The Video4Linux1 (V4L1) ABI is deprecated, and has been for a long time; it
was ostensibly replaced by Video4Linux2 in the 2.5 development series.
But, as has been discovered many times, an ABI is a hard thing to get rid
of. So the kernel still supports V4L1 applications; indeed, there are
still V4L1-only drivers in current kernels. That situation has persisted
for a long time, but it may now be coming to an end.
Hans Verkuil has posted a multi-stage proposal for the removal of V4L1 from
the kernel. The first phase involves the
conversion of the remaining V4L1 drivers - of which there are several - to
the newer ABI. Some of those drivers have since been supplanted by GSPCA
and may just be deleted outright. All told, this is a bit of much-needed
Phase 2 may be a bit more controversial,
though, in that it calls for the removal of the V4L1 compatibility layer in
the kernel. This code allows V4L1 applications to work with V4L2 drivers -
most of the time. It was an important bit of backward compatibility
support, but it has also helped to delay the updating of a number of old
V4L1 applications. Given that these applications do still exist (many
distributions still ship xawtv, for example), it might be a bit surprising
that this layer is slated for removal, perhaps as soon as 2.6.36.
There are problems with the compatibility layer. It cannot provide access
to much of the functionality of contemporary hardware and drivers, it
cannot always do the right thing in response to application requests, and
it has been a long time since anybody had any interest in maintaining this
code. So the V4L developers would like to push it out into user space, and
into the libv4l1 library in particular. Supporting old applications would
then be a matter of a quick edit (replacing ioctl() calls with
v4l1_ioctl(), for example) and a rebuild against the library.
Some old applications may be pulled into the V4L project, since their
original maintainers have almost certainly long since lost interest.
It's not a perfect solution; old, binary applications will cease to work on
newer kernels. It is an ABI break, plain and simple, and it is possible
that there will be enough of an uproar to prevent this change from
happening in the end. But it may also be that nobody really cares about
running binary V4L1 applications on new kernels, and that it is truly time
for this old interface to pass into history.
to post comments)