Re: [PATCH v5 0/6] Support running driver's probe for a device powered off

From: Bingbu Cao <bingbu.cao@linux.intel.com>
To: Sakari Ailus <sakari.ailus@linux.intel.com>, linux-i2c@vger.kernel.org
Cc: Wolfram Sang <wsa@the-dreams.de>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	linux-acpi@vger.kernel.org, Bingbu Cao <bingbu.cao@intel.com>,
	linux-media@vger.kernel.org,
	Chiranjeevi Rapolu <chiranjeevi.rapolu@intel.com>,
	Hyungwoo Yang <hyungwoo.yang@intel.com>,
	Bartosz Golaszewski <bgolaszewski@baylibre.com>,
	Arnd Bergmann <arnd@arndb.de>,
	linux-kernel@vger.kernel.org,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	rajmohan.mani@intel.com, Tomasz Figa <tfiga@chromium.org>,
	"Qiu, Tian Shu" <tian.shu.qiu@intel.com>
Subject: Re: [PATCH v5 0/6] Support running driver's probe for a device powered off
Date: Fri, 14 Aug 2020 12:11:40 +0800	[thread overview]
Message-ID: <5353041e-850f-05ad-3b20-35e91fc9501e@linux.intel.com> (raw)
In-Reply-To: <20200810142747.12400-1-sakari.ailus@linux.intel.com>

On 8/10/20 10:27 PM, Sakari Ailus wrote:
> Hi all,
> 
...snip...
> 
> The use case is such that there is a privacy LED next to an integrated
> user-facing laptop camera, and this LED is there to signal the user that
> the camera is recording a video or capturing images. That LED also happens
> to be wired to one of the power supplies of the camera, so whenever you
> power on the camera, the LED will be lit, whether images are captured from
> the camera --- or not. There's no way to implement this differently
> without additional software control (allowing of which is itself a
> hardware design decision) on most CSI-2-connected camera sensors as they
> simply have no pin to signal the camera streaming state.
> 
> This is also what happens during driver probe: the camera will be powered
> on by the I²C subsystem calling dev_pm_domain_attach() and the device is
> already powered on when the driver's own probe function is called. To the
> user this visible during the boot process as a blink of the privacy LED,
> suggesting that the camera is recording without the user having used an
> application to do that. From the end user's point of view the behaviour is
> not expected and for someone unfamiliar with internal workings of a
> computer surely seems quite suspicious --- even if images are not being
> actually captured.
> 
> I've tested these on linux-next master. They also apply to Wolfram's
> i2c/for-next branch, there's a patch that affects the I²C core changes
> here (see below). The patches apart from that apply to Bartosz's
> at24/for-next as well as Mauro's linux-media master branch.

Sakari, we meet one issue - once the vcm sub-device registered, the user space
will try to open the VCM (I have not figure out who did that), it will also
trigger the acpi pm resume/suspend, as the VCM always shares same power rail
with camera sensor, so the privacy LED still has a blink.

> 
...snip...
-- 
Best regards,
Bingbu Cao