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Date:   Mon, 8 Feb 2021 12:21:22 -0600
From:   Bjorn Andersson <bjorn.andersson@linaro.org>
To:     Geert Uytterhoeven <geert@linux-m68k.org>
Cc:     Arnd Bergmann <arnd@kernel.org>,
        Krzysztof Kozlowski <krzk@kernel.org>,
        Olof Johansson <olof@lixom.net>, Arnd Bergmann <arnd@arndb.de>,
        arm-soc <arm@kernel.org>, SoC Team <soc@kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        "moderated list:ARM/SAMSUNG EXYNOS ARM ARCHITECTURES" 
        <linux-samsung-soc@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Sylwester Nawrocki <snawrocki@kernel.org>,
        DTML <devicetree@vger.kernel.org>,
        Tony Lindgren <tony@atomide.com>,
        Frank Rowand <frowand.list@gmail.com>,
        Rob Herring <robh+dt@kernel.org>,
        Alexandre Belloni <alexandre.belloni@bootlin.com>,
        Gregory Clement <gregory.clement@bootlin.com>,
        Nicolas Ferre <nicolas.ferre@microchip.com>,
        Linus Walleij <linus.walleij@linaro.org>,
        Shawn Guo <shawnguo@kernel.org>,
        Geert Uytterhoeven <geert+renesas@glider.be>,
        Alexandre Torgue <alexandre.torgue@st.com>,
        Kevin Hilman <khilman@baylibre.com>,
        Maxime Ripard <mripard@kernel.org>
Subject: Re: [GIT PULL 2/3] ARM: dts: samsung: DTS for v5.12
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On Sat 06 Feb 13:47 CST 2021, Geert Uytterhoeven wrote:

> Hi Arnd,
> 
> On Sat, Feb 6, 2021 at 3:36 PM Arnd Bergmann <arnd@kernel.org> wrote:
> > That said, I'm still not happy about the patch we discussed in the
> > other email thread[1] and I'd like to handle it a little more strictly in
> > the future, but I agree this wasn't obvious and we have been rather
> > inconsistent about it in the past, with some platform maintainers
> > handling it way more strictly than others.
> >
> > I've added the devicetree maintainers and a few other platform
> > maintainers to Cc here, maybe they can provide some further
> > opinions on the topic so we can come to an approach that
> > works for everyone.
> >
> > My summary of the thread in [1] is there was a driver bug that
> > required a DT binding change. Krzysztof and the other involved
> > parties made sure the driver handles it in a backward-compatible
> > way (an old dtb file will still run into the bug but keep working
> > with new kernels), but decided that they did not need to worry
> > about the opposite case (running an old kernel with an updated
> > dtb). I noticed the compatibility break and said that I would
> > prefer this to be done in a way that is compatible both ways,
> > or at the minimum be alerted about the binding break in the
> > pull request, with an explanation about why this had to be done,
> > even when we don't think anyone is going to be affected.
> >
> > What do others think about this? Should we generally assume
> > that breaking old kernels with new dtbs is acceptable, or should
> > we try to avoid it if possible, the same way we try to avoid
> > breaking new kernels with old dtbs? Should this be a platform
> > specific policy or should we try to handle all platforms the same
> > way?
> 
> For Renesas SoCs, we typically only consider compatibility of new
> kernels with old DTBs, not the other way around.
> However, most DTB updates are due to new hardware support, so using the
> new DTB with an old kernel usually just means no newly documented
> hardware, or new feature, is being used by the old kernel.
> 

This is the case for the Qualcomm tree as well, it's expected that a new
kernel should work with older DT. But, while we don't actively try to
break it, there are plenty of examples where we don't/can't give the
promise in the other direction.

These examples ranges from advancements in power management
(implementation and binding) to DT validation forcing deprecation and
adoption of new bindings.

Regards,
Bjorn