ALSA: snd-aloop - improve the sample copy accurracy

Maintain both streams (playback, capture) synchronized. Previous code
didn't take in account the small byte count drifts caused by the irq
position rounding.

Signed-off-by: Jaroslav Kysela <perex@perex.cz>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/drivers/aloop.c b/sound/drivers/aloop.c
index ad079b6..8b5c36f 100644 (file)
--- a/sound/drivers/aloop.c
+++ b/sound/drivers/aloop.c
@@ -117,6 +117,7 @@ struct loopback_pcm {
        /* timer stuff */
        unsigned int irq_pos;           /* fractional IRQ position */
        unsigned int period_size_frac;
+       unsigned int last_drift;
        unsigned long last_jiffies;
        struct timer_list timer;
 };
@@ -264,6 +265,7 @@ static int loopback_trigger(struct snd_pcm_substream *substream, int cmd)
                        return err;
                dpcm->last_jiffies = jiffies;
                dpcm->pcm_rate_shift = 0;
+               dpcm->last_drift = 0;
                spin_lock(&cable->lock);        
                cable->running |= stream;
                cable->pause &= ~stream;
@@ -444,34 +446,30 @@ static void copy_play_buf(struct loopback_pcm *play,
        }
 }
 
-#define BYTEPOS_UPDATE_POSONLY 0
-#define BYTEPOS_UPDATE_CLEAR   1
-#define BYTEPOS_UPDATE_COPY    2
-
-static void loopback_bytepos_update(struct loopback_pcm *dpcm,
-                                   unsigned int delta,
-                                   unsigned int cmd)
+static inline unsigned int bytepos_delta(struct loopback_pcm *dpcm,
+                                        unsigned int jiffies_delta)
 {
-       unsigned int count;
        unsigned long last_pos;
+       unsigned int delta;
 
        last_pos = byte_pos(dpcm, dpcm->irq_pos);
-       dpcm->irq_pos += delta * dpcm->pcm_bps;
-       count = byte_pos(dpcm, dpcm->irq_pos) - last_pos;
-       if (!count)
-               return;
-       if (cmd == BYTEPOS_UPDATE_CLEAR)
-               clear_capture_buf(dpcm, count);
-       else if (cmd == BYTEPOS_UPDATE_COPY)
-               copy_play_buf(dpcm->cable->streams[SNDRV_PCM_STREAM_PLAYBACK],
-                             dpcm->cable->streams[SNDRV_PCM_STREAM_CAPTURE],
-                             count);
-       dpcm->buf_pos += count;
-       dpcm->buf_pos %= dpcm->pcm_buffer_size;
+       dpcm->irq_pos += jiffies_delta * dpcm->pcm_bps;
+       delta = byte_pos(dpcm, dpcm->irq_pos) - last_pos;
+       if (delta >= dpcm->last_drift)
+               delta -= dpcm->last_drift;
+       dpcm->last_drift = 0;
        if (dpcm->irq_pos >= dpcm->period_size_frac) {
                dpcm->irq_pos %= dpcm->period_size_frac;
                dpcm->period_update_pending = 1;
        }
+       return delta;
+}
+
+static inline void bytepos_finish(struct loopback_pcm *dpcm,
+                                 unsigned int delta)
+{
+       dpcm->buf_pos += delta;
+       dpcm->buf_pos %= dpcm->pcm_buffer_size;
 }
 
 static unsigned int loopback_pos_update(struct loopback_cable *cable)
@@ -481,7 +479,7 @@ static unsigned int loopback_pos_update(struct loopback_cable *cable)
        struct loopback_pcm *dpcm_capt =
                        cable->streams[SNDRV_PCM_STREAM_CAPTURE];
        unsigned long delta_play = 0, delta_capt = 0;
-       unsigned int running;
+       unsigned int running, count1, count2;
        unsigned long flags;
 
        spin_lock_irqsave(&cable->lock, flags);
@@ -500,12 +498,13 @@ static unsigned int loopback_pos_update(struct loopback_cable *cable)
                goto unlock;
                
        if (delta_play > delta_capt) {
-               loopback_bytepos_update(dpcm_play, delta_play - delta_capt,
-                                       BYTEPOS_UPDATE_POSONLY);
+               count1 = bytepos_delta(dpcm_play, delta_play - delta_capt);
+               bytepos_finish(dpcm_play, count1);
                delta_play = delta_capt;
        } else if (delta_play < delta_capt) {
-               loopback_bytepos_update(dpcm_capt, delta_capt - delta_play,
-                                       BYTEPOS_UPDATE_CLEAR);
+               count1 = bytepos_delta(dpcm_capt, delta_capt - delta_play);
+               clear_capture_buf(dpcm_capt, count1);
+               bytepos_finish(dpcm_capt, count1);
                delta_capt = delta_play;
        }
 
@@ -513,8 +512,17 @@ static unsigned int loopback_pos_update(struct loopback_cable *cable)
                goto unlock;
 
        /* note delta_capt == delta_play at this moment */
-       loopback_bytepos_update(dpcm_capt, delta_capt, BYTEPOS_UPDATE_COPY);
-       loopback_bytepos_update(dpcm_play, delta_play, BYTEPOS_UPDATE_POSONLY);
+       count1 = bytepos_delta(dpcm_play, delta_play);
+       count2 = bytepos_delta(dpcm_capt, delta_capt);
+       if (count1 < count2) {
+               dpcm_capt->last_drift = count2 - count1;
+               count1 = count2;
+       } else if (count1 > count2) {
+               dpcm_play->last_drift = count1 - count2;
+       }
+       copy_play_buf(dpcm_play, dpcm_capt, count1);
+       bytepos_finish(dpcm_play, count1);
+       bytepos_finish(dpcm_capt, count1);
  unlock:
        spin_unlock_irqrestore(&cable->lock, flags);
        return running;