| Message ID | 1414411902-5335-1-git-send-email-david.henningsson@canonical.com (mailing list archive) |
|---|---|
| State | Rejected |
| Delegated to: | Takashi Iwai |
| Headers | show |
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. 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 >
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 >> >
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
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(-)