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

Vinod Koul
2019-12-20T10:15:07Z

On 20-12-19, 11:52, Peter Ujfalusi wrote:
> Hi Vinod,
>
> On 20/12/2019 10.28, Vinod Koul wrote:
> > 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?
>
> It is not defined as it could be driver dependent, but afaik we have
> defined (which I'm not sure why it is not here or in the code) that in
> DESC_METADATA_ENGINE case the metadata pointer is valid for the client
> between the time it got the desc (via prep call) and the execution of
> the completion callback.
> Iow, DESC_METADATA_ENGINE does not make any sense if the client want to
> receive metadata back and does not provide a callback.

Make sense and once callback completes driver can free it up!
>
> I will extend the documentation and comment in the code to reflect this.

makes sense, thanks!

>
> >> + 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?
>
> It is up to the client driver what it does with the provided buffer. As
> for what the DMA driver does is not documented as it is not relevant and
> can be different by different HW or SW implementation.

yeah lets document that and the fact the dmaengine driver cant touch it
after the callback
--
~Vinod