[libcamera-devel] Support for hotplugging UVC devices

[Umang Jain]

Hi all,

I have doing some researching regarding hot-plugging UVC devices. The 
goal
is to have CameraManager emit a "camera-added" hotplug signal so that 
applications
like QCam, can detect the newly hotplugged camera and be able to start 
streaming.
I have discussed this with Kieran briefly and we both agree this is 
something worth
pursuing.

As far as I read the implementation, DeviceEnumeratorUdev::udevNotify() 
can
detect new UVC devices being plugged in and add the the list of media 
devices in the
DeviceEnumerator.

But right now, there is nothing done when a new device is being 
plugged-in, so that
it can be brought into use. Below is what I think is a very crude 
version of the code flow is:

1) Emit a signal for DeviceEnumerator::addDevice() that CameraManager 
can connect to.

2) Upon receiving the signal, the CameraManager queries the 
PipelineHandler to see if
     it has a supported pipeline. Initially, this is being done via 
PipelineHandler::match()
     but match() takes a entire DeviceEnumerator as argument and is 
meant to run in a loop(in ::init()).
     We need something similar to  match() which can validate the 
pipeline-handling-support
     but for a specific MediaDevice instance(s?).

3) After the Pipeline Handler can validate the support for a 
MediaDevice, it can go ahead
    and create/register the Camera via 
PipelineHandler::registerCamera() with the CameraManager.

4) The signal-handler in 2), proceeds to emit a 
CameraManager::camera-added signal
    for applications to connect to.

This is what I have researched till now. Of course, this is right now 
being targeted towards
UVC devices but I am sure folks here, will have some generalistic point 
of view to support
hotplugs for various other media-device domains that I might not be 
aware of. Your
thoughts/comments will be helpful.

Let me know what you think!


--
Umang Jain


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