linux-kernel - Re: spi-imx: correct interpretation of num-cs DT property?

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Message-ID: <20200903132241.GB4771@sirena.org.uk>
Date:   Thu, 3 Sep 2020 14:22:41 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Matthias Schiffer <matthias.schiffer@...tq-group.com>
Cc:     Rob Herring <robh+dt@...nel.org>, Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        NXP Linux Team <linux-imx@....com>,
        linux-spi@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: spi-imx: correct interpretation of num-cs DT property?

On Thu, Sep 03, 2020 at 11:22:20AM +0200, Matthias Schiffer wrote:

> - If num-cs is set, use that
> - If num-cs is unset, use the number of cs-gpios
> - If num-cs is unset and no cs-gpios are defined, use a driver-provided 
> default (which is 3 for spi-imx; this matches the number of native CS
> pins in older implementations of this SPI controller; i.MX6 and newer
> support up to 4)

That sounds like what's expected, though we coould just skip the first
step.

> Also, would it make sense to add num-cs to all DTS files for boards
> that actually use fewer than 3 CS pins?

No, it was never a good idea to have that property in the first place
and there should be no case where it helps anything.

> At the moment, the num-cs property is not explicitly documented for the
> spi-imx driver, although the driver understands it. I also suggested to
> add this to the docs, which Fabio didn't deem a good idea (I don't
> quite understand the reasoning here - isn't num-cs generally a useful
> property to have?)

Could you explain what benefit you would expect having num-cs to offer?

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