Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
IronPort-SDR: PwSSrpM7beoRNGtbZuYVQYarDacF81+BP6yekLAu15x7XInnZ+j5yvbR+RmcJR4SgQGfMy3QxF
 +W1I13/hf7uv7PgjkRcXItnriXG56d2qnPNvuMDwP5TT2rKbRL87qo0YrJWVdCrbqnXg6xRfzf
 doKCClYldLzMX+mQPBcJnZA6Dvz+zQ5Lr1GCrroAPKS8Mx4cGGhMgKw87yxskV3KabPuiYNNeU
 HK/iHvecc8tGVcNFPfxJ45nNmcCZWPMtDaZYSvhFb43LTmeJBpaYKI6cYWI22gusuFIG8S9n+2
 A1k=
IronPort-SDR: zDp6qokfh+UZ5a3ETjcvlv4kekHz7nAQLLDIdpowFcBTrgZGK7bVWsZawcpM6af3jkn2ZirSL+
 xiYF4B0FjTeQEOaqUrbDsjYDlp5zzNTt8x6A2qaU9TbnTj+m+d4F507FtMUS8rYhkK0r0FcaZ2
 3R3juBoV0vLvMpOzGa/ATiyM1uMEndtaz8TpDz+FLFU2/ic8Szsk318uheKdvVFW6bqVK1t0m7
 S++Z/We4I3MD7SRVT8cjBv34z3oZ3hm9NGMIgVKwU7J9voCKjrdHBBdawFMa00QeAu3dx5F4qY
 xn8=
Message-ID: <ce08696b6bf2b3eaa84d6f056f47a8240c7479f4.camel@ew.tq-group.com>
Subject: Re: spi-imx: correct interpretation of num-cs DT property?
From:   Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
To:     Mark Brown <broonie@kernel.org>
Cc:     Rob Herring <robh+dt@kernel.org>, Shawn Guo <shawnguo@kernel.org>,
        Sascha Hauer <s.hauer@pengutronix.de>,
        Fabio Estevam <festevam@gmail.com>,
        NXP Linux Team <linux-imx@nxp.com>,
        linux-spi@vger.kernel.org, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org
Date:   Thu, 03 Sep 2020 15:40:43 +0200
In-Reply-To: <20200903132241.GB4771@sirena.org.uk>
References: <ecfa135b7b83a31bed821ec0740ab3cf1d39da15.camel@ew.tq-group.com>
         <20200903132241.GB4771@sirena.org.uk>
Content-Type: text/plain; charset="UTF-8"
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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