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

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Message-ID: <20160602142649.GJ2282@sirena.org.uk>
Date:	Thu, 2 Jun 2016 15:26:49 +0100
From:	Mark Brown <broonie@...nel.org>
To:	Michal Suchanek <hramrach@...il.com>
Cc:	Priit Laes <plaes@...es.org>,
	Maxime Ripard <maxime.ripard@...e-electrons.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 Thu, Jun 02, 2016 at 02:14:26PM +0200, Michal Suchanek wrote:
> On 2 June 2016 at 06:42, Priit Laes <plaes@...es.org> wrote:
> > On Wed, 2016-06-01 at 20:00 +0200, Maxime Ripard wrote:

> > 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

> And the code added in that patch will never run unless you

> 1) use long spi transfers
> 2) compile in/load SPI without DMA support

> There is no reason for doing 2) since we have do DMA support for sunxi.

Well, presumably such code exists and is being worked on?

> So that's another code path that needs maintenance and testing and
> likely will not get it.

Oh, come on.  You might not want to use it yourself but the chances are
that someone will want to use it just like the situation with all the
other SPI drivers.  It's a perfectly reasonable and sensible feature to
support upstream.

I really do not understand why there is such a strong desire to have
these devices be a special snowflake here, the worst that's likely to
happen here is that you're going to end up having to either remove the
DMA controller from the DT or load the driver for it neither of which
seem like the end of the world.

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