[pulseaudio-discuss] [PATCH] alsa: Disable timer-scheduling for PCMs with the BATCH flag

[Lars-Peter Clausen]

PCM Devices which have the BATCH flag set update the PCM pointer only with
period size granularity. Using timer based scheduling does not have any
advantage in this mode. For one devices which have that flag set usually update
the position pointer in software after getting the period interrupt. So
disabling the period interrupt is not possible for this kind of devices.
Furthermore writing to or reading from the buffer slice for the current period
is not possible since the position inside the buffer is not known. On the other
hand the tsched algorithm seems to get easily confused for this kind of
hardware, which results in garbled audio output. This typically means that timer
based scheduling needs to be manually disabled on systems with such devices.
Auto disabling tsched in this case allows these systems to run with the default
configuration.

Signed-off-by: Lars-Peter Clausen <lars at metafoo.de>
---
 src/modules/alsa/alsa-util.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/modules/alsa/alsa-util.c b/src/modules/alsa/alsa-util.c
index 75f5858..4b24e47 100644
--- a/src/modules/alsa/alsa-util.c
+++ b/src/modules/alsa/alsa-util.c
@@ -245,6 +245,10 @@ int pa_alsa_set_hw_params(
     if (!pa_alsa_pcm_is_hw(pcm_handle))
         _use_tsched = false;
 
+    /* The PCM pointer is only updated with period granularity */
+    if (snd_pcm_hw_params_is_batch(hwparams))
+        _use_tsched = false;
+
 #if (SND_LIB_VERSION >= ((1<<16)|(0<<8)|24)) /* API additions in 1.0.24 */
     if (_use_tsched) {
 
-- 
1.8.0