hosted: Improve buffer underrun handling in the ALSA driver

* Bump internal mix buffer size by 4x, to 1K frames (matching ALSA period)
 * Handle an underrun that occurs when filling the audio buffer
 * Log underruns and make them available in the debug info

Change-Id: I28d56dd35d88851fa167ad92368a5882937a758f

firmware/export/pcm_mixer.h

@@ -40,6 +40,10 @@
 /* iBasso Devices: Match Rockbox PCM buffer size to ALSA PCM buffer size
    to minimize memory transfers. */
 #define MIX_FRAME_SAMPLES 2048
+#elif (CONFIG_PLATFORM & PLATFORM_HOSTED)
+/* Hosted targets need larger buffers for decent performance due to
+   OS locking/scheduling overhead */
+#define MIX_FRAME_SAMPLES 1024
 #else
 /* Assume HW DMA engine is available or sufficient latency exists in the
    PCM pathway */

firmware/target/hosted/agptek/debug-agptek.c

@@ -62,6 +62,7 @@ bool dbg_hw_info(void)
         }
 
         lcd_putsf(0, line++, "pcm srate: %d", pcm_alsa_get_rate());
+        lcd_putsf(0, line++, "pcm xruns: %d", pcm_alsa_get_xruns());
 #ifdef HAVE_HEADPHONE_DETECTION
         lcd_putsf(0, line++, "hp: %d", headphones_inserted());
 #endif

firmware/target/hosted/pcm-alsa.c

@@ -62,6 +62,12 @@
 #define DEFAULT_PLAYBACK_DEVICE "plughw:0,0"
 #define DEFAULT_CAPTURE_DEVICE "default"
 
+#if MIX_FRAME_SAMPLES < 512
+#error "MIX_FRAME_SAMPLES needs to be at least 512!"
+#elif MIX_FRAME_SAMPLES < 1024
+#warning "MIX_FRAME_SAMPLES <1024 may cause dropouts!"
+#endif
+
 static const snd_pcm_access_t access_ = SND_PCM_ACCESS_RW_INTERLEAVED; /* access mode */
 #if defined(SONY_NWZ_LINUX) || defined(HAVE_FIIO_LINUX_CODEC)
 /* Sony NWZ must use 32-bit per sample */
@@ -83,6 +89,8 @@ static sample_t *frames = NULL;
 static const void  *pcm_data = 0;
 static size_t       pcm_size = 0;
 
+static unsigned int xruns = 0;
+
 static snd_async_handler_t *ahandler = NULL;
 static pthread_mutex_t pcm_mtx;
 static char signal_stack[SIGSTKSZ];
@@ -117,17 +125,21 @@ static int set_hwparams(snd_pcm_t *handle)
     snd_pcm_hw_params_malloc(&params);
 
     /* Size playback buffers based on sample rate.
-       Note these are in FRAMES, and are sized to be about 10ms
+
-       for the buffer and 2.5ms for the period */
+       Buffer size must be at least 4x period size!
+
+       Note these are in FRAMES, and are sized to be about 8.5ms
+       for the buffer and 2.1ms for the period
+     */
     if (pcm_sampr > SAMPR_96) {
-        buffer_size = MIX_FRAME_SAMPLES * 16 * 4; /* 32k */
+        buffer_size = MIX_FRAME_SAMPLES * 4 * 4;
-        period_size = MIX_FRAME_SAMPLES * 4 * 4;  /*  4k */
+        period_size = MIX_FRAME_SAMPLES * 4;
     } else if (pcm_sampr > SAMPR_48) {
-        buffer_size = MIX_FRAME_SAMPLES * 16 * 2; /* 16k */
+        buffer_size = MIX_FRAME_SAMPLES * 2 * 4;
-        period_size = MIX_FRAME_SAMPLES * 4 * 2;  /*  2k */
+        period_size = MIX_FRAME_SAMPLES * 2;
     } else {
-        buffer_size = MIX_FRAME_SAMPLES * 16; /*  4k */
+        buffer_size = MIX_FRAME_SAMPLES * 4;
-        period_size = MIX_FRAME_SAMPLES * 4;  /*  1k */
+        period_size = MIX_FRAME_SAMPLES;
     }
 
     /* choose all parameters */
@@ -407,7 +419,8 @@ static void async_callback(snd_async_handler_t *ahandler)
 
     if (state == SND_PCM_STATE_XRUN)
     {
-        logf("underrun!");
+        xruns++;
+        logf("initial underrun!");
         err = snd_pcm_recover(handle, -EPIPE, 0);
         if (err < 0) {
             logf("XRUN Recovery error: %s", snd_strerror(err));
@@ -432,8 +445,20 @@ static void async_callback(snd_async_handler_t *ahandler)
         {
             if (copy_frames(false))
             {
+            retry:
                 err = snd_pcm_writei(handle, frames, period_size);
-                if (err < 0 && err != period_size && err != -EAGAIN)
+                if (err == -EPIPE)
+                {
+                    logf("mid underrun!");
+                    xruns++;
+                    err = snd_pcm_recover(handle, -EPIPE, 0);
+                    if (err < 0) {
+                       logf("XRUN Recovery error: %s", snd_strerror(err));
+                       goto abort;
+                    }
+                    goto retry;
+                }
+                else if (err != period_size)
                 {
                     logf("Write error: written %i expected %li", err, period_size);
                     break;
@@ -452,7 +477,18 @@ static void async_callback(snd_async_handler_t *ahandler)
         while (snd_pcm_avail_update(handle) >= period_size)
         {
             int err = snd_pcm_readi(handle, frames, period_size);
-            if (err < 0 && err != period_size && err != -EAGAIN)
+            if (err == -EPIPE)
+            {
+                logf("rec mid underrun!");
+                xruns++;
+                err = snd_pcm_recover(handle, -EPIPE, 0);
+                if (err < 0) {
+                   logf("XRUN Recovery error: %s", snd_strerror(err));
+                   goto abort;
+                }
+		continue;  /* buffer contents trashed, no sense in trying to copy */
+            }
+            else if (err != period_size)
             {
                 logf("Read error: read %i expected %li", err, period_size);
                 break;
@@ -751,11 +787,16 @@ void pcm_play_dma_postinit(void)
 #endif
 }
 
-int pcm_alsa_get_rate(void)
+unsigned int pcm_alsa_get_rate(void)
 {
     return real_sample_rate;
 }
 
+unsigned int pcm_alsa_get_xruns(void)
+{
+    return xruns;
+}
+
 #ifdef HAVE_RECORDING
 void pcm_rec_lock(void)
 {

firmware/target/hosted/pcm-alsa.h

@@ -1,10 +1,10 @@
 /***************************************************************************
- *             __________               __   ___.                  
+ *             __________               __   ___.
- *   Open      \______   \ ____   ____ |  | _\_ |__   _______  ___  
+ *   Open      \______   \ ____   ____ |  | _\_ |__   _______  ___
- *   Source     |       _//  _ \_/ ___\|  |/ /| __ \ /  _ \  \/  /  
+ *   Source     |       _//  _ \_/ ___\|  |/ /| __ \ /  _ \  \/  /
- *   Jukebox    |    |   (  <_> )  \___|    < | \_\ (  <_> > <  <   
+ *   Jukebox    |    |   (  <_> )  \___|    < | \_\ (  <_> > <  <
- *   Firmware   |____|_  /\____/ \___  >__|_ \|___  /\____/__/\_ \  
+ *   Firmware   |____|_  /\____/ \___  >__|_ \|___  /\____/__/\_ \
- *                     \/            \/     \/    \/            \/ 
+ *                     \/            \/     \/    \/            \/
  *
  * Copyright (C) 2016 Amaury Pouly
  *
@@ -34,6 +34,7 @@ void pcm_alsa_set_playback_device(const char *device);
 void pcm_alsa_set_capture_device(const char *device);
 #endif
 
-int pcm_alsa_get_rate(void);
+unsigned int pcm_alsa_get_rate(void);
+unsigned int pcm_alsa_get_xruns(void);
 
 #endif /* __PCM_ALSA_RB_H__ */