On Mon, Feb 08, 2021 at 12:21:22PM -0600, Bjorn Andersson wrote: > 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@xxxxxxxxxx> 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. Thanks everyone for comments. Let me steer the discussion to original topic - it's about old kernel and new DTB, assuming that mainline kernel bisectability is not affected. Flow looks like this: 0. You have existing bidings and drivers. 1. Patch changing bindings (with new compatible) and drivers gets accepted by maintainer. 2. Patch above (bindings+drivers) goes during merge window to v5.11-rc1. 3. Patch changing in-tree DTS to new compatible gets accepted by maintainer and it is sent as v5.12-rc1 material to SoC maintainers. So again: old kernel, using old bindings, new DTB. Another case is where out-of-tree user of bindings, e.g. FreeBSD, takes new DTS (at point of #3 above or later) but did not take the bindings. Such system would be broken but it's their fault - they took DTS without taking the bindings (which were there already for one release!). Best regards, Krzysztof
