soc_camera, v4l2 api, gstreamer: setting errno ?

["Philipp Wiesner" <p.wiesner@gmx.net>]

I'm having some trouble using gstreamer with soc_camera and am a
modified tw9910 driver. I had difficulties compiling the latest sources
for my target so I'm using old kernel and gstreamer versions. But my
question may still be valid, because the problem doesn't seem to be fixed
and this may be interesting for driver programming in the future.

The part I'm suspecting is

  if (v4l2_ioctl (fd, VIDIOC_S_FMT, &format) < 0) {
    if (errno != EINVAL)
      goto set_fmt_failed;

[v4l2src_calls.c,1223]
According to V4L2 api documentation drivers should set errno, but all drivers I've seen in the soc_camera framework (including soc_camera.c)
only 'return -errno'. Should device drivers (like tw9910) set errno or
should soc_camera use return values and set errno? Is it correct that
none of them happens at the moment?
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