[PATCH 19/21 v2] loopback: Add low_device_latency parameter

[georg@xxxxxxxx (Georg Chini)]

For USB devices the latency jitter strongly depends on device latency. Therefore
a boolean low_device_latency parameter is introduced to half the device latency.
Normally 1/3 of the configured end-to-end latency is used, with the parameter
this is changed to 1/6. In many situations the parameter can improve latency
stability but it will also lead to significantly higher CPU consumption.

---
 src/modules/module-loopback.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/src/modules/module-loopback.c b/src/modules/module-loopback.c
index a9b1b04..4d85ee5 100644
--- a/src/modules/module-loopback.c
+++ b/src/modules/module-loopback.c
@@ -47,6 +47,7 @@ PA_MODULE_USAGE(
         "sink=<sink to connect to> "
         "adjust_time=<how often to readjust rates in s> "
         "latency_msec=<latency in ms> "
+        "low_device_latency=<boolean, use half of the normal device latency> "
         "adjust_threshold_usec=<threshold for latency adjustment in usec> "
         "format=<sample format> "
         "rate=<sample rate> "
@@ -98,6 +99,7 @@ struct userdata {
     pa_usec_t latency;
     pa_usec_t adjust_time;
     uint32_t adjust_threshold;
+    bool low_device_latency;
 
     /* Latency boundaries and current values */
     pa_usec_t min_source_latency;
@@ -190,6 +192,7 @@ static const char* const valid_modargs[] = {
     "sink",
     "adjust_time",
     "latency_msec",
+    "low_device_latency",
     "adjust_threshold_usec",
     "format",
     "rate",
@@ -771,11 +774,14 @@ static void get_effective_source_latency(struct userdata *u, pa_source *source,
  * The choice of one third is rather arbitrary somewhere between the minimum
  * possible latency (which would cause a lot of CPU load) and half the configured
  * latency (which would lead to an empty memblockq if the sink is configured
- * likewise). */
+ * likewise). In low device latency mode set source to one sixth of the overall
+ * latency*/
 static void set_source_output_latency(struct userdata *u, pa_source *source) {
     pa_usec_t latency, requested_latency;
 
     requested_latency = u->latency / 3;
+    if (u->low_device_latency)
+        requested_latency = u->latency / 6;
 
     latency = PA_CLAMP(requested_latency , u->min_source_latency, u->max_source_latency);
     u->configured_source_latency = pa_source_output_set_requested_latency(u->source_output, latency);
@@ -1151,11 +1157,14 @@ static int sink_input_process_msg_cb(pa_msgobject *obj, int code, void *data, in
  * The choice of one third is rather arbitrary somewhere between the minimum
  * possible latency (which would cause a lot of CPU load) and half the configured
  * latency (which would lead to an empty memblockq if the source is configured
- * likewise). */
+ * likewise). In low device latency mode set sink to one sixth of the overall
+ * latency. */
 static void set_sink_input_latency(struct userdata *u, pa_sink *sink) {
      pa_usec_t latency, requested_latency;
 
     requested_latency = u->latency / 3;
+    if (u->low_device_latency)
+        requested_latency = u->latency / 6;
 
     latency = PA_CLAMP(requested_latency , u->min_sink_latency, u->max_sink_latency);
     u->configured_sink_latency = pa_sink_input_set_requested_latency(u->sink_input, latency);
@@ -1484,6 +1493,7 @@ int pa__init(pa_module *m) {
     uint32_t adjust_time_sec;
     const char *n;
     bool remix = true;
+    bool low_device_latency = false;
 
     pa_assert(m);
 
@@ -1565,6 +1575,11 @@ int pa__init(pa_module *m) {
         goto fail;
     }
 
+    if (pa_modargs_get_value_boolean(ma, "low_device_latency", &low_device_latency) < 0) {
+        pa_log("Invalid boolean device latency parameter");
+        goto fail;
+    }
+
     m->userdata = u = pa_xnew0(struct userdata, 1);
     u->core = m->core;
     u->module = m;
@@ -1584,6 +1599,7 @@ int pa__init(pa_module *m) {
     u->latency_error = 0;
     u->adjust_threshold = adjust_threshold;
     u->target_latency_cross_counter = 0;
+    u->low_device_latency = low_device_latency;
     u->initial_adjust_pending = true;
 
     adjust_time_sec = DEFAULT_ADJUST_TIME_USEC / PA_USEC_PER_SEC;
-- 
2.10.1