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

From: Tomasz Figa
Date: Fri Aug 14 2020 - 09:23:54 EST

On Fri, Aug 14, 2020 at 6:12 AM Bingbu Cao <bingbu.cao@xxxxxxxxxxxxxxx> 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.

It's not always the case, as on some designs there are multiple power
rails to the sensor and one drives the LED, while the other drives the
VCM. That said, it would be still good to solve it in either case.

Perhaps we need some more general discussion on the side effects of
simply opening and querying a device. Most of V4L2 drivers these days
are designed to avoid powering up the hardware until it's absolutely
needed to do so. However, for non-streaming subdevs that are directly
controlled by the userspace, like VCM, it's a common practice to power
up on open and down on release. This is because they don't have a
"streaming" state, so the driver has no way to determine when the
power is needed. I wonder if there is a way to improve this.

Best regards,