On Sat, 14 Feb 2009 10:08:31 +0100
Hans Verkuil <hverk...@xs4all.nl> wrote:
> On Saturday 14 February 2009 09:52:06 Mauro Carvalho Chehab wrote:
> > On Fri, 13 Feb 2009 13:57:45 +0100
> >
> > Hans Verkuil <hverk...@xs4all.nl> wrote:
> > > Hi Hans,
> > >
> > > I've developed a converter for the HM12 format (produced by Conexant
> > > MPEG encoders as used in the ivtv and cx18 drivers).
> > >
> > > But libv4l2 has a problem in its implementation of v4l2_read: it
> > > assumes that the driver can always do streaming. However, that is not
> > > the case for some drivers, including cx18 and ivtv. These drivers only
> > > implement read() functionality and no streaming.
> > >
> > > Can you as a minimum modify libv4l2 so that it will check for this
> > > case? The best solution would be that libv4l2 can read HM12 from the
> > > driver and convert it on the fly. But currently it tries to convert
> > > HM12 by starting to stream, and that produces an error.
> > >
> > > This bug needs to be fixed first before I can contribute my HM12
> > > converter.
> >
> > Hans Verkuil,
> >
> > I think it would be valuable if you could convert the drivers to use
> > videobuf. There's currently a videobuf variation for devices that don't
> > support scatter/gather dma transfers. By using videobuf, the mmap()
> > methods (and also overlay, if you want) will be supported.
>
> It's been on my todo list for ages, but I don't see it happening anytime
> soon. It will be difficult to do and the simple fact of the matter is that
> the read() interface is much more suitable for MPEG streams than mmap, and
> almost nobody is using the raw video streams where mmap would make sense.
>
> The only reason for doing this would be to make the driver consistent with
> the other drivers in V4L2. Which is a valid argument, but as long as we
> still have V4L1 drivers to convert I'd argue that this is definitely a low
> prio task.
I suspect that the only two drivers that don't support mmap() are ivtv and
cx18. All other drivers support it, including other drivers that also provides
compressed data (like jpeg webcams). In a matter of fact, most applications
work only with mmap() interface (being mythtv and mplayer capable of supporting
both read() and mmap()). So, by providing mmap(), other applications will
benefit of it.
Also, there is a sort of chicken and egg trouble: almost nobody uses raw
formats, since it uses a non-standard format that it is not supported by
userspace apps. The libv4l2 is the proper way for handling it, but only works
with mmap().
The usage of read() for raw formats is possible, but, read() method doesn't
provide any meta-data info. For example, there's no timestamp that would be
useful for syncing audio and video and detect frame losses. Also, if, for some
reason, you loose a half frame, the result would be a disaster if you're using
the read() method.
So, IMO, adding read() support to libv4l2 would be just a hack and will likely
cause more troubles than benefits. This is just my 2 cents.
> BTW, it would help if someone would actually document videobuf.
> Documentation should be much more important than it currently is.
Videobuf usage is not that complicate. You just need to provide ops for four
handlers:
q->ops->buf_setup - calculates the size of the video buffers and avoid they to
waste more than some maximum limit of RAM;
q->ops->buf_prepare - fills the video buffer structs and calls
videobuf_iolock() to alloc and prepare mmaped memory;
q->ops->buf_queue - advices the driver that another buffer were
requested (by read() or by QBUF);
q->ops->buf_release - frees any buffer that were allocated.
In order to use it, the driver need to have a code (generally called at
interrupt context) that will properly handle the buffer request lists,
announcing that a new buffer were filled.
There are a number of videobuf methods that practically matches the video
buffer ioctls. for example videobuf_streamon() should be called for streaming
the video on (VIDIOC_STREAMON).
The better way to understand it is to take a look at vivi driver.
Anyway, I just documented it, from the driver authors POV:
http://linuxtv.org/hg/v4l-dvb/rev/6f4cff0e7f16
Cheers,
Mauro
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