linux-kernel - Re: [BUG] SPI broken for SPI based panel drivers

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Message-ID: <CAGngYiUPaR=_1NKZSjUQRK9+zUw3ztUpro7NV-O=sGAC2eOzUw@mail.gmail.com>
Date:   Fri, 4 Dec 2020 08:46:33 -0500
From:   Sven Van Asbroeck <thesven73@...il.com>
To:     "H. Nikolaus Schaller" <hns@...delico.com>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Lukas Wunner <lukas@...ner.de>,
        Mark Brown <broonie@...nel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Discussions about the Letux Kernel 
        <letux-kernel@...nphoenux.org>,
        Andreas Kemnade <andreas@...nade.info>
Subject: Re: [BUG] SPI broken for SPI based panel drivers

On Fri, Dec 4, 2020 at 5:11 AM H. Nikolaus Schaller <hns@...delico.com> wrote:
>
> Anyways it is debatable if this is a bug at all. It is just a definition.

I respectfully disagree. Prior to the fix, your panel's active-low chip select
needed to be described in the devicetree with 'spi-cs-high'. That sounds
very much like a bug to me.

> Which is not well documented anywhere.

I agree that documentation can be improved here.
Would you like to submit a patch that improves:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/spi/spi-controller.yaml?h=v5.10-rc6#n28
?
This way, we also get Rob Herring involved, which may lead to more elegant
documentation. He is more likely to respond to a patch than to a question.

>
> What I especially wonder is why you fix that at all in drivers/spi/spi.c
> if polarity inversion is handled in gpiolib.

The reason for that is described in the commit message of
766c6b63aa04 ("spi: fix client driver breakages when using GPIO descriptors")