[FFmpeg-devel] PATCH grab.c, adding support for hardware based VIDIOSFPS

[Trent Piepho]

On Mon, 4 Jun 2007, Benoit Fouet wrote:
> mmh wrote:
> >  >
> >  > this is wrong.
> >  > from my videodev2.h:
> >  > #define VIDIOC_S_RDS            _IOWR ('V', BASE_VIDIOC_PRIVATE+11,
> >  > struct v4l2_radio_rds_set)
> >  >
> >  > i can (if i want) change the BASE_VIDIOC_PRIVATE+11 by
> >  > BASE_VIDIOC_PRIVATE+20
> >  > this ioctl will be defined, but will not do what you want...
> >  >
> >  > moreover, it can just return without any error, and lead you to believe
> >  > that framerate is accepted by the camera, even though this is not true...

Not exactly.  The size of the argument to the ioctl (struct
v4l2_radio_rds_set) is part of the ioctl number.  You could assign two
different ioctls to BASE_VIDIOC_PRIVATE+20 and they won't be the same if
the size of the arugment and the direction (R, W, WR) doesn't match.

> > #ifndef VIDIOSFPS
> > #define VIDIOSFPS		_IOW('v',BASE_VIDIOCPRIVATE+20, int)			/* Set fps */
> > #endif
> >
> yes, i really think this should be removed
> and, FWIW, syntax in v4l2 is, most of the time, as follows:
> VIDIOC_S_*
> i don't know if it's planned to add such a feature in future videodev
> version, though...

This isn't a v4l2 ioctl, but a v4l1 ioctl.  V4L2 uses the ioctl range 'V',
while v4l1 uses 'v'.  Notice the case difference.  Also BASE_VIDIOCPRIVATE
is the base for v4l1 private ioctls and BASE_VIDIOC_PRIVATE is the base for
v4l2.  It should still be VIDIOCSFPS to have correct v4l1 naming
conventions.

The whole ioctl name clash thing isn't solved by just removing the #define
from the ffmpeg code.  What happens if someone compiles on a kernel where
'v',private+20 is SFPS and then runs on a kernel where it's something else?

This is why changing ioctls once they make it into the kernel isn't
allowed.