linux-kernel - Re: [PATCH v7 03/12] dmaengine: doc: Add sections for per descriptor metadata support

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Message-ID: <20191220082810.GJ2536@vkoul-mobl>
Date:   Fri, 20 Dec 2019 13:58:10 +0530
From:   Vinod Koul <vkoul@...nel.org>
To:     Peter Ujfalusi <peter.ujfalusi@...com>
Cc:     robh+dt@...nel.org, nm@...com, ssantosh@...nel.org,
        dan.j.williams@...el.com, dmaengine@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, grygorii.strashko@...com,
        lokeshvutla@...com, t-kristo@...com, tony@...mide.com,
        j-keerthy@...com, vigneshr@...com
Subject: Re: [PATCH v7 03/12] dmaengine: doc: Add sections for per descriptor
 metadata support

Hi Peter,

On 09-12-19, 11:43, Peter Ujfalusi wrote:

> +  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

and when would it be freed?

> +    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

This is simpler, txn finished the metadata would be freed up right?
> +    - 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

and when and how would driver free that up :)

> +
> +  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::
> +
> +     Mixed use of DESC_METADATA_CLIENT / DESC_METADATA_ENGINE is not allowed,
> +     client drivers must use either of the modes per descriptor.

We should check that if not done already!
-- 
~Vinod