Re: [PATCH] media: v4l2-async: link ancillary device runtime PM to the sensor's

[Kieran Bingham <kieran.bingham@ideasonboard.com>]

Quoting Sakari Ailus (2026-09-23 07:34:54)
> Hi C�dric,
> 
> On Tue, Sep 22, 2026 at 09:39:12PM +0200, C�dric Bellegarde wrote:
> > When a sensor's fwnode references an ancillary lens or flash device
> > (e.g. via the "lens-focus" or "flash-leds" properties),
> > v4l2_async_create_ancillary_links() already creates a media controller
> > link between the two entities, but their runtime PM states remain
> > independent.
> > 
> > This is a problem for devices such as VCM lens actuators, which are
> > typically spring-loaded: holding a position away from the spring's
> > rest point requires continuous power, and the position is not retained
> > once power is cut. If such an actuator is allowed to runtime-suspend
> > independently of the sensor, the lens can drift back to its rest
> > position during an otherwise active capture session.
> > 
> > Add V4L2_SUBDEV_FL_PM_LINK to allow an ancillary subdevice to request
> > that its runtime PM state be linked to the associated sensor.
> 
> Interesting idea.
> 
> The IPU bridge has created such a device link between the VCM and the
> sensor as on some ACPI systems the VCM is in fact relying on the power
> resources of the sensor. But to do this everywhere?

Same on any Raspberry Pi camera module. The VCM is powered by the same
enable lines as the Sensor there I think which has made things
interesting in handling VCMs for those devices.


> VCMs traditionally have been powered through opening their sub-device node
> and that hasn't been exactly neat API-wise. It has been practical still,
> AFAIK, as in order to control the VCM, you have to have a sub-device node
> open.

This is an issue though, as libcamera opens the device nodes and holds
the file descriptor when it has a camera. This means that even if the
camera isn't streaming, the VCM is powered on and causes power
consumption on mobile devices which people then attribute to pipewire.

 
> This change also does mean that if the sensor is powered, even for
> always-on use cases that generally consume very little power, the VCM is
> powered on as well. VCMs still typically consume very little power if the

Do you foresee use cases where a linked VCM shouldn't be  powered while
a camera is streaming? That effectively means the lens position seen by
the camera is 'arbitrary' / undefined ?

> current is configured to zero. Maybe this won't be an issue? Backtracking
> from such a change wouldn't be simple, and might not be possible at all.
> 
> There wouldn't be a need for a sub-device flag and this would be done for
> all VCMs based on the ancillary link.
> 
> I wonder what others think.
> 
> Cc Laurent and Hans as well.

Cc Dave too as he's looked at similar things in the past.

--
Kieran

> 
> -- 
> Kind regards,
> 
> Sakari Ailus