On Tuesday, May 24, 2011 16:57:22 Devin Heitmueller wrote:
> On Tue, May 24, 2011 at 2:50 AM, Hans Verkuil <[email protected]> wrote:
> > On Monday, May 23, 2011 22:17:06 Mauro Carvalho Chehab wrote:
> >> Due to the alsa detection code that I've added at libv4l2util (at
> >> v4l2-utils)
> >> during the weekend, I decided to add alsa support also on xawtv3, basically
> >> to provide a real usecase example. Of course, for it to work, it needs the
> >> very latest v4l2-utils version from the git tree.
> >
> > Please, please add at the very least some very big disclaimer in libv4l2util
> > that the API/ABI is likely to change. As mentioned earlier, this library is
> > undocumented, has not gone through any peer-review, and I am very unhappy
> > with
> > it and with the decision (without discussion it seems) to install it.
> >
> > Once you install it on systems it becomes much harder to change.
I wanted to do a review of this library, but Devin did it for me in his
comments below.
I completely agree with his comments.
Once I have all the control framework stuff that is in my queue done, then
I want to go through as many drivers as I can and bring them all up to
the latest V4L2 standards (using v4l2-compliance to verify correctness).
It is my intention to create some helper functions to implement a MC node for
these simple legacy drivers. Eventually all V4L drivers should have a MC node.
Writing a library like the one proposed here would then be much easier and
it would function as a front-end for the MC.
The last few months I wasn't able to really spend the time on V4L that I
wanted, but that is changing for the better.
Regards,
Hans
> I share Hans' concern on this. This is an API that seems pretty
> immature, and I worry that it's adoption will cause lots of problems
> as people expect it to work with a wide variety of tuners.
>
> For example, how does the sysfs approach handle PCI boards that have
> multiple video and audio devices? The sysfs layout will effectively
> be:
>
> PCI DEVICE
> -> video0
> -> video1
> -> alsa hw:1,0
> -> alsa hw:1,1
>
> The approach taken in this library will probably help with the simple
> cases such as a USB tuner that only has a single video device, audio
> device, and VBI device. But it's going to fall flat on its face when
> it comes to devices that have multiple capture sources (since sysfs
> will represent this as a tree with all the nodes on the same level).
>
> Oh, and how is it expected to handle informing the application about
> device contention between DVB and V4L? Some devices share the tuner
> and therefore you cannot use both the V4L and DVB device at the same
> time. Other products have two independent input paths on the same
> board, allowing both to be used simultaneously (the HVR-1600 is a
> popular example of this). Sysfs isn't going to tell you this
> information, which is why in the MC API we explicitly added the notion
> of device groups (so the driver author can explicitly state the
> relationships).
>
> Today MythTV users accomplish this by manually specifying "Input
> Groups". I say that's what they do, but in reality they don't realize
> that they need to configure MythTV this way until they complain that
> MythTV recordings fail when trying to record programs on both inputs,
> at which point an advanced user points it out to them. End users
> shouldn't have to understand the internal architecture of their
> capture card just to avoid weird crashy behavior (which is what often
> happens if you try to use both devices simultaneously since almost no
> hybrid drivers do proper locking).
>
> I am in favor of this finally getting some attention, but the reality
> is that sysfs isn't going to cut it. It just doesn't expose enough
> information about the underlying hardware layout.
>
> Devin
>
>
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