Re: [patch] voluntary-preempt-2.6.8-rc3-O4

Takashi Iwai
2004-08-16T13:35:19Z

At Tue, 10 Aug 2004 11:22:49 +0200,
Ingo Molnar wrote:
>
>
> * Ingo Molnar <mingo@elte.hu> wrote:
>
> > another idea: you are running this on a C3, using CONFIG_MCYRIXIII,
> > correct? That is one of the rare configs that triggers X86_USE_3DNOW
> > and MMX ops. If 3dnow is in any way handicapped in that CPU then that
> > could cause trouble. Could you compile for e.g. CONFIG_M586TSC? [that
> > option should be fully compatible with a C3.] - this will exclude the
> > MMX page clearing ops.
>
> another (more remote) possibility is that the timestamp counter gets
> somehow messed up during MMX ops. Does the ALSA detector use the
> timestamp counter, or does it only use jiffies?

No, neither of them. It checks the DMA buffer position, so the
accuracy of the latency measurement depends on the size of DMA buffer
fragment (called "period" in ALSA).

The only thing related with the time in the pcm layer is the call of
do_gettimeofday() when status ioctl is called (or at each DMA
interrupt, depending on the running mode).

Takashi