[PATCH v6 23/25] 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 77a5c04..35a817b 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, values >= 100 are in ms> "
         "latency_msec=<latency in ms> "
+        "low_device_latency=<boolean, use half of the normal device latency> "
         "format=<sample format> "
         "rate=<sample rate> "
         "channels=<number of channels> "
@@ -97,6 +98,7 @@ struct userdata {
     /* Values from command line configuration */
     pa_usec_t latency;
     pa_usec_t adjust_time;
+    bool low_device_latency;
 
     /* Latency boundaries and current values */
     pa_usec_t min_source_latency;
@@ -149,6 +151,7 @@ static const char* const valid_modargs[] = {
     "sink",
     "adjust_time",
     "latency_msec",
+    "low_device_latency",
     "format",
     "rate",
     "channels",
@@ -590,11 +593,14 @@ static void update_latency_boundaries(struct userdata *u, pa_source *source, pa_
     update_minimum_latency(u);
 }
 
-/* Set source output latency to one third of the overall latency if possible */
+/* In low device latency mode set source to one sixth of the overall latency,
+ * else use one third to save CPU */
 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);
@@ -887,11 +893,14 @@ static int sink_input_process_msg_cb(pa_msgobject *obj, int code, void *data, in
     return pa_sink_input_process_msg(obj, code, data, offset, chunk);
 }
 
-/* Set sink input latency to one third of the overall latency if possible */
+/* In low device latency mode set sink to one sixth of the overall latency,
+ * else use one third to save CPU */
 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);
@@ -1110,6 +1119,7 @@ int pa__init(pa_module *m) {
     uint32_t adjust_time;
     const char *n;
     bool remix = true;
+    bool low_device_latency = false;
 
     pa_assert(m);
 
@@ -1185,6 +1195,11 @@ 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;
@@ -1196,6 +1211,7 @@ int pa__init(pa_module *m) {
     u->underrun_occured = false;
     u->extra_latency = 0;
     u->latency_error = 0;
+    u->low_device_latency = low_device_latency;
 
     /* The default adjust time is set to 1s. The adjust time given on the
      * command line is considered to be in seconds if it is below 100,
-- 
2.8.1