v4l2: v4l2_camera_proxy: Don't return -EINVAL for zero sizeimage in REQBUFS

If VIDIOC_REQBUFS returns -EINVAL, it signals to the application that
the requested buffer or memory type is not supported. If we return
-EINVAL due to a zero sizeimage, then the application will think that we
don't support a memory type that we actually do. We cannot error on a
zero sizeimage, because reqbufs could be called merely to probe what IO
methods we support; qv4l2, for example, called reqbufs once with userptr
and once more with mmap, both times with count=1.

On the other hand, sizeimage will be zero for formats whose size we
don't know how to calculate, such as MJPEG. If we try to stream such
formats anyway, we will get a floating point exception and crash. Issue
a warning for now, and don't return -EINVAL, so that we can continue
operation.

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

src/v4l2/v4l2_camera_proxy.cpp

@@ -352,8 +352,18 @@ int V4L2CameraProxy::vidioc_reqbufs(struct v4l2_requestbuffers *arg)
 		return -EINVAL;
 
 	sizeimage_ = calculateSizeImage(streamConfig_);
+	/*
+	 * If we return -EINVAL here then the application will think that we
+	 * don't support streaming mmap. Since we don't support readwrite and
+	 * userptr either, the application will get confused and think that
+	 * we don't support anything.
+	 * On the other hand, if a format has a zero sizeimage (eg. MJPEG),
+	 * we'll get a floating point exception when we try to stream it.
+	 */
 	if (sizeimage_ == 0)
-		return -EINVAL;
+		LOG(V4L2Compat, Warning)
+			<< "sizeimage of at least one format is zero. "
+			<< "Streaming this format will cause a floating point exception.";
 
 	setFmtFromConfig(streamConfig_);