linux-kernel - Re: [LINUX RFC v2 1/4] spi: add support of two chip selects & data stripe

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Message-ID: <20150904153718.GO5313@sirena.org.uk>
Date:	Fri, 4 Sep 2015 16:37:18 +0100
From:	Mark Brown <broonie@...nel.org>
To:	Martin Sperl <martin@...rl.org>
Cc:	Ranjit Waghmode <ranjit.waghmode@...inx.com>, dwmw2@...radead.org,
	computersforpeace@...il.com, michal.simek@...inx.com,
	soren.brinkmann@...inx.com, zajec5@...il.com, ben@...adent.org.uk,
	marex@...x.de, b32955@...escale.com, knut.wohlrab@...bosch.com,
	juhosg@...nwrt.org, beanhuo@...ron.com,
	linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-spi@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	harinik@...inx.com, punnaia@...inx.com
Subject: Re: [LINUX RFC v2 1/4] spi: add support of two chip selects & data
 stripe

On Fri, Sep 04, 2015 at 02:35:52PM +0200, Martin Sperl wrote:
> > On 03.09.2015, at 14:12, Mark Brown <broonie@...nel.org> wrote:

> > This isn't saying that the controller supports more than one chip, it's
> > saying that the controller supports asserting more than one chip select
> > at once which isn't the same thing.  I'm also not entirely sure that
> > this makes sense as a separate feature to the data striping one - I'm
> > struggling to think of a way to use this sensibly separately to that.

> Well - there is one use-case that I can think of:
> fbtft has the requirement for some devices to control a GPIO to
> differentiate between command and data getting transferred
> - sort of 9 bit.

That's another thing again, isn't it?  It's one device switching between
two different control interfaces at runtime rather than two devices
controlled in lockstep.

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