5.1 video(4) mmap interface?

[Mouse <mouse%Rodents-Montreal.ORG@localhost>]

I'm trying to use the video(4) interface under 5.1.  The read(2)-based
interface seems to work fine, but when I try the mmap(2)-based
interface, everything goes fine except that VIDIOC_DQBUF never returns.
I did a little UTSLing and didn't see anything obviously wrong, but I
already found at least a few things I had to do which weren't
documented, so I'm wondering if it's just another such that's not quite
as obvious, or if that interface is just broken in 5.1, or what.  The
webpage view of -current's video(4) doesn't lead me to think it's just
a documentation bug, for what that may be worth.

The program I'm using is up for ftp from ftp.rodents-montreal.org in
/mouse/misc/video-mmap.c.  It's not long, 107 lines.  The underlying
hardware in my case is i386 with a uvideo:

uvideo0 at uhub3 port 1 configuration 1 interface 0: Vimicro Corp. Venus USB2.0 
Camera, rev 2.00/1.02, addr 2
video0 at uvideo0: Vimicro Corp. Venus USB2.0 Camera, rev 2.00/1.02, addr 2

The hardware clearly works, since the read(2) interface works fine.

I'd welcome any thoughts.

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