linux-kernel - Re: [PATCH 2/2] ARM: dts: imx6qdl: tqma6: minor fixes

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Date:   Thu, 27 Aug 2020 09:31:09 +0200
From:   Matthias Schiffer <matthias.schiffer@...tq-group.com>
To:     Fabio Estevam <festevam@...il.com>
Cc:     Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        NXP Linux Team <linux-imx@....com>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] ARM: dts: imx6qdl: tqma6: minor fixes

On Wed, 2020-08-26 at 10:49 -0300, Fabio Estevam wrote:
> On Wed, Aug 26, 2020 at 10:13 AM Matthias Schiffer
> <matthias.schiffer@...tq-group.com> wrote:
> 
> > Using GPIOs for chipselect would require different pinmuxing. Also,
> > why
> > use GPIOs, when the SPI controller has this built in?
> 
> In the initial chips with the ECSPI controller there was a bug with
> the native chipselect controller and we had to use GPIO.

Ah, I see.

Nevertheless, hardware that uses the native chipselects of newer chips
exists (for example our TQ-Systems starterkit mainboards, the DTS of
which we're currently preparing for mainline submission). Shouldn't
num-cs be set for boards (or SoM DTSI) where not all CS pins of the SoC
are usable?

In any case, my original question was about the intended logic for
num_chipselects for SPI drivers. My proposal was:

- 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

How do other SPI controller drivers deal with this?