pulse: make tlength parameter configurable, reduce fragsize

PipeWire audio agent synchronizes to pacat-simple-vchan rate of taking
data out of vchan and wants to ensure it never ends empty. Pulseaudio
and PipeWire seems to handle them differently (looks like Pulseaudio is
less accurate in this regard). With PipeWire in dom0, it is closer to
what is configured, and theoretically could lead to underruns. Make it
configurable (beyond just disabling low latency mode), so it's easier to
mitigate issues. Note also that PA_STREAM_ADJUST_LATENCY is enabled, so
the tlength avalue includes hardware buffer size, which may differ from
system to system. And also, tlength affects default prebuf value.

While at it, reduce fragsize for recording. Pulseaudio agent (used in
older distributions, and in a stubdomain) uses vchan of 2048 bytes, so
record stream chunked into 4096 doesn't fit. It works only because
pacat-simple-vchan uses blocking vchan writes, but that may negatively
impact parallel playback stream. And also, smaller fargsize may reduce
latency (although it was good enough already).

QubesOS/qubes-issues#8955

Author: marmarek

pulse/pacat-simple-vchan.c

@@ -81,11 +81,11 @@ static pa_sample_spec sample_spec = {
     .rate = 44100,
     .channels = 2
 };
-static const pa_buffer_attr custom_bufattr ={
+static pa_buffer_attr custom_bufattr ={
     .maxlength = (uint32_t)-1,
     .minreq = (uint32_t)-1,
     .prebuf = (uint32_t)-1,
-    .fragsize = 4096,
+    .fragsize = 2048,
     .tlength = 4096
 };
 const pa_buffer_attr * bufattr = NULL;
@@ -1032,12 +1032,14 @@ static void vchan_rec_async_connect(pa_mainloop_api *UNUSED(a),
 
 static _Noreturn void usage(char *arg0, int arg) {
     FILE *stream = arg ? stderr : stdout;
-    fprintf(stream, "usage: %s [-l] [--] domid domname\n",
+    fprintf(stream, "usage: %s [options] [--] domid domname\n",
             arg0 ? arg0 : "pacat-simple-vchan");
     fprintf(stream, "  -l - low-latency mode (higher CPU usage)\n");
     fprintf(stream, "  -n - never block on vchan I/O (overrides previous -b option)\n");
     fprintf(stream, "  -b - always block on vchan I/O (default, overrides previous -n option)\n");
     fprintf(stream, "  -v - verbose logging (a lot of output, may affect performance)\n");
+    fprintf(stream, "  -t size - target playback buffer fill, implies -l, default %d\n",
+            custom_bufattr.tlength);
     fprintf(stream, "  -h - print this message\n");
     if (fflush(NULL) || ferror(stdout) || ferror(stderr))
         exit(1);
@@ -1055,11 +1057,12 @@ int main(int argc, char *argv[])
     int pidfile_fd;
     int play_watch_fd, rec_watch_fd;
     int i;
+    char *endptr;
 
     memset(&u, 0, sizeof(u));
     if (argc <= 2)
         usage(argv[0], 1);
-    while ((i = getopt(argc, argv, "+lnbvh")) != -1) {
+    while ((i = getopt(argc, argv, "+lnbvt:h")) != -1) {
         switch (i) {
             case 'l':
                 bufattr = &custom_bufattr;
@@ -1073,6 +1076,13 @@ int main(int argc, char *argv[])
             case 'v':
                 verbose += 1;
                 break;
+            case 't':
+                bufattr = &custom_bufattr;
+                errno = 0;
+                custom_bufattr.tlength = strtoul(optarg, &endptr, 0);
+                if (*endptr || errno == ERANGE)
+                    err(1, "Invalid -t argument: %s", optarg);
+                break;
             case 'h':
                 usage(argv[0], 0);
             default:
@@ -1082,7 +1092,6 @@ int main(int argc, char *argv[])
     if (argc - optind != 2)
         usage(argv[0], 1);
     const char *domid_str = argv[optind], *domname = argv[optind + 1];
-    char *endptr;
     errno = 0;
     long l_domid = strtol(domid_str, &endptr, 10);
     /* 0x7FF0 is DOMID_FIRST_RESERVED */