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
