linux-kernel - Re: [PATCH v14 2/2] leds: Add driver for Qualcomm LPG

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Date:   Wed, 4 May 2022 07:51:46 -0700
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Pavel Machek <pavel@....cz>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Uwe Kleine-K?nig <u.kleine-koenig@...gutronix.de>,
        Lee Jones <lee.jones@...aro.org>, Luca Weiss <luca@...tu.xyz>,
        Doug Anderson <dianders@...omium.org>,
        Rob Herring <robh+dt@...nel.org>,
        Jonathan Corbet <corbet@....net>, linux-leds@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-pwm@...r.kernel.org,
        linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH v14 2/2] leds: Add driver for Qualcomm LPG

On Wed 04 May 00:30 PDT 2022, Pavel Machek wrote:

> Hi!
> 
> > +/sys/class/leds/<led>/hw_pattern
> > +--------------------------------
> > +
> > +Specify a hardware pattern for a Qualcomm LPG LED.
> > +
> > +The pattern is a series of brightness and hold-time pairs, with the hold-time
> > +expressed in milliseconds. The hold time is a property of the pattern and must
> > +therefor be identical for each element in the pattern (except for the pauses
> > +described below).
> 
> therefore?
> 

Yes

> > +Simple pattern::
> > +
> > +    "255 500 0 500"
> > +
> > +        ^
> > +        |
> > +    255 +----+    +----+
> > +        |    |    |    |      ...
> > +      0 |    +----+    +----
> > +        +---------------------->
> > +        0    5   10   15     time (100ms)
> > +
> > +The LPG supports specifying a longer hold-time for the first and last element
> > +in the pattern, the so called "low pause" and "high pause".
> 
> Please see
> Documentation/devicetree/bindings/leds/leds-trigger-pattern.txt . This
> should really be compatible.
> 

Unfortunately the LPG hardware only supports fixed duration (except for
the ability to hold/extend the first and last duration in the pattern)
and it also does not support gradual transition between the brightness
levels.

As such the pattern sequence provided to hw_pattern looks to be the
smae, but I don't see that it can be made compatible.

> Can I get either patch to disable pattern infrastructure for now or to
> get it compatible?
> 

I'd be happy to get this updated to your liking, but this was one of the
drivers we discussed when we introduced the pattern trigger and led to
the conclusion that we need the ability to do hw-specific patterns.

As such this document provides the hardware specific documentation, as
we describe under "hw_pattern" in
Documentation/ABI/testing/sysfs-class-led-trigger-pattern.

Please advice on what you would like me to do.

Regards,
Bjorn