add rewind-safeguard parameter

Rewinding the ring buffer completely causes audible issues with DMAs.
Previous solution didn't work with tsched=0, and used tsched_watermark
for guardband, which isn't linked to hardware and could become really high
if underflows occurred.

Added separate parameter that can be tuned to hardware limitations and size
of DMA bursts.

2 files changed, 17 insertions, 5 deletions

diff --git a/src/modules/alsa/alsa-sink.c b/src/modules/alsa/alsa-sink.c
index 3b13cc27..41ddce71 100644
--- a/src/modules/alsa/alsa-sink.c
+++ b/src/modules/alsa/alsa-sink.c
@@ -83,6 +83,8 @@
 
 #define VOLUME_ACCURACY (PA_VOLUME_NORM/100)  /* don't require volume adjustments to be perfectly correct. don't necessarily extend granularity in software unless the differences get greater than this level */
 
+#define DEFAULT_REWIND_SAFEGUARD_BYTES (256) /* 1.33ms @48kHz, should work for most hardware */
+
 struct userdata {
     pa_core *core;
     pa_module *module;
@@ -112,7 +114,8 @@ struct userdata {
         watermark_inc_step,
         watermark_dec_step,
         watermark_inc_threshold,
-        watermark_dec_threshold;
+        watermark_dec_threshold,
+        rewind_safeguard;
 
     pa_usec_t watermark_dec_not_before;
 
@@ -1320,8 +1323,8 @@ static int process_rewind(struct userdata *u) {
 
     unused_nbytes = (size_t) unused * u->frame_size;
 
-    if (u->use_tsched)
-        unused_nbytes += u->tsched_watermark;
+    /* make sure rewind doesn't go too far, can cause issues with DMAs */
+    unused_nbytes += u->rewind_safeguard;
 
     if (u->hwbuf_size > unused_nbytes)
         limit_nbytes = u->hwbuf_size - unused_nbytes;
@@ -1676,7 +1679,7 @@ pa_sink *pa_alsa_sink_new(pa_module *m, pa_modargs *ma, const char*driver, pa_ca
     pa_channel_map map;
     uint32_t nfrags, frag_size, buffer_size, tsched_size, tsched_watermark;
     snd_pcm_uframes_t period_frames, buffer_frames, tsched_frames;
-    size_t frame_size;
+    size_t frame_size, rewind_safeguard;
     pa_bool_t use_mmap = TRUE, b, use_tsched = TRUE, d, ignore_dB = FALSE;
     pa_sink_new_data data;
     pa_alsa_profile_set *profile_set = NULL;
@@ -1730,6 +1733,12 @@ pa_sink *pa_alsa_sink_new(pa_module *m, pa_modargs *ma, const char*driver, pa_ca
         goto fail;
     }
 
+    rewind_safeguard = DEFAULT_REWIND_SAFEGUARD_BYTES;
+    if (pa_modargs_get_value_u32(ma, "rewind_safeguard", &rewind_safeguard) < 0) {
+        pa_log("Failed to parse rewind_safeguard argument");
+        goto fail;
+    }
+
     use_tsched = pa_alsa_may_tsched(use_tsched);
 
     u = pa_xnew0(struct userdata, 1);
@@ -1738,6 +1747,7 @@ pa_sink *pa_alsa_sink_new(pa_module *m, pa_modargs *ma, const char*driver, pa_ca
     u->use_mmap = use_mmap;
     u->use_tsched = use_tsched;
     u->first = TRUE;
+    u->rewind_safeguard = rewind_safeguard;
     u->rtpoll = pa_rtpoll_new();
     pa_thread_mq_init(&u->thread_mq, m->core->mainloop, u->rtpoll);
 
diff --git a/src/modules/alsa/module-alsa-sink.c b/src/modules/alsa/module-alsa-sink.c
index 3aa89b2a..a73274fd 100644
--- a/src/modules/alsa/module-alsa-sink.c
+++ b/src/modules/alsa/module-alsa-sink.c
@@ -54,7 +54,8 @@ PA_MODULE_USAGE(
         "tsched_buffer_size=<buffer size when using timer based scheduling> "
         "tsched_buffer_watermark=<lower fill watermark> "
         "ignore_dB=<ignore dB information from the device?> "
-        "control=<name of mixer control>");
+        "control=<name of mixer control>"
+        "rewind_safeguard=<number of bytes that cannot be rewound");
 
 static const char* const valid_modargs[] = {
     "name",
@@ -74,6 +75,7 @@ static const char* const valid_modargs[] = {
     "tsched_buffer_watermark",
     "ignore_dB",
     "control",
+    "rewind_safeguard",
     NULL
 };