linux-kernel - Re: [PATCH] spi: dt-bindings: clarify CS behavior for spi-cs-high and gpio descriptors

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Message-ID: <20201211131810.GB4929@sirena.org.uk>
Date:   Fri, 11 Dec 2020 13:18:10 +0000
From:   Mark Brown <broonie@...nel.org>
To:     Sven Van Asbroeck <thesven73@...il.com>
Cc:     "H. Nikolaus Schaller" <hns@...delico.com>,
        Rob Herring <robh+dt@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        linux-spi <linux-spi@...r.kernel.org>,
        linux-gpio@...r.kernel.org,
        devicetree <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Discussions about the Letux Kernel 
        <letux-kernel@...nphoenux.org>, kernel@...a-handheld.com,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Lukas Wunner <lukas@...ner.de>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Andreas Kemnade <andreas@...nade.info>
Subject: Re: [PATCH] spi: dt-bindings: clarify CS behavior for spi-cs-high
 and gpio descriptors

On Wed, Dec 09, 2020 at 12:36:40PM -0500, Sven Van Asbroeck wrote:
> On Wed, Dec 9, 2020 at 4:57 AM H. Nikolaus Schaller <hns@...delico.com> wrote:

> > +      device node     | cs-gpio       | CS pin state active | Note
> > +      ================+===============+=====================+=====
> > +      spi-cs-high     | -             | H                   |
> > +      -               | -             | L                   |
> > +      spi-cs-high     | ACTIVE_HIGH   | H                   |
> > +      -               | ACTIVE_HIGH   | L                   | 1
> > +      spi-cs-high     | ACTIVE_LOW    | H                   | 2
> > +      -               | ACTIVE_LOW    | L                   |
> > +

> Doesn't this table simply say:
> - specify   'spi-cs-high' for an active-high chip select
> - leave out 'spi-cs-high' for an active-low  chip select
> - the gpio active high/active low consumer flags are ignored
> ?

It seems to, yes.

> If so, then I would simply document it that way.
> Simple is beautiful.

Yeah, it'd definitely be easier to read and clearer what people should
actually do.  As Linus said it'd also be a good idea to explicitly say
that this is not great design or particularly intentional since it could
be pretty confusing for someone trying to understand why the bindings
are the way they are.

I'm going to apply this anyway to make sure we get this documentated but
some incremental improvements along these lines would be good.

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