Hi Arnd,

On Sat, Feb 6, 2021 at 3:36 PM Arnd Bergmann [off-list ref] 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.

In case there was a real issue fixed, and using the new DTB with the old
kernel would cause a regression, and we're aware of it, we do make sure
the DTS update is postponed until the corresponding driver update has
hit upstream.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel