linux-kernel - Re: [PATCH] backlight: pwm_bl: apply the initial backlight state with sane defaults

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Message-ID: <aQ4HY5Hncv1fvxVk@aspen.lan>
Date: Fri, 7 Nov 2025 14:51:15 +0000
From: Daniel Thompson <daniel@...cstar.com>
To: Uwe Kleine-König <ukleinek@...nel.org>
Cc: Daniel Thompson <danielt@...nel.org>, Lee Jones <lee@...nel.org>,
	Michael Grzeschik <m.grzeschik@...gutronix.de>,
	Jingoo Han <jingoohan1@...il.com>, Helge Deller <deller@....de>,
	Pengutronix <kernel@...gutronix.de>, linux-pwm@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] backlight: pwm_bl: apply the initial backlight state
 with sane defaults

On Fri, Nov 07, 2025 at 09:00:33AM +0100, Uwe Kleine-König wrote:
> On Thu, Oct 30, 2025 at 11:51:07AM +0000, Daniel Thompson wrote:
> > On Thu, Jul 31, 2025 at 10:47:18AM +0200, Michael Grzeschik wrote:
> > > Currently when calling pwm_apply_might_sleep in the probe routine
> > > the pwm will be configured with an not fully defined state.
> > >
> > > The duty_cycle is not yet set in that moment. There is a final
> > > backlight_update_status call that will have a properly setup state.
> > > However this change in the backlight can create a short flicker if the
> > > backlight was already preinitialised.
> > >
> > > We fix the flicker by moving the pwm_apply after the default duty_cycle
> > > can be calculated.
> > >
> > > Signed-off-by: Michael Grzeschik <m.grzeschik@...gutronix.de>
> >
> > Reviewed-by: Daniel Thompson (RISCstar) <danielt@...nel.org>
>
> I guess this tag resulted in Lee picking up the change. I wonder if you
> share my concern and who's responsibility it is now to address it.

You mean the ordering the backlight registration versus setting the
properties in the probe method?

I definitely share the concern that there's a short window where
something could request a brightness via sysfs and then have it
overwritten by the remains of the probe method. Likewise I can't see
why there would be any problem moving the call to
pwm_backlight_initial_power_state() before the backlight is registered.
Thus I'd be happy to see the backlight registered later in the probe
method.

On the other hand I don't see any problem calling
backlight_update_status() from the probe method. This is a relatively
common approach in backlight drivers to impose the initial brightness
on the hardware without needing extra code paths.
backlight_update_status() is guarded with a mutex and should be
idempotent for most drivers. Therefore it is OK even if something gets
in via sysfs and provokes an update before the probe method has started
it's own update.

In terms of who should follow up I've got no strong opinions on that.
It's worth noting that I don't own any hardware that uses a PWM
backlight so I'm not in a position to test it!

Daniel.