[pulseaudio-discuss] [PATCH] alsa: Don't disable timer-based scheduling on USB devices

[arun at accosted.net]

From: Arun Raghavan <git at arunraghavan.net>

This isn't a great fix, but we need ALSA API to do this right. In the
mean time, USB devices work fine with timer-based scheduling, so there's
no reason to force a large minimum latency by disabling tsched on them.
---
 src/modules/alsa/alsa-util.c | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/src/modules/alsa/alsa-util.c b/src/modules/alsa/alsa-util.c
index bb79e71..a4bb449 100644
--- a/src/modules/alsa/alsa-util.c
+++ b/src/modules/alsa/alsa-util.c
@@ -261,8 +261,25 @@ int pa_alsa_set_hw_params(
 
     /* The PCM pointer is only updated with period granularity */
     if (snd_pcm_hw_params_is_batch(hwparams)) {
-        pa_log_info("Disabling tsched mode since BATCH flag is set");
-        _use_tsched = false;
+        bool is_usb = false;
+        const char *id;
+        snd_pcm_info_t* pcm_info;
+        snd_pcm_info_alloca(&pcm_info);
+
+        if (snd_pcm_info(pcm_handle, pcm_info) == 0 &&
+            (id = snd_pcm_info_get_id(pcm_info))) {
+            /* This horrible hack makes sure we don't disable tsched on USB
+             * devices, which have a low enough transfer size for timer-based
+             * scheduling to work. This can go away when the ALSA API supprots
+             * querying the block transfer size. */
+            if (pa_streq(id, "USB Audio"))
+                is_usb = true;
+        }
+
+        if (!is_usb) {
+            pa_log_info("Disabling tsched mode since BATCH flag is set");
+            _use_tsched = false;
+        }
     }
 
 #if (SND_LIB_VERSION >= ((1<<16)|(0<<8)|24)) /* API additions in 1.0.24 */
-- 
2.5.0