On Wed, 9 Apr 2008, Lennart Poettering wrote:
Hey!
Hi,
The problem with this is that there seems to be no way to determine from an application if monotonic timestamps are enabled in an snd_pcm_t or not, ALSA just switches over to them, making the timestamps relatively useless, because we cannot reliable relate them to timestamps we query from the kernel ourselves -- because we just don't know if we need to use CLOCK_MONOTONIC or CLOCK_REALTIME.
Application can just do a simple comparsion between ALSA timestamp and gettimeofday() output. The gettimeofday() returns value since the Epoch, but CLOCK_MONOTONIC returns time since start of system. Thus, it's really easy to see if time matches or not (difference will be very big) to detect the time source. Value from gettimeofday() does not make much sense in audio (real time) environment.
I find it very strange that ALSA just switches to monotonic timestamps just like that, anyway. Programs written for wallclack timestamps will break if they run on a system where ALSA uses monotonic timestamps!
It's true, but so far - I don't know about any program using timestamps in serious way. Also, timestamps from gettimeofday() are not reliable (for example when NTP time synchronization is used in system).
Could anybody please explain the difference between status->tstamp and status->trigger_tstamp for me, please? The doxygen docs are bit too terse on this, I fear.
I added just these words to trigger tstamp:
+ * Trigger means a PCM state transition (from stopped to running or + * versa vice). It applies also to pause and suspend. In other words, + * timestamp contains time when stream started or when it was stopped.
The "now" tstamp is obvious, or not? It's just timestamp related to current stream position reported in other ALSA functions.
Oh, and the code I pasted above will not compile if the #ifdef check fails, because of the "else". The "else" should be moved inside of #ifddef block.
Thanks. Fixed in our repo.
Jaroslav
----- Jaroslav Kysela perex@perex.cz Linux Kernel Sound Maintainer ALSA Project, Red Hat, Inc.