linux-kernel - Re: [PATCH 2/2] dmaengine: xdmac: Add scatter gathered memset support

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Date:	Fri, 17 Jul 2015 08:39:26 +0530
From:	Vinod Koul <vinod.koul@...el.com>
To:	Maxime Ripard <maxime.ripard@...e-electrons.com>
Cc:	Nicolas Ferre <nicolas.ferre@...el.com>,
	Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
	Ludovic Desroches <ludovic.desroches@...el.com>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Boris Brezillon <boris.brezillon@...e-electrons.com>,
	dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 2/2] dmaengine: xdmac: Add scatter gathered memset support

On Thu, Jul 16, 2015 at 04:11:05PM +0200, Maxime Ripard wrote:
> Hi Vinod,
> 
> > > +		/*
> > > +		 * The scatterlist API gives us only the address and
> > > +		 * length of each elements.
> > > +		 *
> > > +		 * Unfortunately, we don't have the stride, which we
> > > +		 * will need to compute.
> > > +		 *
> > > +		 * That make us end up in a situation like this one:
> > > +		 *    len    stride    len    stride    len
> > > +		 * +-------+        +-------+        +-------+
> > > +		 * |  N-2  |        |  N-1  |        |   N   |
> > > +		 * +-------+        +-------+        +-------+
> > > +		 *
> > > +		 * We need all these three elements (N-2, N-1 and N)
> > > +		 * to actually take the decision on whether we need to
> > > +		 * queue N-1 or reuse N-2.
> > > +		 *
> > > +		 * We will only consider N if it is the last element.
> > > +		 */
> >
> > Why do you need stride?
> > 
> > This is scatterlist so the computation of stride sounds odd here. Ideally
> > you should take the scatterlist and program the lli for controller.
> 
> Because it is sub-optimal if the length and stride are equals from one
> descriptors to another. The XDMAC is able to repeat any given
> descriptor a given number of time (which is one by default), which
> means that if the parameters of the transfer don't change, we simply
> have to increment the number of time the descriptor has to be used,
> instead of creating a new one that the controller will have to fetch.
> 
> In the non-optimal case (ie the length and/or stride change from one
> scatterlist element to another), we simply fallback to a one LLI per
> scatter list element.
Sound fine to me. The optimization is a good one then...

Another question though, do you expect stride to be linear for your cases,
if so have you actually though about using interleaved API, for these cases

-- 
~Vinod


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