linux-kernel - Re: [PATCH 05/10] backlight: qcom-wled: Fix off-by-one maximum with default num

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Date:   Wed, 6 Oct 2021 15:44:44 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Marijn Suijten <marijn.suijten@...ainline.org>
Cc:     phone-devel@...r.kernel.org, Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Lee Jones <lee.jones@...aro.org>,
        Jingoo Han <jingoohan1@...il.com>,
        ~postmarketos/upstreaming@...ts.sr.ht,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...ainline.org>,
        Konrad Dybcio <konrad.dybcio@...ainline.org>,
        Martin Botka <martin.botka@...ainline.org>,
        Jami Kettunen <jami.kettunen@...ainline.org>,
        Pavel Dubrova <pashadubrova@...il.com>,
        Kiran Gunda <kgunda@...eaurora.org>,
        Courtney Cavin <courtney.cavin@...ymobile.com>,
        Bryan Wu <cooloney@...il.com>, linux-arm-msm@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 05/10] backlight: qcom-wled: Fix off-by-one maximum with
 default num_strings

On Tue, Oct 05, 2021 at 07:34:00PM +0200, Marijn Suijten wrote:
> On 2021-10-05 17:24:53, Daniel Thompson wrote:
> > On Tue, Oct 05, 2021 at 05:23:26PM +0200, Marijn Suijten wrote:
> > > Since there don't seem to be any substantial platforms/PMICs using this
> > > functionality in a working manner, can I talk you into agreeing with
> > > fixing the DT instead?
> > 
> > I've no objections to seeing the DT updated. However I don't really see
> > what benefit we get from breaking existing DTs in order to do so.
> > 
> > "Cleaning up annoying legacy" is seldom a good reason to break existing
> > DTs since, if we could break DTs whenever we choose, there would never
> > be any annoying legacy to worry about. When conflicting properties
> > result in uninterpretable DTs then a break may be justified but that is
> > not the case here.
> 
> As mentioned in my message and repeated by Konrad, the only "existing
> DT" that could possibly be broken is a platform that's brought up by us
> (SoMainline) and we're more than happy to improve the driver and leave
> legacy DT behind us, unless there's more DT in circulation that hasn't
> landed in Linux mainline but should be taken into account?

Devicetrees are supposed to be the domain of firmware (e.g. not part of
the kernel).

I'm therefore reluctant to adopt an "it only exists if it is upstream"
approach for documented DT bindings. Doubly so when it is our bugs that
causes DTs to be written in a manner which we then retrospectively
declare to be wrong.


> Anyway the plan is to leave qcom,num-strings in place so that the
> default enabled_strings list in this driver actually serves a purpose.
> Then, if num-strings and enabled-strings is provided the former has
> precedence (assuming it doesn't exceed the size of the latter) but
> we'll print a warning about this (now unnecessary) ambiguity, and if
> possible at all - haven't found an example yet - make the properties
> mutually exclusive in dt-bindings.
> 
> Disallowing both cases would only simplify the code in the end but we
> can spend a few lines to support the desired legacy.

Yes, warning is OK for me.


Daniel.