alsa: Set the rewind safeguard proportionally to sample spec

Currently when rewinding alsa, a fixed value of 256 bytes is used,
which represents 1.33ms @ 48kHz (2ch, 16bit). This is typically fine
and due to DMA constraints we would not want to rewind less than this.

However with more demanding sample specs, (e.g. 8ch 192kHz 32bit)
256 bytes is likely not sufficient, so calculate what 1.33ms would
be and use which ever value is bigger.

Discussed with David Henningsson and Pierre-Louis Bossart here:
 http://thread.gmane.org/gmane.comp.audio.pulseaudio.general/7286

diff --git a/src/modules/alsa/alsa-sink.c b/src/modules/alsa/alsa-sink.c
index e92c6e5..e62dcc0 100644 (file)
--- a/src/modules/alsa/alsa-sink.c
+++ b/src/modules/alsa/alsa-sink.c
@@ -86,7 +86,8 @@
 
 #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 */
+#define DEFAULT_REWIND_SAFEGUARD_BYTES (256U) /* 1.33ms @48kHz, we'll never rewind less than this */
+#define DEFAULT_REWIND_SAFEGUARD_USEC (1330) /* 1.33ms, depending on channels/rate/sample we may rewind more than 256 above */
 
 struct userdata {
     pa_core *core;
@@ -1740,7 +1741,7 @@ 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;
+    rewind_safeguard = PA_MAX(DEFAULT_REWIND_SAFEGUARD_BYTES, pa_usec_to_bytes(DEFAULT_REWIND_SAFEGUARD_USEC, &ss));
     if (pa_modargs_get_value_u32(ma, "rewind_safeguard", &rewind_safeguard) < 0) {
         pa_log("Failed to parse rewind_safeguard argument");
         goto fail;