Re: [PATCH v9 1/6] dmaengine: Add API function dmaengine_prep_peripheral_dma_vec()

[Paul Cercueil <paul@crapouillou.net>]

Hi Vinod,

Le jeudi 28 mars 2024 à 11:53 +0530, Vinod Koul a écrit :
> On 10-03-24, 13:48, 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>
> > Signed-off-by: Nuno Sa <nuno.sa@analog.com>
> > 
> > ---
> > 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?
> > 
> > v7:
> >   - Renamed *device_prep_slave_dma_vec() ->
> > device_prep_peripheral_dma_vec();
> >   - Added a new flag parameter to the function as agreed between
> > Paul
> >     and Vinod. I renamed the first parameter to prep_flags as it's
> > supposed to
> >     be used (I think) with enum dma_ctrl_flags. I'm not really sure
> > how that API
> >     can grow but I was thinking in just having a bool cyclic
> > parameter (as the
> >     first intention of the flags is to support cyclic transfers)
> > but ended up
> >     "respecting" the previously agreed approach.
> > ---
> >  include/linux/dmaengine.h | 27 +++++++++++++++++++++++++++
> >  1 file changed, 27 insertions(+)
> > 
> > diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
> > index 752dbde4cec1..856df8cd9a4e 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;
> > +};
> > +
> >  /**
> >   * 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_peripheral_dma_vec)(
> > +		struct dma_chan *chan, const struct dma_vec *vecs,
> > +		size_t nents, enum dma_transfer_direction
> > direction,
> > +		unsigned long prep_flags, 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,
> > @@ -973,6 +987,19 @@ static inline struct dma_async_tx_descriptor
> > *dmaengine_prep_slave_single(
> >  						  dir, flags,
> > NULL);
> >  }
> >  
> > +static inline struct dma_async_tx_descriptor
> > *dmaengine_prep_peripheral_dma_vec(
> > +	struct dma_chan *chan, const struct dma_vec *vecs, size_t
> > nents,
> > +	enum dma_transfer_direction dir, unsigned long prep_flags,
> > +	unsigned long flags)
> > +{
> > +	if (!chan || !chan->device || !chan->device-
> > >device_prep_peripheral_dma_vec)
> > +		return NULL;
> > +
> > +	return chan->device->device_prep_peripheral_dma_vec(chan,
> > vecs, nents,
> > +							    dir,
> > prep_flags,
> > +							   
> > flags);
> > +}
> 
> API looks good to me, thanks
> Few nits though:
> - Can we add kernel-doc for this new API please
> - Also update the documentation adding this new api
> - Lastly, we seem to have two flags, I know you have added a comment
> but
>   I dont seem to recall the discussion (looked at old threads for
> clue
>   as well), can you please remind me why we need both? And in your
> case,
>   what is the intended usage of these flags, i would prefer single
>   clean one...
> 

The "prep_flags" is a mask of "enum dma_ctrl_flags".

The second "flags" was supposed to be specific to this function, and
was to future-proof the API as we eventually want to have a "cyclic"
flag, which would emulate a cyclic transfer by linking the SG hardware
descriptors accordingly.

However - I think we can already do that with DMA_PREP_REPEAT and
DMA_PREP_LOAD_EOT, right? So we can probably drop the second "flags".

Cheers,
-Paul