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

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Message-ID: <aSVnulk0yfAd4UCx@pengutronix.de>
Date: Tue, 25 Nov 2025 09:24:26 +0100
From: Michael Grzeschik <mgr@...gutronix.de>
To: Daniel Thompson <danielt@...nel.org>
Cc: Mark Brown <broonie@...nel.org>, linux-pwm@...r.kernel.org,
	linux-fbdev@...r.kernel.org, Jingoo Han <jingoohan1@...il.com>,
	Helge Deller <deller@....de>, Lee Jones <lee@...nel.org>,
	linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	Uwe Kleine-König <ukleinek@...nel.org>,
	Pengutronix <kernel@...gutronix.de>
Subject: Re: [PATCH] backlight: pwm_bl: apply the initial backlight state
 with sane defaults

On Tue, Nov 18, 2025 at 12:52:14PM +0000, Daniel Thompson wrote:
>On Fri, Nov 14, 2025 at 02:09:56PM +0000, Mark Brown 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.
>>
>> I'm seeing the libre.computer Renegade Elite producing warnings during
>> boot in -next which bisect to this patch.  The warnings are:
>>
>> [   24.175095] input: adc-keys as /devices/platform/adc-keys/input/input1
>> [   24.176612] ------------[ cut here ]------------
>> [   24.177048] WARNING: CPU: 0 PID: 0 at kernel/context_tracking.c:127 ct_kernel_exit.constprop.0+0x98/0xa0
>>
>> ...
>>
>> [   24.190106] Call trace:
>> [   24.190325]  ct_kernel_exit.constprop.0+0x98/0xa0 (P)
>> [   24.190775]  ct_idle_enter+0x10/0x20
>> [   24.191096]  cpuidle_enter_state+0x1fc/0x320
>> [   24.191476]  cpuidle_enter+0x38/0x50
>> [   24.191802]  do_idle+0x1e4/0x260
>> [   24.192094]  cpu_startup_entry+0x34/0x3c
>> [   24.192444]  rest_init+0xdc/0xe0
>> [   24.192734]  console_on_rootfs+0x0/0x6c
>> [   24.193082]  __primary_switched+0x88/0x90
>> [   24.193445] ---[ end trace 0000000000000000 ]---
>>
>> which seems a little surprising but there is some console stuff there
>> that looks relevant.
>>
>> Full log:
>>
>>     https://lava.sirena.org.uk/scheduler/job/2086528#L897
>
>Michael, reading these logs it looks to me like the underlying oops
>is this backtrace (which makes a lot more sense given the code you
>altered):
>
>[   24.133631] Call trace:
>[   24.133853]  pwm_backlight_probe+0x830/0x868 [pwm_bl] (P)
>[   24.134341]  platform_probe+0x5c/0xa4
>[   24.134679]  really_probe+0xbc/0x2c0
>[   24.135001]  __driver_probe_device+0x78/0x120
>[   24.135391]  driver_probe_device+0x3c/0x154
>[   24.135765]  __driver_attach+0x90/0x1a0
>[   24.136111]  bus_for_each_dev+0x7c/0xdc
>[   24.136462]  driver_attach+0x24/0x38
>[   24.136785]  bus_add_driver+0xe4/0x208
>[   24.137124]  driver_register+0x68/0x130
>[   24.137468]  __platform_driver_register+0x24/0x30
>[   24.137888]  pwm_backlight_driver_init+0x20/0x1000 [pwm_bl]
>[   24.138389]  do_one_initcall+0x60/0x1d4
>[   24.138735]  do_init_module+0x54/0x23c
>[   24.139073]  load_module+0x1760/0x1cf0
>[   24.139407]  init_module_from_file+0x88/0xcc
>[   24.139787]  __arm64_sys_finit_module+0x1bc/0x338
>[   24.140207]  invoke_syscall+0x48/0x104
>[   24.140549]  el0_svc_common.constprop.0+0x40/0xe0
>[   24.140970]  do_el0_svc+0x1c/0x28
>[   24.141268]  el0_svc+0x34/0xec
>[   24.141548]  el0t_64_sync_handler+0xa0/0xf0
>[   24.141920]  el0t_64_sync+0x198/0x19c
>
>Should we back out the patch for now?

I would be fine with that. But actually I would like to see the
proof that without the patch, this backtrace will not trigger.
Looking through the codepath, I could not directly find a case
where this should happen.

Mark, is there a way to rerun this without my patch?

Michael

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