[Ffmpeg-devel] video4linux2 input format?

Erik Slagter erik
Thu Jan 26 12:57:52 CET 2006

On Thu, 2006-01-26 at 12:56 +0100, Luca Abeni wrote:
> Thanks; I'll have a look at it today, to check if I got the v4l2 abi
> right.

The problem is actually that the api is not 100% specified and some
things can be interpreted in various ways. Only the source knows
exactly ;-)

Positive thing here is that the drivers for bttv, saa7134 and cx88 are
all written by Gerd Knorr and behave very similarly, although not always
as expected ;-)

> > Please note: the current implementation uses memory mapped i/o, which
> > means memory is allocated by the kernel and passed to the user process.
> Yes, this is what I am doing too: first, I want to have mmap working,
> then I'll implement read-based grabbing (for very old card or broken
> drivers?), and then I'll try the pointer-based I/O. The problem with it
> is that the demo program included in the v4l2 specifications just
> crashed my computer (debian unstable, 2.6.8 kernel, and a Bt878-based
> card)... So, I assume the specifications are not very reliable regarding
> pointer-based I/O ;-)

Or maybe the demo program is obsolete, who knows. The way you setup
stuff is very critical indeed, because of the whole nature, this close
to the hardware, although a perfect implementation off course never
crashes ;-) I had it working using 2.6.6 or something like that. The
read-based grabbing is actually not workable, it delivers about 1 frame
per second. Also afaik there are no drivers that only support the read()
interface. Please reconsider using it.

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