Re: Re: [Jackit-devel] irq handler top half timestamps

[Florian Schmidt <mista.tapas@gmx.net>]

On Sun, 19 Dec 2004 18:38:19 -0500
Lee Revell <rlrevell@joe-job.com> wrote:

> OK the interrupt handler calls snd_pcm_period_elapsed, which calls
> snd_pcm_update_hw_ptr_interrupt, which calls snd_pcm_update_hw_ptr_pos.
> snd_pcm_update_hw_ptr_pos does this (from alsa-kernel/core/pcm-lib.c):
> 
> 151         pos = substream->ops->pointer(substream);
> 152         if (pos == SNDRV_PCM_POS_XRUN)
> 153                 return pos; /* XRUN */
> 154         if (runtime->tstamp_mode & SNDRV_PCM_TSTAMP_MMAP)
> 155                 snd_timestamp_now((snd_timestamp_t*)&runtime->status->tstamp, runtime->tstamp_timespec);

I see. And all these calls have constant execution time?

> 
> Here is snd_timestamp_now (from alsa-kernel/include/core.h):
> 
> 464 static inline void snd_timestamp_now(struct timespec *tstamp, int timespec)
> 465 {
> 466         struct timeval val;
> 467         /* FIXME: use a linear time source */
> 468         do_gettimeofday(&val);
> 469         tstamp->tv_sec = val.tv_sec;
> 470         tstamp->tv_nsec = val.tv_usec;
> 471         if (timespec)
> 472                 tstamp->tv_nsec *= 1000L;
> 473 }
> 
> Note the FIXME.
> 
> So this timestamp should be OK for our purposes.  Note the pointer
> callback is called before the timestamp is taken, so this will introduce
> variable delay as the pointer callback is different in every driver and
> requires accessing the PCI bus to read the pointer from the hardware.
> 
> The API function to get the timestamp is snd_pcm_status_get_tstamp,
> defined in alsa-lib/src/pcm/pcm.c.

Hmm,

how is do_gettimeofday implemented? Is it based on the TSC? It seems it
is for X86 which has a TSC. into arch/i386/kernel/time.c also points out
that it is subject to adjustments due to NTP.

I suppose it would be better to get the TSC timestamp directly as Ingo's
patch produces for RP kernels. OTOH gettimeofday is a more general API
and i suppose it should provide the best timestamping mechanism a
platform can offer, right?

I might be talking out of my behind here though. As i don't know pretty
much anything about ALSA nor kernel internals..

Flo

-- 
Palimm Palimm!
http://affenbande.org/~tapas/


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