v4l2: v4l2_compat: Add eventfd signaling to support polling

To support polling, we need to be able to signal when data is
available to be read (POLLIN), as well as events (POLLPRI). Add the
necessary calls to eventfd to allow signaling POLLIN. We signal POLLIN
by writing writing to the eventfd, and clear it by reading from the
eventfd, upon VIDIOC_DQBUF.

Note that eventfd does not support signaling POLLPRI, so we don't yet
support V4L2 events.

Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

src/v4l2/v4l2_camera_proxy.cpp

@@ -13,6 +13,7 @@
 #include <linux/videodev2.h>
 #include <string.h>
 #include <sys/mman.h>
+#include <unistd.h>
 
 #include <libcamera/camera.h>
 #include <libcamera/object.h>
@@ -451,6 +452,9 @@ int V4L2CameraProxy::vidioc_dqbuf(struct v4l2_buffer *arg)
 
 	currentBuf_ = (currentBuf_ + 1) % bufferCount_;
 
+	uint64_t data;
+	::read(efd_, &data, sizeof(data));
+
 	return 0;
 }
 
@@ -529,6 +533,12 @@ int V4L2CameraProxy::ioctl(unsigned long request, void *arg)
 	return ret;
 }
 
+void V4L2CameraProxy::bind(int fd)
+{
+	efd_ = fd;
+	vcam_->bind(fd);
+}
+
 struct PixelFormatPlaneInfo {
 	unsigned int bitsPerPixel;
 	unsigned int hSubSampling;