linux-kernel - Re: [PATCH v2 1/4] spi: dt-bindings: Introduce spi-mosi-idle-low flag

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Date:   Wed, 17 May 2023 10:26:07 +0200
From:   Börge Strümpfel <boerge.struempfel@...il.com>
To:     Mark Brown <broonie@...nel.org>
Cc:     bstruempfel@...ratronik.de, andy.shevchenko@...il.com,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...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, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 1/4] spi: dt-bindings: Introduce spi-mosi-idle-low flag

Thank you for your feedback

Am Mo., 15. Mai 2023 um 02:34 Uhr schrieb Mark Brown <broonie@...nel.org>:
>
> On Fri, May 12, 2023 at 08:54:19AM +0200, Börge Strümpfel wrote:
> > Am Fr., 12. Mai 2023 um 05:30 Uhr schrieb Mark Brown <broonie@...nel.org>:
>
> > > If this is always required for a given device (which I'd expect to be
> > > the case) why configure it through DT?  I know we've got some legacy
> > > stuff like that but not all legacy DT choices were good and no need to
> > > continue the pattern.
>
> > Yes this will always be the case for specific spi-device, spi-controller
> > combinations. Just to make sure, that I understand your suggestion
> > correctly: You propose to check from the specific spi-device-driver, if
> > the spi-controller supports this particular mode-bit, and then set it if
> > it does and thereby loose the need for the DT entry completely?
>
> Yes, we shouldn't need DT here.  Though the device should just be
> setting this unconditionally if it's always required.

I agree with you, that we should not need DT here. I will remove the
dt-binding in the next patch version.

However I am not so sure about setting it unconditionally, since this
is dependent on the spi-controller. Not all spi-controller show this
behavior, that they use a high mosi line in idle mode and have the
ability to change this. As far as I know, another common behavior
is that the mosi just keeps the last state which it transmitted. In this
case, devices like Neopixel would still work without this mode bit.