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

From: Bingbu Cao
Date: Fri Aug 14 2020 - 02:19:32 EST

On 8/14/20 12:11 PM, Bingbu Cao wrote:
> 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.
Sakari, please ignore my previous comment, it is not related to this change. I
see the sub device open is caused by v4l_id program from udev.

> ...snip...

Best regards,
Bingbu Cao