linux-kernel - Re: [PATCH v5 3/8] dmaengine: Add API function dmaengine_prep_slave_dma

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Message-ID: <fdfa480e2e702e4e0e96269ac579f2ee750f1fc1.camel@crapouillou.net>
Date: Thu, 21 Dec 2023 16:29:27 +0100
From: Paul Cercueil <paul@...pouillou.net>
To: Vinod Koul <vkoul@...nel.org>
Cc: Jonathan Cameron <jic23@...nel.org>, Lars-Peter Clausen
 <lars@...afoo.de>,  Sumit Semwal <sumit.semwal@...aro.org>, Christian
 König <christian.koenig@....com>,  Jonathan Corbet
 <corbet@....net>, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
 dmaengine@...r.kernel.org, linux-iio@...r.kernel.org, 
 linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org, 
 linaro-mm-sig@...ts.linaro.org, Nuno Sá
 <noname.nuno@...il.com>, Michael Hennerich <Michael.Hennerich@...log.com>
Subject: Re: [PATCH v5 3/8] dmaengine: Add API function
 dmaengine_prep_slave_dma_vec()

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

Well I think I answer that in the commit message, don't I?

> 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...

So if I want to transfer 16 bytes from 0x10, then 16 bytes from 0x0,
then 16 bytes from 0x20, how should I configure the
dma_interleaved_template?

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
>