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[104.57.184.186]) by smtp.gmail.com with ESMTPSA id t2sm3937004otj.47.2021.02.08.10.21.23 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 08 Feb 2021 10:21:24 -0800 (PST) Date: Mon, 8 Feb 2021 12:21:22 -0600 From: Bjorn Andersson To: Geert Uytterhoeven List-Id: Cc: Arnd Bergmann , Krzysztof Kozlowski , Olof Johansson , Arnd Bergmann , arm-soc , SoC Team , Linux ARM , "moderated list:ARM/SAMSUNG EXYNOS ARM ARCHITECTURES" , "linux-kernel@vger.kernel.org" , Marek Szyprowski , Sylwester Nawrocki , DTML , Tony Lindgren , Frank Rowand , Rob Herring , Alexandre Belloni , Gregory Clement , Nicolas Ferre , Linus Walleij , Shawn Guo , Geert Uytterhoeven , Alexandre Torgue , Kevin Hilman , Maxime Ripard Subject: Re: [GIT PULL 2/3] ARM: dts: samsung: DTS for v5.12 Message-ID: References: <20210125191240.11278-1-krzk@kernel.org> <20210125191240.11278-3-krzk@kernel.org> <20210206134531.l5vpzlmev4v3f3uo@kozik-lap> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: devicetree@vger.kernel.org On Sat 06 Feb 13:47 CST 2021, Geert Uytterhoeven wrote: > Hi Arnd, > > On Sat, Feb 6, 2021 at 3:36 PM Arnd Bergmann 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