METADATA_CLIENT
> +
> +  The metadata buffer is allocated/provided by the client driver and it is
> +  attached (via the dmaengine_desc_attach_metadata() helper to the descriptor.
> +
> +  From the DMA driver the following is expected for this mode:
> +  - DMA_MEM_TO_DEV / DEV_MEM_TO_MEM
> +    The data from the provided metadata buffer should be prepared for the DMA
> +    controller to be sent alongside of the payload data. Either by copying to a
> +    hardware descriptor, or highly coupled packet.
> +  - DMA_DEV_TO_MEM
> +    On transfer completion the DMA driver must copy the metadata to the client
> +    provided metadata buffer.
> +
> +- 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. dmaengine_desc_get_metadata_ptr()
> +  and dmaengine_desc_set_metadata_len() is provided as helper functions.
> +
> +  From the DMA driver the following is expected for this mode:
> +  - get_metadata_ptr
> +    Should return a pointer for the metadata buffer, the maximum size of the
> +    metadata buffer and the currently used / valid (if any) bytes in the buffer.
> +  - set_metadata_len
> +    It is called by the clients after it have placed the metadata to the buffer
> +    to let the DMA driver know the number of valid bytes provided.
> +
> +  Note: since the client will ask for the metadata pointer in the completion
> +  callback (in DMA_DEV_TO_MEM case) the DMA driver must ensure that the
> +  descriptor is not freed up prior the callback is called.
> +
>   Device operations
>   -----------------
>   

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