Setting overlay parameters on v4l2sink fails

[Andreas Naumann]

Hi,

i observed positioning of video overlays using the v4l2sink parameters 
like 'overlay-left' doesnt work. Digging a little I found out the 
VIDIOC_G_FMT/VIDIOC_S_FMT ioctl's dont reach the overlay-driver due to 
being filtered in the kernels v4l2 ioctl dispatcher functions because my 
driver is not a VFL_DIR_RX but TX type.

After reading the Linux for Video API docu, it seems that the S/G_FMT 
function are indeed intended for the Capture/Rx part of the Video 
Overlay Interface. Somehow I cant image such a device but anyway, the 
code that checks for RX is in place since 2012.

Now, the gstreamer v4l2sink does need a TX device doesnt it? So with 
current kernels, the video-overlay ioctls cant ever work, or what am I 
missing?

Digging further I came across another type, that is 
V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY. This is per API doc intended for 
output and shares the data structures for S/G_FMT ioctls with 
V4L2_BUF_TYPE_VIDEO_OVERLAY. So I tried telling my driver that he 
supports such an interface and redirected the existing s/g_fmt function 
pointers (as I saw in some other implementation, I think omap).
On v4l2sink side its a oneliner to change to using that interface ( 
gst_v4l2sink_sync_overlay_fields()).

Voila, the positioning works!

Now, is this the way to go or what do the experts think?


cheers,
Andreas