From mboxrd@z Thu Jan  1 00:00:00 1970
From: Tim Cussins <timcussins@eml.cc>
Subject: Re: [PATCH v2 3/3] snd_pcm_start_at: Implement
 snd_pcm_start_at for system and audio tstamp types.
Date: Fri, 19 Dec 2014 11:41:17 +0000
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To: Raymond Yau <superquad.vortex2@gmail.com>
Cc: nstoughton@aether.com, tiwai@suse.de, alsa-devel@alsa-project.org, pierre-louis.bossart@linux.intel.com
List-Id: alsa-devel@alsa-project.org

On 17/12/14 23:37, Raymond Yau wrote:
>  >
>  > System tstamp types rely on high-res timers.
>  > Audio tstamp types delegate to the pcm driver.
>  > Current start_at timer is cancelled on attempt to change stream state.
>
> http://git.alsa-project.org/?p=alsa-lib.git;a=blob_plain;f=test/timer.c;hb=HEAD
>
> Are there any difference between sound card's timer with high res
> timer/wall clock time stamp ?
>
> It seem those sound cards timer have resolution and max tick
>
> https://git.kernel.org/cgit/linux/kernel/git/tiwai/sound.git/commit/sound/pci/ymfpci?id=6e2efaacb3579fd9643d0dc59963b58b801c03a1
>
> snd_timer_status_get_resolution
>
> The clock tick counters are 32 bits and wrap around
>
> This may mean that snd_pcm_start_at can only start within certian time
> interval
>

[and this time I'll post to the list :)]

I had presumed it would be pretty simple to use the timer-wrap interrupt 
to maintain a monotonic sense of time. Hardware that can't support this 
might not be able to support start_at.

T