[RFC] dmaengine: Add metadat_ops for dma_async_tx_descriptor

[Vinod Koul <vkoul@kernel.org>]

On 30-07-18, 12:46, Peter Ujfalusi wrote:
> Vinod,
> 
> On 2018-07-24 14:14, Vinod wrote:
> >>>> Clients must not mix the two way of handling the metadata.
> >>>> The set_len() is intended to tell the DMA driver the client provided
> >>>> metadata size (in MEM_TO_DEV case mostly).
> >>>>
> >>>> MEM_TO_DEV flow on client side:
> >>>> get_ptr()
> >>>> fill in the metadata to the pointer (not exceeding max_len)
> >>>> set_len() to tell the DMA driver the amount of valid bytes written
> >>>>
> >>>> DEV_TO_MEM flow on client side:
> >>>> In the completion callback, get_ptr()
> >>>> the metadata is payload_len bytes and can be accessed in the return pointer.
> >>>
> >>> I would think to unify this..
> >>
> >> I have tried it, but the attach mode and the pointer mode is hard to
> >> handle with a generic API.
> >> I will try to find a way to unify things in a sane way.
> > 
> > Hmmm, looking from the description they will be for different methods,
> > so lets make them orthogonal and not allow driver to register both.
> 
> I would allow DMA drivers to register both, but somehow enforce that
> clients are not mixing the two distinct way of dealing with the metadata.
> 
> The reason for that is for example the attach mode is the simplest (I
> implemented it first and I have a client using it), but if the pointer
> mode is found to be more efficient and feasible for the DMA then the DMA
> driver can implement that mode and the client can move as well w/o
> breaking anything.

Sounds reasonable...