linux-kernel - Re: [PATCH 1/2] spi: sun4i: add DMA support

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Message-ID: <20160317115839.GG2566@sirena.org.uk>
Date:	Thu, 17 Mar 2016 11:58:39 +0000
From:	Mark Brown <broonie@...nel.org>
To:	Michal Suchanek <hramrach@...il.com>
Cc:	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	Priit Laes <plaes@...es.org>, 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 Mailing List <linux-kernel@...r.kernel.org>,
	Emilio López <emilio@...pez.com.ar>
Subject: Re: [PATCH 1/2] spi: sun4i: add DMA support

On Thu, Mar 17, 2016 at 12:54:08PM +0100, Michal Suchanek wrote:

> That's what the driver does. The discussion revolves around the fact
> that the driver does not attempt to work (even for very short
> transfers) when the DMA channels are not configured and just bails
> out. AFAICT the channels are always available when the system is
> properly configured and the dmaengine driver loaded.

The driver should tell the core about this constraint so the core can
split the transfers for it.

> Very few device drivers would work with 63byte transfers only and the
> code for manually driving the CS line in case the DMA engine fails to
> configure will necessarily go untested most of the time since most
> systems will have DMA configured properly.

A lot of devices will be perfectly happy with 63 byte transfers,
register accesses for example tend to be much smaller than that.  The
manual /CS might be an issue but for most SoCs that is easily addressed
by driving the pin as a GPIO if there's an issue.

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