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

[Marijn Suijten]

On 2021-10-05 15:03:49, Daniel Thompson wrote:
[..]
> > I much prefer doing that instead of trying to wrangle enumeration
> > parsing around integer values that are supposed to be used as-is.  After
> > all this variable is already named to set the `+ 1` override currently,
> > and `qcom,enabled_strings` has "custom" handling as well.  I'll extend
> > the validation to ensure num_strings>=1 too.
> 
> Great.
> 
> 
> > In addition, and this needs some investigation on the dt-bindings side
> > too, it might be beneficial to make both properties mutually exclusive.
> > When specifying qcom,enabled_strings it makes little sense to also
> > provide qcom,num_strings and we want the former to take precedence.
> 
> If we are designing a "fix" for that then my view is that if both are
> passed then num-strings should take precedence because it is an
> explicit statement about the number of strings where enabled_strings
> is implicit. In other words, if num-strings <= len(enabled_strings) then
> we should do what we are told, otherwise report error.

IMO both should be identical (num-strings == len(enabled-strings)) to
avoid ambiguity, but do read on.

> > 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?

PS. In -next pmi8994_wled is only enabled for sony-xperia-tone, and
pm660l_wled has yet to be enabled by anything.

- Marijn