Re: [PATCH v5 3/8] dmaengine: Add API function dmaengine_prep_slave_dma_vec()

[Paul Cercueil <paul@crapouillou.net>]

Hi Vinod,

Le jeudi 21 décembre 2023 à 20:44 +0530, Vinod Koul a écrit :
> On 19-12-23, 18:50, Paul Cercueil wrote:
> > This function can be used to initiate a scatter-gather DMA
> > transfer,
> > where the address and size of each segment is located in one entry
> > of
> > the dma_vec array.
> > 
> > The major difference with dmaengine_prep_slave_sg() is that it
> > supports
> > specifying the lengths of each DMA transfer; as trying to override
> > the
> > length of the transfer with dmaengine_prep_slave_sg() is a very
> > tedious
> > process. The introduction of a new API function is also justified
> > by the
> > fact that scatterlists are on their way out.
> > 
> > Note that dmaengine_prep_interleaved_dma() is not helpful either in
> > that
> > case, as it assumes that the address of each segment will be higher
> > than
> > the one of the previous segment, which we just cannot guarantee in
> > case
> > of a scatter-gather transfer.
> > 
> > Signed-off-by: Paul Cercueil <paul@crapouillou.net>
> > 
> > ---
> > v3: New patch
> > 
> > v5: Replace with function dmaengine_prep_slave_dma_vec(), and
> > struct
> >     'dma_vec'.
> >     Note that at some point we will need to support cyclic
> > transfers
> >     using dmaengine_prep_slave_dma_vec(). Maybe with a new "flags"
> >     parameter to the function?
> > ---
> >  include/linux/dmaengine.h | 25 +++++++++++++++++++++++++
> >  1 file changed, 25 insertions(+)
> > 
> > diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
> > index 3df70d6131c8..ee5931ddb42f 100644
> > --- a/include/linux/dmaengine.h
> > +++ b/include/linux/dmaengine.h
> > @@ -160,6 +160,16 @@ struct dma_interleaved_template {
> >  	struct data_chunk sgl[];
> >  };
> >  
> > +/**
> > + * struct dma_vec - DMA vector
> > + * @addr: Bus address of the start of the vector
> > + * @len: Length in bytes of the DMA vector
> > + */
> > +struct dma_vec {
> > +	dma_addr_t addr;
> > +	size_t len;
> > +};

I don't want to be pushy, but I'd like to know how to solve this now,
otherwise I'll just send the same patches for my v6.

> so you want to transfer multiple buffers, right? why not use
> dmaengine_prep_slave_sg(). If there is reason for not using that one?

The reason is that we want to have the possibility to transfer less
than the total size of the scatterlist, and that's currently very hard
to do - scatterlists were designed to not be tampered with.

Christian König then suggested to introduce a "dma_vec" which had been
on his TODO list for a while now.

> Furthermore I missed replying to your email earlier on use of
> dmaengine_prep_interleaved_dma(), my apologies.
> That can be made to work for you as well. Please see the notes where
> icg
> can be ignored and it does not need icg value to be set
> 
> Infact, interleaved api can be made to work in most of these cases I
> can
> think of...

Interleaved API only supports incrementing addresses, I see no way to
decrement the address (without using crude hacks e.g. overflowing
size_t). I can't guarantee that my DMABUF's pages are ordered in
memory.

Cheers,
-Paul

> > +
> >  /**
> >   * enum dma_ctrl_flags - DMA flags to augment operation
> > preparation,
> >   *  control completion, and communicate status.
> > @@ -910,6 +920,10 @@ struct dma_device {
> >  	struct dma_async_tx_descriptor
> > *(*device_prep_dma_interrupt)(
> >  		struct dma_chan *chan, unsigned long flags);
> >  
> > +	struct dma_async_tx_descriptor
> > *(*device_prep_slave_dma_vec)(
> > +		struct dma_chan *chan, const struct dma_vec *vecs,
> > +		size_t nents, enum dma_transfer_direction
> > direction,
> > +		unsigned long flags);
> >  	struct dma_async_tx_descriptor *(*device_prep_slave_sg)(
> >  		struct dma_chan *chan, struct scatterlist *sgl,
> >  		unsigned int sg_len, enum dma_transfer_direction
> > direction,
> > @@ -972,6 +986,17 @@ static inline struct dma_async_tx_descriptor
> > *dmaengine_prep_slave_single(
> >  						  dir, flags,
> > NULL);
> >  }
> >  
> > +static inline struct dma_async_tx_descriptor
> > *dmaengine_prep_slave_dma_vec(
> > +	struct dma_chan *chan, const struct dma_vec *vecs, size_t
> > nents,
> > +	enum dma_transfer_direction dir, unsigned long flags)
> > +{
> > +	if (!chan || !chan->device || !chan->device-
> > >device_prep_slave_dma_vec)
> > +		return NULL;
> > +
> > +	return chan->device->device_prep_slave_dma_vec(chan, vecs,
> > nents,
> > +						       dir,
> > flags);
> > +}
> > +
> >  static inline struct dma_async_tx_descriptor
> > *dmaengine_prep_slave_sg(
> >  	struct dma_chan *chan, struct scatterlist
> > *sgl,	unsigned int sg_len,
> >  	enum dma_transfer_direction dir, unsigned long flags)
> > -- 
> > 2.43.0
>