i915 - disable vsync?

Eric Anholt eric at anholt.net
Tue Mar 11 10:30:06 PDT 2008

On Tue, 2008-03-11 at 11:11 +0000, James Stone wrote:
> Hi there,
> I have an eeepc and I am trying to get the realtime audio daemon jackd
> working properly on it. I notice there is some conflict between the
> audio and video since moving windows etc. causes audio dropouts
> (xruns). The two devices appear to share an interrupt, and there
> doesn't seem to be much I can do about that. Paul Davis (author of
> Jackd) did not think that sharing an interrupt was necessarily the
> cause of xruns, but rather that the graphics driver may be taking
> control of the pci bus for too long. Another person on the LAU list
> (Lee Revell) suggested that disabling vsync interrupts for the intel
> graphics driver might help with this:
> "Typically the (userspace X11) video driver tries to write to the
> card's command FIFO without first
> checking whether it is full.  If the FIFO is full, this will
> completely stall the PCI bus until the FIFO drains.  Nothing at all to
> do with interrupts."

Not at all.  We don't have a fifo like that, and I don't know any xorg
drivers for fifo-having hardware that have that bug either.  We may be
keeping the PCI bus busy with our spinning on ring tail reads when we're
waiting for hardware idle or otherwise trying to get ring space, but we
do one of those and then a gettimeofday for timeout handling anyway, so
you shouldn't be all that busy I don't think.

That FIFO issue is completely separate from any vertical sync issue.  We
do no syncing to vblank for plain 2d rendering anyway.

Eric Anholt                             anholt at FreeBSD.org
eric at anholt.net                         eric.anholt at intel.com

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