linux-kernel - Re: [PATCH v2 2/2] dmaengine: doc: Add sections for per descriptor metadata support

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Date:   Tue, 11 Sep 2018 13:06:27 +0530
From:   Vinod <vkoul@...nel.org>
To:     Peter Ujfalusi <peter.ujfalusi@...com>
Cc:     dan.j.williams@...el.com, dmaengine@...r.kernel.org,
        linux-kernel@...r.kernel.org, lars@...afoo.de, radheys@...inx.com
Subject: Re: [PATCH v2 2/2] dmaengine: doc: Add sections for per descriptor
 metadata support

On 30-08-18, 15:19, Peter Ujfalusi wrote:
> Update the provider and client documentation with details about the
> metadata support.

It would make sense to make this patch 1 :)

> 
> Signed-off-by: Peter Ujfalusi <peter.ujfalusi@...com>
> ---
>  Documentation/driver-api/dmaengine/client.rst | 70 +++++++++++++++++++
>  .../driver-api/dmaengine/provider.rst         | 46 ++++++++++++
>  2 files changed, 116 insertions(+)
> 
> diff --git a/Documentation/driver-api/dmaengine/client.rst b/Documentation/driver-api/dmaengine/client.rst
> index fbbb2831f29f..584a39347cfe 100644
> --- a/Documentation/driver-api/dmaengine/client.rst
> +++ b/Documentation/driver-api/dmaengine/client.rst
> @@ -151,6 +151,76 @@ 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
> +      4. Read out the metadate from the pointer

Lets put a note here that only one method should be used

> +
>  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..502c59f75302 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

typo architecure

> +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 implment either

implment
-- 
~Vinod