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

[Daniel Thompson <daniel.thompson@linaro.org>]

On Tue, Oct 05, 2021 at 05:23:26PM +0200, Marijn Suijten wrote:
> On 2021-10-05 15:03:49, Daniel Thompson wrote:
> [..]
> > > At that point one might ask why qcom,num_strings remains at all when
> > > DT can use qcom,enabled_strings instead.  We will supposedly have to
> > > keep backwards compatibility with DTs in mind so none of this can be
> > > removed or made mutually exclusive from a driver standpoint, that all
> > > has to be done in dt-bindings yaml to be enforced on checked-in DTs.
> > 
> > So... perhaps I made a make offering a Reviewed-by: to a patch
> > that allows len(enabled-strings) to have precedence. If anything
> > currently uses enabled-strings then it *will* be 4 cells long and
> > is relying on num-strings to ensure the right things happens ;-) .
> 
> Unfortunately Konrad (one of my team members) landed such a patch at the
> beginning of this year because I failed to submit this patchset in time
> while it has been sitting in my queue since 2019 after being used in a
> downstream project.  This is in pmi8994 which doesn't have anything
> widely used / production ready yet, so I'd prefer to fix the DT instead
> and remove / fix his comment:
> 
>     /* Yes, all four strings *have to* be defined or things won't work. */
> 
> But this is mostly because, prior to this patchset, no default was set
> for WLED4 so the 0'th string would get enabled num-strings (3 in
> pmi8994's case) times.
> 
> Aside that there's only one more PMIC (also being worked on by
> SoMainline) that sets qcom,enabled-strings: this is pm660l, pulled from
> our local tree, and it actually has enabled-strings of length 2 which is
> broken in its current form, exactly because of relying on this patchset.
> 
> Finally, we already discussed this inside SoMainline and the
> number/enabled leds should most likely be moved out of the PMIC dtsi's
> as they're probably panel, hence board or even device dependent.
> 
> > We'd like that case to keep working so we must allow num-strings to have
> > precedence. In other words, when you add the new code, please put it at
> > the end of the function!
> 
> 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.


Daniel.