access/alsa.c: snd_pcm_readi() before snd_pcm_wait()

The alsa provided 'arecord' application tries to read first before doing
a snd_pcm_wait() (aka poll). Doing it the other way results in loss of
capture buffers when the system is under load. This is either a bug in
alsa implementation of snd_pcm_wait() or something else. The disadvantage
is that there seems to be a lot more overruns now.

modules/access/alsa.c

@@ -464,51 +464,13 @@ static void *AudioThread( vlc_object_t *p_this )
 
     do
     {
-        /* Wait for data */
-        int i_wait = snd_pcm_wait( p_sys->p_alsa_pcm, 10 ); /* See poll() comment in oss.c */
-#if 0
-        msg_Info( p_demux, "ALSA snd_pcm_wait returned (%d): %s %"PRId64,
-                  i_wait, snd_strerror(i_wait), mdate() );
-#endif
-        switch( i_wait )
+        p_block = GrabAudio( p_demux );
+        if( p_block )
         {
-            case 1:
-            {
-                p_block = GrabAudio( p_demux );
-                if( p_block )
-                {
-                    block_FifoPut( p_sys->p_grab->p_fifo, p_block );
-                    p_block = NULL;
-                }
-            }
-
-            /* FIXME: this is a copy paste from below. Shouldn't be needed
-             * twice. */
-            case -EPIPE:
-            {
-                /* xrun */
-                snd_pcm_prepare( p_sys->p_alsa_pcm );
-                break;
-            }
-            case -ESTRPIPE:
-            {
-                /* suspend */
-                int i_resume = snd_pcm_resume( p_sys->p_alsa_pcm );
-                if( i_resume < 0 && i_resume != -EAGAIN ) snd_pcm_prepare( p_sys->p_alsa_pcm );
-                break;
-            }
-            /* </FIXME> */
-            default:
-            {
-                if (i_wait == 0)
-                {
-                   snd_pcm_prepare( p_sys->p_alsa_pcm );
-                }
-                else
-                    msg_Err(p_demux, "returned: %d (%s)", i_wait, snd_strerror(i_wait));
-                break;
-            }
+            block_FifoPut( p_sys->p_grab->p_fifo, p_block );
+            p_block = NULL;
         }
+
     } while( vlc_object_alive( p_grab ) );
 
     if( p_block )
@@ -533,7 +495,7 @@ static block_t* GrabAudio( demux_t *p_demux )
     if( !p_block )
     {
         msg_Warn( p_demux, "cannot get block" );
-        return 0;
+        return NULL;
     }
 
     p_sys->p_block = p_block;
@@ -542,11 +504,14 @@ static block_t* GrabAudio( demux_t *p_demux )
     i_read = snd_pcm_readi( p_sys->p_alsa_pcm, p_block->p_buffer, p_sys->i_alsa_chunk_size );
     if( i_read <= 0 )
     {
-        int i_resume;
         switch( i_read )
         {
+            case -EINTR:
             case -EAGAIN:
+            {
+                snd_pcm_wait( p_sys->p_alsa_pcm, 10 ); /* See poll() comment in oss.c */
                 break;
+            }
             case -EPIPE:
             {
                 /* xrun */
@@ -557,13 +522,15 @@ static block_t* GrabAudio( demux_t *p_demux )
                 break;
             }
             case -ESTRPIPE:
+            {
                 /* suspend */
-                i_resume = snd_pcm_resume( p_sys->p_alsa_pcm );
+                int i_resume = snd_pcm_resume( p_sys->p_alsa_pcm );
                 if( i_resume < 0 && i_resume != -EAGAIN ) snd_pcm_prepare( p_sys->p_alsa_pcm );
                 break;
+            }
             default:
                 msg_Err( p_demux, "Failed to read alsa frame (%s)", snd_strerror( i_read ) );
-                return 0;
+                return NULL;
         }
     }
     else
@@ -572,10 +539,10 @@ static block_t* GrabAudio( demux_t *p_demux )
         i_read *= p_sys->i_alsa_frame_size;
     }
 
-    if( i_read <= 0 ) return 0;
+    if( i_read <= 0 ) return NULL;
 
     p_block->i_buffer = i_read;
-    p_sys->p_block = 0;
+    p_sys->p_block = NULL;
 
     /* Correct the date because of kernel buffering */
     i_correct = i_read;