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

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Message-ID: <Za5MHkzSjyVUoytI@matsya>
Date: Mon, 22 Jan 2024 16:36:06 +0530
From: Vinod Koul <vkoul@...nel.org>
To: Paul Cercueil <paul@...pouillou.net>
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: [Linaro-mm-sig] Re: [PATCH v5 3/8] dmaengine: Add API function
 dmaengine_prep_slave_dma_vec()

Hi Paul,


On 08-01-24, 13:20, Paul Cercueil wrote:
> 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;
> > > +};
> 
> 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.

Yeah for this interleaved seems overkill. Lets go with this api. I would
suggest change the name of the API replacing slave with peripheral
though

-- 
~Vinod