Re: [ANNOUNCE] experimental alsa stream support at xawtv3

[Emil Meier <emil276me@yahoo.com>]

--- On Tue, 24/5/11, Devin Heitmueller <dheitmueller@kernellabs.com> wrote:

> From: Devin Heitmueller <dheitmueller@kernellabs.com>
> Subject: Re: [ANNOUNCE] experimental alsa stream support at xawtv3
> To: "Hans Verkuil" <hverkuil@xs4all.nl>
> Cc: "Mauro Carvalho Chehab" <mchehab@redhat.com>, "Linux Media Mailing List" <linux-media@vger.kernel.org>
> Date: Tuesday, 24 May, 2011, 16:57
> On Tue, May 24, 2011 at 2:50 AM, Hans
> 
> 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).

Are there mechanisms for device-locking in the v4l api? With my 2 hybrid saa7134 cards I have observed exactly this issues in Mythtv and also in xawtv and kaffeine....
At the moment I disable one device via additional card definitions and modprobe-parameters, so that the other one is "alone" and only one app gets control over the tuner. 

But this driver (saa7134) seams not even lock the /dev/video0 correctly, as starting xawtv twice renders the card inoperable and forces rebooting to get the card working again...
 
Is there a good starting point to implement the 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
> 
> -- 
> Devin J. Heitmueller - Kernel Labs
> http://www.kernellabs.com
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>
Emil