linux-kernel - Re: [PATCH V4 3/8] backlight: qcom-wled: Add new properties for PMI8998

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Date:   Mon, 3 Sep 2018 11:55:10 -0700
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     kgunda@...eaurora.org
Cc:     jingoohan1@...il.com, lee.jones@...aro.org,
        b.zolnierkie@...sung.com, dri-devel@...ts.freedesktop.org,
        daniel.thompson@...aro.org, jacek.anaszewski@...il.com,
        pavel@....cz, robh+dt@...nel.org, mark.rutland@....com,
        linux-leds@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        linux-arm-msm-owner@...r.kernel.org
Subject: Re: [PATCH V4 3/8] backlight: qcom-wled: Add new properties for
 PMI8998

On Wed 15 Aug 22:23 PDT 2018, kgunda@...eaurora.org wrote:

> On 2018-08-07 10:53, Bjorn Andersson wrote:
> > On Mon 09 Jul 03:22 PDT 2018, Kiran Gunda wrote:
> > > diff --git
> > > a/Documentation/devicetree/bindings/leds/backlight/qcom-wled.txt
> > > b/Documentation/devicetree/bindings/leds/backlight/qcom-wled.txt
> > [..]
> > >  - qcom,num-strings
> > >  	Usage:        optional
> > >  	Value type:   <u32>
> > >  	Definition:   #; number of led strings attached;
> > > -		      value from 1 to 3. default: 2
> > > -		      This property is supported only for PM8941.
> > > +		      value: For PM8941 from 1 to 3.
> > > +			     For PMI8998 from 1 to 4.
> > [..]
> > > +- qcom,enabled-strings
> > > +	Usage:        optional
> > > +	Value tyoe:   <u32 array>
> > > +	Definition:   Array of the WLED strings numbered from 0 to 3. Each
> > > +		      string of leds are operated individually. Specify the
> > > +		      list of strings used by the device. Any combination of
> > > +		      led strings can be used.
> > [..]
> > > 
> > >  Example:
> > > 
> > > @@ -99,4 +146,5 @@ pm8941-wled@...0 {
> > >  	qcom,switching-freq = <1600>;
> > >  	qcom,ovp = <29>;
> > >  	qcom,num-strings = <2>;
> > > +	qcom,enabled-strings = <0x00 0x01>;
> > 
> > Nit. I would assume that specifying qcom,num-strings = <2> implies that
> > the first 2 strings are used, so one would not also specify
> > qcom,enabled-strings.
> > 
> Thanks Bjorn for reviewing the series !
> 
> "qcom,enabled-strings" need be specified along with the "qcom,num-strings".
> Because the enabled-strings can be <0, 2> or <0, 3 > also. The driver picks
> the string
> configuration from the enabled-strings array and enable only those
> current-sinks.
> 

The original binding described qcom,num-strings to mean "the first N
strings", requiring qcom,enabled-strings now would break backwards
compatibility with this binding.

In the case that qcom,enabled-strings is specified we can easily derive
num-strings from the listed entires. So I would suggest that you look
for enabled-strings and if not found fall back to checking for
num-strings.

Regards,
Bjorn