While reading alsa-lib code, I discovered that snd_pcm_avail_update does require a user-kernel context switch:
snd_pcm_avail_update() -> snd_pcm_hw_avail_update() -> sync_ptr() -> sync_ptr1() -> ioctl()
The documentation says it does not, so let's fix the documentation.
Signed-off-by: David Henningsson david.henningsson@canonical.com --- src/pcm/pcm.c | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-)
diff --git a/src/pcm/pcm.c b/src/pcm/pcm.c index 4a7be6c..e57e362 100644 --- a/src/pcm/pcm.c +++ b/src/pcm/pcm.c @@ -403,10 +403,12 @@ The function #snd_pcm_avail_update() updates the current available count of samples for writing (playback) or filled samples for reading (capture). This call is mandatory for updating actual r/w pointer. Using standalone, it is a light method to obtain current stream position, -because it does not require the user <-> kernel context switch, but the value -is less accurate, because ring buffer pointers are updated in kernel drivers +because while it does require a user <-> kernel context switch, the value is +just copied from the kernel, and not updated by synchronously asking the hardware +about its current position. The ring buffer pointers are updated in kernel drivers only when an interrupt occurs. If you want to get accurate stream state, -use functions #snd_pcm_avail(), #snd_pcm_delay() or #snd_pcm_avail_delay(). +use functions #snd_pcm_avail(), #snd_pcm_delay(), #snd_pcm_avail_delay() or +#snd_pcm_status(). </p> <p> The function #snd_pcm_avail() reads the current hardware pointer