linux-kernel - Re: [PATCH v4 1/2] PM: domains: Detach on device_unbind

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Message-ID: <f4vj7rab6uftieicz5s3qur7yk26cg3xxaabhcmqnx3nybhshl@j6ahyvw6grik>
Date: Sun, 29 Jun 2025 22:02:45 -0700
From: Dmitry Torokhov <dmitry.torokhov@...il.com>
To: Ulf Hansson <ulf.hansson@...aro.org>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, 
	Wolfram Sang <wsa+renesas@...g-engineering.com>, Claudiu Beznea <claudiu.beznea@...on.dev>, 
	gregkh@...uxfoundation.org, dakr@...nel.org, len.brown@...el.com, pavel@...nel.org, 
	jic23@...nel.org, daniel.lezcano@...aro.org, linux-kernel@...r.kernel.org, 
	linux-pm@...r.kernel.org, bhelgaas@...gle.com, geert@...ux-m68k.org, 
	linux-iio@...r.kernel.org, linux-renesas-soc@...r.kernel.org, 
	fabrizio.castro.jz@...esas.com, Claudiu Beznea <claudiu.beznea.uj@...renesas.com>, 
	linux-i2c <linux-i2c@...r.kernel.org>, "andi.shyti@...nel.org" <andi.shyti@...nel.org>
Subject: Re: [PATCH v4 1/2] PM: domains: Detach on device_unbind_cleanup()

On Thu, Jun 19, 2025 at 02:16:37PM +0200, Ulf Hansson wrote:
> 
> I am not sure it's needed, unless it's especially targeted for the
> ACPI PM domain, which I find hard to believe.
> 
> Also, I find it awkward why the i2c bus should be any different from
> many other types of buses. It's probably just because of legacy and
> that someone took a decision when we added it.

It has nothing to do with I2C and everything to do with ACPI and the
fact that it brings devices into D0 when probing. On ACPI systems
(unlike DT ones) power sequencing is done in firmware so drivers are
unable to control this. And this causes annoying flashing of privacy
leds on webcams.

See details in 1e96078e0ae4 ("at24: Support probing while in non-zero ACPI D state")

If there was a SPI device sharing power rails with a camera we'd need
similar hack in SPI bus.

Thanks.

-- 
Dmitry