linux-kernel - Re: [PATCH RESEND v2] dt-bindings: pwm: mediatek: Add compatible for MT7986

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Message-ID: <Y3/NQBzU/R6XubBf@makrotopia.org>
Date:   Thu, 24 Nov 2022 20:00:00 +0000
From:   Daniel Golle <daniel@...rotopia.org>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc:     linux-mediatek@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org, linux-pwm@...r.kernel.or,
        Matthias Brugger <matthias.bgg@...il.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Thierry Reding <thierry.reding@...il.com>,
        Fabien Parent <fparent@...libre.com>,
        Zhi Mao <zhi.mao@...iatek.com>,
        Sam Shih <sam.shih@...iatek.com>
Subject: Re: [PATCH RESEND v2] dt-bindings: pwm: mediatek: Add compatible for
 MT7986

Hi Krzysztof,

On Thu, Nov 24, 2022 at 02:33:35PM +0100, Krzysztof Kozlowski wrote:
> On 24/11/2022 13:11, Daniel Golle wrote:
> > On Thu, Nov 24, 2022 at 12:30:44PM +0100, Krzysztof Kozlowski wrote:
> >> On 24/11/2022 12:03, Daniel Golle wrote:
> >>> Add new compatible string for MT7986 PWM and list compatible units for
> >>> existing entries. Also make sure the number of pwm1-X clocks is listed
> >>> for all supported units.
> >>>
> >>> Signed-off-by: Daniel Golle <daniel@...rotopia.org>
> >>> ---
> >>> Changes since v1: list compatibles, fix pwm1-n clocks for all SoCs
> >>>
> >>> Rebased on linux-next and re-run scripts/get_maintainers.pl on patch to
> >>> makes sure dt maintainers are included. This has been requested by
> >>> Krzysztof Kozlowski.
> >>>
> >>>  .../devicetree/bindings/pwm/pwm-mediatek.txt  | 20 +++++++++++--------
> >>>  1 file changed, 12 insertions(+), 8 deletions(-)
> >>>
> >>> diff --git a/Documentation/devicetree/bindings/pwm/pwm-mediatek.txt b/Documentation/devicetree/bindings/pwm/pwm-mediatek.txt
> >>> index 554c96b6d0c3..952a338e06e7 100644
> >>> --- a/Documentation/devicetree/bindings/pwm/pwm-mediatek.txt
> >>> +++ b/Documentation/devicetree/bindings/pwm/pwm-mediatek.txt
> >>> @@ -2,14 +2,15 @@ MediaTek PWM controller
> >>>  
> >>>  Required properties:
> >>>   - compatible: should be "mediatek,<name>-pwm":
> >>> -   - "mediatek,mt2712-pwm": found on mt2712 SoC.
> >>> +   - "mediatek,mt2712-pwm", "mediatek,mt6795-pwm": found on mt2712 SoC.
> >>>     - "mediatek,mt6795-pwm": found on mt6795 SoC.
> >>> -   - "mediatek,mt7622-pwm": found on mt7622 SoC.
> >>> -   - "mediatek,mt7623-pwm": found on mt7623 SoC.
> >>> +   - "mediatek,mt7622-pwm", "mediatek,mt8195-pwm", "mediatek,mt8183-pwm", "mediatek,mt7986-pwm": found on mt7622 SoC.
> >>
> >> This does not look right. What you are saying is mt7622 is compatible
> >> with mt8195, which is compatible with mt8183, which is compatible with
> >> mt7986. It could be true, but I feel you wanted to say something else -
> >> mt7622 is compatible with one SoC which is generic and common to all
> >> other implementations.
> > 
> > MT7622 has 6 PWM channels, it does have CK_26M_SEL register and does
> > not need pwm45_fixup. Hence, when using a driver made for MT8195, only
> > 4 out of 6 channels woukd work. MT8183 PWM is identical to MT8195,
> > hence also compatible. When using driver for MT7986, only 2 channels
> > would work, but otherwise it is also compatible.
> > 
> > So unfortunately, that one generic implementation ("common ancestor")
> > does not exist and development of the PWM unit found in MediaTek SoCs
> > did not necessarily increase features in more recent iterations, but
> > rather just reduce or increase the number of PWM channels available.
> > Ironically, the unit with least features (only 2 channels) is found in
> > the most recent SoC (MT7986).
> 
> None of these explain listing four compatibles.

So do I understand correctly that in this case only the newly
introduced "mediatek,mt7986-pwm" should be listed as more generic
compatible after the more specific "mediatek,mt7622-pwm", everything in
between should be dropped? Or only drop "mediatek,mt8195-pwm" here?

I'm asking because in your message from 23/10/2022 you were quoting
Devicetree specification:
> "The property value consists of a concatenated list of null terminated
> strings, from most specific to most general. They allow a device to
> express its compatibility with a family of similar devices, potentially
> allowing a single device driver to match against several devices."

And we have discussed in great length (as I had misunderstood it) that
this should mean that units with the lowest number of channels are to
be considered the "most general" if otherwise identical.


> 
> > 
> >>
> >>> +   - "mediatek,mt7623-pwm", "mediatek,mt7628-pwm": found on mt7623 SoC.
> >>>     - "mediatek,mt7628-pwm": found on mt7628 SoC.
> >>>     - "mediatek,mt7629-pwm": found on mt7629 SoC.
> >>> -   - "mediatek,mt8183-pwm": found on mt8183 SoC.
> >>> -   - "mediatek,mt8195-pwm", "mediatek,mt8183-pwm": found on mt8195 SoC.
> >>> +   - "mediatek,mt7986-pwm": found on mt7986 SoC.
> >>> +   - "mediatek,mt8183-pwm", "mediatek,mt7986-pwm": found on mt8183 SoC.
> >>> +   - "mediatek,mt8195-pwm", "mediatek,mt8183-pwm", "mediatek,mt7986-pwm": found on mt8195 SoC.
> >>
> >> This as well looks excessive.
> > 
> > I agree. But it's difficult to say which one should be ommitted.

So are you suggesting to drop the "mediatek,mt8183-pwm" string here?

Thank you for your patience!


Daniel