linux-kernel - Re: [Letux-kernel] [PATCH 2/2] DTS: ARM: gta04: introduce legacy spi-cs-high to make display work again

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Message-ID: <CACRpkdZvpPOM1Ug-=GHf7Z-2VEbJz3Cuo7+0yDFuNm5ShXK8=Q@mail.gmail.com>
Date:   Fri, 20 Sep 2019 10:55:24 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     "H. Nikolaus Schaller" <hns@...delico.com>
Cc:     Discussions about the Letux Kernel <letux-kernel@...nphoenux.org>,
        Rob Herring <robh@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        devicetree <devicetree@...r.kernel.org>,
        Linux-OMAP <linux-omap@...r.kernel.org>,
        Tony Lindgren <tony@...mide.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>,
        linux-spi <linux-spi@...r.kernel.org>,
        Mark Brown <broonie@...nel.org>,
        Benoît Cousson <bcousson@...libre.com>
Subject: Re: [Letux-kernel] [PATCH 2/2] DTS: ARM: gta04: introduce legacy
 spi-cs-high to make display work again

On Tue, Sep 17, 2019 at 4:26 PM H. Nikolaus Schaller <hns@...delico.com> wrote:
> > Am 17.09.2019 um 00:52 schrieb Linus Walleij <linus.walleij@...aro.org>:
> > On Mon, Sep 16, 2019 at 12:59 PM H. Nikolaus Schaller <hns@...delico.com> wrote:
> >
> >> ping.
> >>
> >> Device omap3-gta04 is neither working with v5.3 nor linux-next quite a while and we need a solution.
> >
> > Can't we just apply the last part of the patch in this thread:
> >
> > diff --git a/arch/arm/boot/dts/omap3-gta04.dtsi
> > b/arch/arm/boot/dts/omap3-gta04.dtsi
> > index 9a9a29fe88ec..47bab8e1040e 100644
> > --- a/arch/arm/boot/dts/omap3-gta04.dtsi
> > +++ b/arch/arm/boot/dts/omap3-gta04.dtsi
> > @@ -124,6 +124,7 @@
> >                        spi-max-frequency = <100000>;
> >                        spi-cpol;
> >                        spi-cpha;
> > +                       spi-cs-high;
> >
> >                        backlight= <&backlight>;
> >                        label = "lcd";
> >
> >
> > Surely this fixes the problem?
>
> yes, it is a workaround, but appears to violate some policies.
> E.g. the spi-cs-high; is undocumented but DT bindings maintainer
> seems to be against documenting it as I had proposed in my
> other patch.

It is documented as a boolean in
Documentation/devicetree/bindings/spi/spi-controller.yaml
with the following description:

      spi-cs-high:
        $ref: /schemas/types.yaml#/definitions/flag
        description:
          The device requires the chip select active high.

So I don't think it is about it being undocumented.

> Rather he seems to have proposed a white-list in the driver code.
> So that the legacy mode is only becoming active for those systems
> which really need the legacy mode instead of everyone.

Yeah that seems like a plausible way forward if we want to
move away from the legacy way of specifying polarity.

> Then, we do not need this patch for GTA04.

We don't need to implement the perfect solution up front.
We can aim for that in the long run. I usually go by the IETF
motto "rough consensus and running code".

> So its up to you to decide which way to go. We are happy with
> any one that makes mainline work again asap...

I suggest to go both way:
apply this oneliner and tag for stable so that GTA04 works
again.

Then for the next kernel think about a possible more abitious
whitelist solution and after adding that remove *all* "spi-cs-high"
flags from all device trees in the kernel after fixing them
all up.

Yours,
Linus Walleij