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

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Message-ID: <20160602081824.GH4908@lukather>
Date:	Thu, 2 Jun 2016 10:18:24 +0200
From:	Maxime Ripard <maxime.ripard@...e-electrons.com>
To:	Michal Suchanek <hramrach@...il.com>
Cc:	Mark Brown <broonie@...nel.org>,
	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: [PATCH 5/5] RFC spi: sun4i: add DMA support

On Tue, May 31, 2016 at 04:19:28PM +0200, Michal Suchanek wrote:
> On 31 May 2016 at 15:27, Mark Brown <broonie@...nel.org> wrote:
> > On Tue, May 31, 2016 at 12:44:54PM +0200, Michal Suchanek wrote:
> >> On 30 May 2016 at 17:50, Mark Brown <broonie@...nel.org> wrote:
> >> > On Mon, May 30, 2016 at 05:28:10PM +0200, Michal Suchanek wrote:
> >
> >> >> 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.
> >
> >> Of course, the solution is to compile in the DMA driver.
> >
> >> It's been argued that some drivers which use only short transfers will
> >> just work.
> >
> > With nothing else in the system that needs DMA?  It's making the
> > performance of the system less reliable for the benefit of a very narrow
> > use case.
> 
> Some of the platform devices have dedicated DMA *controller* built
> into the device IP so the DMA engine really is optional on many sunxi
> devices. Besides SPI you definitely need the DMA engine for audio. You
> probably don't need it for storage and graphics. I don't have any idea
> if it's used for USB and Ethernet.

USB and Ethernet have their own dedicated DMA engines. So currently,
the only driver that requires it is the audio codec.

Maxime

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

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