11 Mar
2008
11 Mar
'08
7:37 p.m.
On Mon, Mar 10, 2008 at 08:30:50PM -0400, Lee Revell wrote:
On Mon, Mar 10, 2008 at 4:36 PM, Paul Davis
<paul(a)linuxaudiosystems.com> wrote:
I don't think this is the problem. I asked (to the best of my
knowledge) about this on the xorg list and got this reply:
On Mon, 2008-03-10 at 19:53 +0000, James Stone
wrote:
Paul, you're absolutely right. I encountered this exact issue years
ago with the open source Via Unichrome driver, and fortunately was
able to get it fixed by the developers. 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.
This is done by vendors to get better benchmark scores. You can also
prove that this is your issue by running a simple userspace latency
tester that does not touch the audio hardware; it will also report
large delays when moving windows around.
If this is your problem, your only hope is to use the open source
driver for your video hardware, and if that driver has the same
problem, to report it to the maintainers.
I'd still like to see if disabling the VSYNC interrupt changes things,
if only to disprove the common perception that the problem is caused
by interrupt sharing.
Yes. but IRQ tuning is not much use with the
video on the same
interrupt as the sound card. Basically whenever a widget moves,
sound dropout occurs.
this is not how video display works. they are a lot like audio
interfaces actually. periodically, they interrupt the host CPU to say
"i'm about to start redrawing the screen from the framebuffer you last
told me about". that's about it really. they never redraw the screen at
any other time.
when your GUI toolkit decides to redraw a widget, it doesn't interact
with the h/w. all that happens is that eventually the new image of the
screen gets into the framebuffer, and on the next "VSYNC" cycle, the
video card will push it out to the actual display.
what is an issue is that the drivers and/or h/w associated with the
video card sometimes do not play fair with the PCI bus - they claim
owernship over it for too long while doing a redraw, and this *does*
lead to xruns at low latencies.
"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.
James