Re: [Xen-devel] Xen 4.2.0-rc4 issues with DVB tuner

[Javier Marcet <jmarcet@xxxxxxxxx>]

On Tue, Sep 18, 2012 at 2:08 AM, Javier Marcet <jmarcet@xxxxxxxxx> wrote:

Hi,

> Still nothing. Not even enabling the debug options within the DVB
> subsystem I get
> anything which doesn't look perfectly normal, exactly like when it's
> working fine.
>
> I have just posted a message on the linux media mailing list looking
> for some help
> from the dvb maintainers.

I have some news from the linux-media mailing list. There, Devin Heitmueller,
said this:

"""
> Initially I thought Xen would be the cause of the problem, but after
> having written on
> the Xen development mailing list and talked about it with a couple
> developers, it isn't
> very clear where the problem is. So far I haven't been able to get the
> smallest warning
> or error.

This is a very common problem when attempting to use any PCI/PCIe
tuner under a hypervisor.  Essentially the issue is all of the
virtualization solutions provide very poor interrupt latency, which
results in the data being lost.

Devices delivering a high bitrate stream of data in realtime are much
more likely for this problem to be visible since such devices have
very little buffering (it's not like a hard drive controller where it
can just deliver the data slower).  The problem is not specific to the
cx23885 - pretty much all of the PCI/PCIe bridges used in tuner cards
work this way, and they cannot really be blamed for expecting to run
in an environment with really crappy interrupt latency.

I won't go as far as to say, "abandon all hope", but you're not really
likely to find any help in this forum.
"""

What I donÂt understand is how graphics pass through even works
and a tuner card has problems due to the introduced latencies in the
IRQs.

Do you have any more info about this?


-- 
Javier Marcet <jmarcet@xxxxxxxxx>

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