[pulseaudio-discuss] [PATCH v2] alsa-util: Set ALSA report_delay flag in pa_alsa_safe_delay()

[Georg Chini]

The current code does not call snd_pcm_status_set_audio_htstamp_config()
to configure the way timestamps are updated in ALSA. In kernel 4.14 and
above a bug in ALSA has been fixed which changes timmestamp behavior.
This leads to inconsistencies in the delay reporting because the time
stamp no longer reflects the time when the delay was updated if the
ALSA report_delay flag is not set. Therefore latencies are not calculated
correctly.

This patch uses snd_pcm_status_set_audio_htstamp_config() to set the
ALSA report_delay flag to 1 before the call to snd_pcm_status(). With
this, time stamps are updated as expected.
---

Changes in v2:
 - updated commit message

 src/modules/alsa/alsa-util.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/src/modules/alsa/alsa-util.c b/src/modules/alsa/alsa-util.c
index 61fb4903..b91a0e98 100644
--- a/src/modules/alsa/alsa-util.c
+++ b/src/modules/alsa/alsa-util.c
@@ -1187,6 +1187,7 @@ int pa_alsa_safe_delay(snd_pcm_t *pcm, snd_pcm_status_t *status, snd_pcm_sframes
     size_t abs_k;
     int err;
     snd_pcm_sframes_t avail = 0;
+    snd_pcm_audio_tstamp_config_t tstamp_config;
 
     pa_assert(pcm);
     pa_assert(delay);
@@ -1200,6 +1201,12 @@ int pa_alsa_safe_delay(snd_pcm_t *pcm, snd_pcm_status_t *status, snd_pcm_sframes
      * avail, delay and timestamp values in a single kernel call to improve
      * timer-based scheduling */
 
+    /* The time stamp configuration needs to be set so that the
+     * ALSA code will use the internal delay reported by the driver */
+    tstamp_config.type_requested = 1; /* ALSA default time stamp type */
+    tstamp_config.report_delay = 1;
+    snd_pcm_status_set_audio_htstamp_config(status, &tstamp_config);
+
     if ((err = snd_pcm_status(pcm, status)) < 0)
         return err;
 
-- 
2.16.2