linux-kernel - Re: [PATCH 1/5] spi: do not fail if the CS line is not connected

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Message-ID: <20160627130657.GV28202@sirena.org.uk>
Date:	Mon, 27 Jun 2016 14:06:57 +0100
From:	Mark Brown <broonie@...nel.org>
To:	Andi Shyti <andi.shyti@...sung.com>
Cc:	Andi Shyti <andi@...zian.org>, Kukjin Kim <kgene@...nel.org>,
	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	linux-spi@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/5] spi: do not fail if the CS line is not connected

On Mon, Jun 27, 2016 at 07:57:16PM +0900, Andi Shyti wrote:

> What I meant is that if we do not like num-cs = <0>, the
> unlinked CS line can be handled only this way (case of the
> s3c64xx driver):

> +- broken-cs: the CS line is disconnected, therefore the device should not wait
> +  for the CS protocol to be established

So what you're saying here is that you just need a property for the
inability to read back the chip select status?  That seems like a
totally reasonable thing to have which fits in idiomatically with the
rest of the subsystem.  I might call it no-cs-readback or something.

> Which is not something I like, because it means adding a new
> flag in the dts.

> What I want to suggest, instead, is to slightly change the logic
> behind the num-cs property: i.e. if "num-cs = <0>", doesn't
> necessarily mean that we don't have a CS controller, but, while
> we can have as many as we wish, non of them is connected.

I disagree, I think from a system integration point of view this is just
a chip select which can't be changed and it's less likely that we will
run into nasty surprises later on with things assuming that chip selects
exist.  AFAICT you only need this property in your case because this
controller has some features that rely on readback of the chip select
status, that's not very common - normally it'd be write only.  I'd
expect most controllers would just say they have one chip select and
that'd be that.

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