linux-kernel - Re: [PATCH 2/2] dt-bindings: pwm: mediatek: Add compatible string for MT7986

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Message-ID: <eab019f7-f801-848e-80a3-5bb526d95d53@linaro.org>
Date:   Sun, 23 Oct 2022 08:39:34 -0400
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Daniel Golle <daniel@...rotopia.org>
Cc:     Rob Herring <robh@...nel.org>, linux-pwm@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        Thierry Reding <thierry.reding@...il.com>,
        linux-mediatek@...ts.infradead.org,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Uwe Kleine-König <u.kleine-koenig@...gutronix.de>,
        Matthias Brugger <matthias.bgg@...il.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 2/2] dt-bindings: pwm: mediatek: Add compatible string for
 MT7986

On 23/10/2022 08:24, Daniel Golle wrote:
> Hi Krzysztof,
> 
> On Sat, Oct 22, 2022 at 12:35:25PM -0400, Krzysztof Kozlowski wrote:
>> On 21/10/2022 18:58, Daniel Golle wrote:
>>> On Fri, Oct 21, 2022 at 05:23:38PM -0500, Rob Herring wrote:
>>>> On Fri, Oct 21, 2022 at 04:25:18PM +0100, Daniel Golle wrote:
>>>>> Add new compatible string for MT7986 PWM.
>>>>>
>>>>> Signed-off-by: Daniel Golle <daniel@...rotopia.org>
>>>>> ---
>>>>>  Documentation/devicetree/bindings/pwm/pwm-mediatek.txt | 1 +
>>>>>  1 file changed, 1 insertion(+)
>>>>>
>>>>> diff --git a/Documentation/devicetree/bindings/pwm/pwm-mediatek.txt b/Documentation/devicetree/bindings/pwm/pwm-mediatek.txt
>>>>> index 554c96b6d0c3e0..6f4e60c9e18b81 100644
>>>>> --- a/Documentation/devicetree/bindings/pwm/pwm-mediatek.txt
>>>>> +++ b/Documentation/devicetree/bindings/pwm/pwm-mediatek.txt
>>>>> @@ -8,6 +8,7 @@ Required properties:
>>>>>     - "mediatek,mt7623-pwm": found on mt7623 SoC.
>>>>>     - "mediatek,mt7628-pwm": found on mt7628 SoC.
>>>>>     - "mediatek,mt7629-pwm": found on mt7629 SoC.
>>>>> +   - "mediatek,mt7986-pwm": found on mt7986 SoC.
>>>>
>>>> This version of the PWM h/w is not compatible with any of the existing 
>>>> chips? If it is, it should have a fallback compatible.
>>>
>>> No, it is unique because it comes with just 2 PWM channels.
>>> Otherwise the driver behaves just like for MT8183 (4 channels) or
>>> MT8365 (3 channels) which also got distinct compatible strings.
>>
>> Then something would be here compatible. E.g. If you bound MT8183 with
>> mt7986-pwm compatible, would you get working device with two channels?
> 
> Yes, but I'd see another 2 channels which do not work, accessing them
> may even cause problems (I haven't tried that) as it means accessing
> an undocumented memory range of the SoC which we in general we
> shouldn't be messing around with.

Why on MT8183 there would be undocumented memory? Where is undocumented
memory?

> 
> Also note that this case is the same as MT8183 vs. MT8365, they got
> distinct compatible strings and also for those two the only difference
> is the number of channels.

So why they are not made compatible?

> 
>>
>> If so, they are compatible.
> 
> By that definition you should remove the additional compatible for
> MT8365 or rather, it should have been rejected for the same argument.
> 
> I'm talking about
> commit fe00faee8060402a3d85aed95775e16838a6dad2
> commit 394b517585da9fbb2eea2f2103ff47d37321e976

This is a pattern spreading in several Mediatek bindings and we already
commented on new patches. I don't know why people working on Mediatek do
not mark pieces compatible.

Best regards,
Krzysztof