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: <7DF102BC-C818-4D27-988F-150C7527E6CC@goldelico.com>
Date:   Fri, 20 Sep 2019 11:19:11 +0200
From:   "H. Nikolaus Schaller" <hns@...delico.com>
To:     Linus Walleij <linus.walleij@...aro.org>
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


> Am 20.09.2019 um 10:55 schrieb Linus Walleij <linus.walleij@...aro.org>:
> 
> 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.

Yes, the basic property is documented. But incomplete.

The strange inversion side-effect on the third gpio parameter
is undocumented and not understandable from this description
alone.

> 
>> 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.

Ok, that looks like a viable path.

BR and thanks,
Nikolaus