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

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Date:	Tue, 21 Jul 2015 14:19:00 +0200
From:	Maxime Ripard <maxime.ripard@...e-electrons.com>
To:	Vinod Koul <vinod.koul@...el.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 Fri, Jul 17, 2015 at 08:39:26AM +0530, Vinod Koul wrote:
> 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

What do you mean by linear? I guess, at least from an API point of
view, we should expect any stride and length. The scatter gather
variants of the dmaengine API were already using scatterlist, and the
interleaved template doesn't really fit our use case here, so I chose
to use a scatterlist here, mostly for consistency.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com

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