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.cpp

@@ -8,6 +8,7 @@
 #include "v4l2_camera.h"
 
 #include <errno.h>
+#include <unistd.h>
 
 #include "libcamera/internal/log.h"
 
@@ -53,6 +54,11 @@ void V4L2Camera::close()
 	camera_->release();
 }
 
+void V4L2Camera::bind(int efd)
+{
+	efd_ = efd;
+}
+
 void V4L2Camera::getStreamConfig(StreamConfiguration *streamConfig)
 {
 	*streamConfig = config_->at(0);
@@ -84,6 +90,9 @@ void V4L2Camera::requestComplete(Request *request)
 	completedBuffers_.push_back(std::move(metadata));
 	bufferLock_.unlock();
 
+	uint64_t data = 1;
+	::write(efd_, &data, sizeof(data));
+
 	bufferSema_.release();
 }