linux-kernel - Re: [PATCH v2 1/4] dt-bindings: rtc: ingenic: Rework compatible strings and add #clock-cells

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Date:   Tue, 1 Nov 2022 12:32:05 -0500
From:   Rob Herring <robh@...nel.org>
To:     Paul Cercueil <paul@...pouillou.net>
Cc:     Alessandro Zummo <a.zummo@...ertech.it>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        linux-rtc@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mips@...r.kernel.org,
        od@...ndingux.net
Subject: Re: [PATCH v2 1/4] dt-bindings: rtc: ingenic: Rework compatible
 strings and add #clock-cells

On Mon, Oct 31, 2022 at 2:41 PM Paul Cercueil <paul@...pouillou.net> wrote:
>
> Hi Rob,
>
> Le lun. 31 oct. 2022 à 14:23:41 -0500, Rob Herring <robh@...nel.org> a
> écrit :
> > On Fri, Oct 28, 2022 at 11:55:16PM +0100, Paul Cercueil wrote:
> >>  The RTC in the JZ4770 is compatible with the JZ4760, but has an
> >> extra
> >>  register that permits to configure the behaviour of the CLK32K pin.
> >> The
> >>  same goes for the RTC in the JZ4780.
> >>
> >>  Therefore, the ingenic,jz4770-rtc and ingenic,jz4780-rtc strings do
> >> not
> >>  fall back anymore to ingenic,jz4760-rtc. The ingenic,jz4780-rtc
> >> string
> >>  now falls back to the ingenic,jz4770-rtc string.
> >
> > This is a compatibility mess. There is no driver support in v6.1-rc
> > for
> > ingenic,jz4770-rtc, so a new DT would not work with existing kernels.
> > It
> > sounds like you need 3 compatibles for 4780.
>
> Do newer DTs need to work with older kernels? I always assumed the
> compatibility was only enforced for the other way around.

For a stable platform, yes. Would you want a firmware update carrying
the DT to break an existing OS install?

Compatibility either way ultimately is up to the platform whether you
care or not. I just ask that the commit msg spell that out. In this
case, it is easily avoided.

Rob