libcamera: v4l2_device: add frame start event helpers

Add helper to check if frame start event are supported by subdevice.
Since kernel does not have interface to query supported events
use subscribe interface.

Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com> # v3
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com> # v5
Signed-off-by: Stanislaw Gruszka <stanislaw.gruszka@linux.intel.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

include/libcamera/internal/v4l2_device.h

@@ -45,6 +45,7 @@ public:
 	const std::string &deviceNode() const { return deviceNode_; }
 	std::string devicePath() const;
 
+	bool supportsFrameStartEvent();
 	int setFrameStartEnabled(bool enable);
 	Signal<uint32_t> frameStart;