On 2014-10-27 13:17, Takashi Iwai wrote:
At Mon, 27 Oct 2014 13:11:42 +0100, David Henningsson wrote:
While reading alsa-lib code, I discovered that snd_pcm_avail_update does require a user-kernel context switch:
Not "does" but "may"...
snd_pcm_avail_update() -> snd_pcm_hw_avail_update() -> sync_ptr() -> sync_ptr1() -> ioctl()
... since this depends on the setup. sync_ptr() calls sync_ptr1() only when the status mmap isn't done.
Ah, that's what I was missing. Thanks.
Takashi
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 -- 1.9.1