linux-kernel - Re: [PATCH v11 2/2] leds: Add driver for Qualcomm LPG

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Date:   Wed, 2 Feb 2022 16:08:29 -0600
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Marijn Suijten <marijn.suijten@...ainline.org>
Cc:     Pavel Machek <pavel@....cz>,
        Thierry Reding <thierry.reding@...il.com>,
        "Uwe Kleine-K?nig" <u.kleine-koenig@...gutronix.de>,
        Lee Jones <lee.jones@...aro.org>,
        Satya Priya Kakitapalli <c_skakit@....qualcomm.com>,
        Luca Weiss <luca@...tu.xyz>, Rob Herring <robh+dt@...nel.org>,
        linux-leds@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-pwm@...r.kernel.org
Subject: Re: [PATCH v11 2/2] leds: Add driver for Qualcomm LPG

On Wed, Feb 2, 2022 at 2:04 PM Bjorn Andersson
<bjorn.andersson@...aro.org> wrote:
>
> On Wed 02 Feb 03:18 PST 2022, Marijn Suijten wrote:
>
> > On 2022-01-28 16:54:29, Bjorn Andersson wrote:
> > > The Light Pulse Generator (LPG) is a PWM-block found in a wide range of
> > > PMICs from Qualcomm. These PMICs typically comes with 1-8 LPG instances,
> > > with their output being routed to various other components, such as
> > > current sinks or GPIOs.
> > >
> > > Each LPG instance can operate on fixed parameters or based on a shared
> > > lookup-table, altering the duty cycle over time. This provides the means
> > > for hardware assisted transitions of LED brightness.
> > >
> > > A typical use case for the fixed parameter mode is to drive a PWM
> > > backlight control signal, the driver therefor allows each LPG instance
> > > to be exposed to the kernel either through the LED framework or the PWM
> > > framework.
> > >
> > > A typical use case for the LED configuration is to drive RGB LEDs in
> > > smartphones etc, for which the driver support multiple channels to be
> > > ganged up to a MULTICOLOR LED. In this configuration the pattern
> > > generators will be synchronized, to allow for multi-color patterns.
> > >
> > > Signed-off-by: Bjorn Andersson <bjorn.andersson@...aro.org>
> >
> > Reviewed-by: Marijn Suijten <marijn.suijten@...ainline.org>
> >
> > There may still be some things to improve based on whether lo_pause
> > works in non-ping-pong mode - to alleviate the requirement for the first
> > delta_t to be at most 512ms - but this patch should not be delayed much
> > longer and that's perhaps for a followup patch.  Same for my request for
> > documentation and examples which at the same time serve as some form of
> > tests to see if everything works as desired.
> >
>
> I've been considering lopause to be the value before we start the
> pattern, but I think you're right in that it denotes how long we should
> hold the first value.
>
> So I think it might make sense in the predefined "<value> <delay> <value>
> <delay>" scheme to use first <delay> as to calculate lo-pause. I think
> it has to be calculated, because the first value will iiuc be held
> for (lopause + 1) * delay ms.
>
> > I also vaguely remember other (downstream) drivers to support more than
> > 512ms per step by (drastically?) changing PWM period, but not sure how
> > that worked again nor if it was reliable.
> >
>
> Thinking about it again, while 512 is the 9th bit, we should be able to
> represent [0..1023] with 9 bits...
>

Sorry, my mind was elsewhere as I wrote that. [0..511] is what we got.

Regards,
Bjorn