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

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



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