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

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Date:   Thu, 03 Sep 2020 15:40:43 +0200
From:   Matthias Schiffer <matthias.schiffer@...tq-group.com>
To:     Mark Brown <broonie@...nel.org>
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, 2020-09-03 at 14:22 +0100, Mark Brown wrote:
> * PGP Signed by an unknown key
> 
> 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.


Oh, thank you for the clarification.

As currently no in-tree DTs use the num-cs property for spi-imx and
it's not documented, should support for it be dropped from the driver
altogether?


> 
> > 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?
> 
> * Unknown Key
> * 0x5D5487D0