Date:   Sun, 23 Oct 2022 11:29:45 -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 11:01, Daniel Golle wrote:
> On Sun, Oct 23, 2022 at 08:39:34AM -0400, Krzysztof Kozlowski wrote:
>> 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?
> 
> So we were talking about using the MT8183 compatible for MT7986 SoC, as
> the PWM units are identical apart from the number of channels they
> offer:

No, we talk about MT8183 with mt7986-pwm compatible. Read again my message.

> 
> MT7986 got 2 PWM channels. The MMIO registers used for those two
> channels start at offsets 0x10 (pwm0) and 0x50 (pwm1)
> 
> MT8183 got 4 PWM channels. The MMIO registers used for those four
> channels start of offsets 0x10 (pwm0), 0x50 (pwm1), 0x90 (pwm2) and
> 0xd0 (pwm3).
> 
> Hence, when using the MT8183 compatible with MT7986, the driver will
> access offsets 0x90 and 0xd0 in case the users enables the (bogus)
> outputs pwm2 and pwm3. These offsets, however, are not mentioned in the
> datasheet, so it has to be considered that writing things to these
> undocumented offsets could cause unknown behavior.
> 
> I hope it's more clear now what I mean.

But even your case is not correct. On MT7986 the device would still have
2 channels, how the heck he would get 4? Driver binds to ,t7986-pwm
compatible, which defines 2 channels.

> 
>>
>>>
>>> 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?
> 
> My guess is that it's for this very reason:
> To correctly communicate the capabilities (in this case: number of
> channels) to the driver and not have bogus pwmX show up in the OS
> which then causes undocumented MMIO register access in case anyone
> tries to actually use them.

No, that's not correct reason. There would be no wrong MMIO access and
capabilities would be still correctly communicated.

> 
>>
>>>
>>>>
>>>> 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.
> 
> Others will have to answer that for you.
> 
> To me this looks a bit like a structural shortcoming of the PWM controller
> bindings: if there was a way to tell the driver "hey, this is like MT8183,
> but it got only two channels" that would solve it nicely.
> This could either be done using child-nodes for each PWM channel or by
> simply adding a 'nr-pwms' property.

No, it's rather someone did not think about Devicetree compatibles or
did not care to design the Mediatek bindings and just copy-paste
existing pattern...

Best regards,
Krzysztof

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