Re: [Alsa-devel] alsa timer slippage

[Jaroslav Kysela]

On Mon, 15 Dec 2003, Chris Cannam wrote:

> On Monday 15 Dec 2003 9:16 am, Chris Cannam wrote:
> > This seems to suggest that the default system-based timer is doing
> > something a bit naive, like assuming it was called back on time
> > without actually checking.
> 
> Hmm yes, even this crappy little patch to alsa-kernel/core/timer.c 
> (gross adjustment to each system timer tick according to how late the 
> previous one was) improves things for me.  With this the timer still 
> slips, but never quite enough to be audible in a MIDI-only stream.  
> The PCM clock is still much much better though.
> 
> @@ -940,9 +940,16 @@
>         return 1000000000L / HZ;
>  }
> 
> +static long lateness;
> +
>  static void snd_timer_s_function(unsigned long data)
>  {
> +       struct timer_list *tlist;
>         snd_timer_t *timer = (snd_timer_t *)data;
> +
> +       tlist = (struct timer_list *) timer->private_data;
> +       lateness = (long)jiffies - (long)tlist->expires;
> +
>         snd_timer_interrupt(timer, timer->sticks);
>  }
> 
> @@ -951,7 +958,7 @@
>         struct timer_list *tlist;
> 
>         tlist = (struct timer_list *) timer->private_data;
> -       tlist->expires = jiffies + timer->sticks;
> +       tlist->expires = jiffies + timer->sticks - lateness;
>         add_timer(tlist);
>         return 0;
>  }

Thanks for insigh to the problem. Can you try the attached patch?
(It's also in CVS.)

                                                Jaroslav

-----
Jaroslav Kysela <[EMAIL PROTECTED]>
Linux Kernel Sound Maintainer
ALSA Project, SuSE Labs
Index: timer.c
===================================================================
RCS file: /cvsroot/alsa/alsa-kernel/core/timer.c,v
retrieving revision 1.52
retrieving revision 1.55
diff -u -r1.52 -r1.55
--- timer.c     23 Oct 2003 14:34:52 -0000      1.52
+++ timer.c     17 Dec 2003 15:14:33 -0000      1.55
@@ -935,6 +935,14 @@
  *  System timer
  */
 
+struct snd_timer_system_private {
+       struct timer_list tlist;
+       struct timer * timer;
+       unsigned long last_expires;
+       unsigned long last_jiffies;
+       unsigned long correction;
+};
+
 unsigned int snd_timer_system_resolution(void)
 {
        return 1000000000L / HZ;
@@ -943,26 +951,44 @@
 static void snd_timer_s_function(unsigned long data)
 {
        snd_timer_t *timer = (snd_timer_t *)data;
-       snd_timer_interrupt(timer, timer->sticks);
+       struct snd_timer_system_private *priv = timer->private_data;
+       unsigned long jiff = jiffies;
+       if (time_after(jiff, priv->last_expires))
+               priv->correction = (long)jiff - (long)priv->last_expires;
+       snd_timer_interrupt(timer, (long)jiff - (long)priv->last_jiffies);
 }
 
 static int snd_timer_s_start(snd_timer_t * timer)
 {
-       struct timer_list *tlist;
+       struct snd_timer_system_private *priv;
+       unsigned long njiff;
 
-       tlist = (struct timer_list *) timer->private_data;
-       tlist->expires = jiffies + timer->sticks;
-       add_timer(tlist);
+       priv = (struct snd_timer_system_private *) timer->private_data;
+       njiff = (priv->last_jiffies = jiffies);
+       if (priv->correction > timer->sticks - 1) {
+               priv->correction -= timer->sticks - 1;
+               njiff++;
+       } else {
+               njiff += timer->sticks - priv->correction;
+               priv->correction -= timer->sticks;
+       }
+       priv->last_expires = priv->tlist.expires = njiff;
+       add_timer(&priv->tlist);
        return 0;
 }
 
 static int snd_timer_s_stop(snd_timer_t * timer)
 {
-       struct timer_list *tlist;
+       struct snd_timer_system_private *priv;
+       unsigned long jiff;
 
-       tlist = (struct timer_list *) timer->private_data;
-       del_timer(tlist);
-       timer->sticks = tlist->expires - jiffies;
+       priv = (struct snd_timer_system_private *) timer->private_data;
+       del_timer(&priv->tlist);
+       jiff = jiffies;
+       if (time_before(jiff, priv->last_expires))
+               timer->sticks = priv->last_expires - jiff;
+       else
+               timer->sticks = 1;
        return 0;
 }
 
@@ -984,22 +1010,22 @@
 static int snd_timer_register_system(void)
 {
        snd_timer_t *timer;
-       struct timer_list *tlist;
+       struct snd_timer_system_private *priv;
        int err;
 
        if ((err = snd_timer_global_new("system", SNDRV_TIMER_GLOBAL_SYSTEM, &timer)) 
< 0)
                return err;
        strcpy(timer->name, "system timer");
        timer->hw = snd_timer_system;
-       tlist = (struct timer_list *) snd_kcalloc(sizeof(struct timer_list), 
GFP_KERNEL);
-       if (tlist == NULL) {
+       priv = (struct snd_timer_system_private *) snd_kcalloc(sizeof(struct 
snd_timer_system_private), GFP_KERNEL);
+       if (priv == NULL) {
                snd_timer_free(timer);
                return -ENOMEM;
        }
-       init_timer(tlist);
-       tlist->function = snd_timer_s_function;
-       tlist->data = (unsigned long) timer;
-       timer->private_data = tlist;
+       init_timer(&priv->tlist);
+       priv->tlist.function = snd_timer_s_function;
+       priv->tlist.data = (unsigned long) timer;
+       timer->private_data = priv;
        timer->private_free = snd_timer_free_system;
        return snd_timer_global_register(timer);
 }