linux-kernel - Re: [GIT PULL 2/3] ARM: dts: samsung: DTS for v5.12

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Date:   Tue, 9 Feb 2021 13:19:29 -0600
From:   Frank Rowand <frowand.list@...il.com>
To:     Arnd Bergmann <arnd@...nel.org>,
        Krzysztof Kozlowski <krzk@...nel.org>
Cc:     Olof Johansson <olof@...om.net>, Arnd Bergmann <arnd@...db.de>,
        arm-soc <arm@...nel.org>, SoC Team <soc@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "moderated list:ARM/SAMSUNG EXYNOS ARM ARCHITECTURES" 
        <linux-samsung-soc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Sylwester Nawrocki <snawrocki@...nel.org>,
        DTML <devicetree@...r.kernel.org>,
        Tony Lindgren <tony@...mide.com>,
        Rob Herring <robh+dt@...nel.org>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Gregory Clement <gregory.clement@...tlin.com>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Alexandre Torgue <alexandre.torgue@...com>,
        Kevin Hilman <khilman@...libre.com>,
        Maxime Ripard <mripard@...nel.org>,
        Frank Rowand <frowand.list@...il.com>
Subject: Re: [GIT PULL 2/3] ARM: dts: samsung: DTS for v5.12

On 2/6/21 8:35 AM, Arnd Bergmann wrote:
> On Sat, Feb 6, 2021 at 2:45 PM Krzysztof Kozlowski <krzk@...nel.org> wrote:
>> On Mon, Jan 25, 2021 at 08:12:39PM +0100, Krzysztof Kozlowski wrote:
>>>
>>> ----------------------------------------------------------------
>>> Samsung DTS ARM changes for v5.12
>>>
>>> 1. Use new compatile to properly configure Exynos5420 USB2 PHY, fixing
>>>    it suspend/resume cycle.
>>> 2. Correct Samsung PMIC interrupt trigger levels on multiple boards.
>>> 3. Correct the voltages of Samsung GT-I9100 charger and add top-off
>>>    charger.
>>>
>>
>> Hi everyone,
>>
>> Any progress or new comments about this pull request?
> 
> Hi Krzysztof,
> 
> Sorry for not getting back to you on this earlier. I discussed this with
> Olof the other day and we decided to merge this, I just haven't
> gone through the pull requests over the past few days. My plan is
> to do the next round on Monday.
> 
> 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?

The current policy (since before 2013) is that newer kernels,
implementing new bindings, do not break with old existing dtbs.

Old existing kernels are not required to work with new dtbs.

See Documentation/devicetree/bindings/ABI.rst

We can choose to change the rules, so I'm not saying that the
discussion should not occur.  I'm just pointing out the
current policy.

I think that ABI.rst does not state "Old existing kernels are
not required to work with new dtbs" clearly enough, and
should be updated to do so.

I also think it would be good to explicitly say that care
should be taken with new bindings to not break existing
kernels, if reasonably possible.

-Frank

> 
>           Arnd
> 
> [1] https://lore.kernel.org/lkml/20210130143949.aamac2724esupt7v@kozik-lap/
>