linux-kernel - Re: [linux-sunxi] Re: [PATCH 5/5] RFC spi: sun4i: add DMA support

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Date:	Thu, 02 Jun 2016 07:42:26 +0300
From:	Priit Laes <plaes@...es.org>
To:	maxime.ripard@...e-electrons.com, Mark Brown <broonie@...nel.org>
Cc:	Michal Suchanek <hramrach@...il.com>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	linux-sunxi <linux-sunxi@...glegroups.com>,
	Chen-Yu Tsai <wens@...e.org>,
	linux-spi <linux-spi@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [linux-sunxi] Re: [PATCH 5/5] RFC spi: sun4i: add DMA support

On Wed, 2016-06-01 at 20:00 +0200, Maxime Ripard wrote:
> Hi,
> 
> On Mon, May 30, 2016 at 04:50:16PM +0100, Mark Brown wrote:
> > 
> > On Mon, May 30, 2016 at 05:28:10PM +0200, Michal Suchanek wrote:
> > > 
> > > On 30 May 2016 at 17:03, Mark Brown <broonie@...nel.org> wrote:
> > > 
> > > > 
> > > > I really don't think it's worth caring too much about cases
> > > > where the
> > > > DMA driver hasn't been compiled in, it's not like SPI is the
> > > > only thing
> > > 
> > > It's what the driver did to start with and it was requested to
> > > fall
> > > back to non-DMA in the case DMA is not available.
> > Why?  I really can't see any sensible use case for this that
> > doesn't
> > have a better solution available.
> SPI works just fine without DMA, which might just be considered an
> (optional) optimisation.
> 
> We've been using it without DMA for years now, and it was working
> just
> fine, and it will work even better with the other patches in this
> serie. There's no reason to add a hard dependency on something that
> we
> don't really need.
> 

Actually it non-DMA case works fine if you don't need SPI transfers
larger than SUN4I_FIFO_DEPTH - 1, which is 63 bytes.

This was addressed by this patch, but was never applied:
http://permalink.gmane.org/gmane.linux.kernel.spi.devel/18950