ALSA: Enable PCM hw_ptr_jiffies check only in xrun_debug mode

The PCM hw_ptr jiffies check results sometimes in problems when a
hardware doesn't give smooth hw_ptr updates.  So far, au88x0 and some
other drivers appear not working due to this strict check.
However, this check is a nice debug tool, and the capability should be
still kept.

Hence, we disable this check now as default unless the user enables it
by setting the xrun_debug mode to the specific stream via a proc file.

Signed-off-by: Takashi Iwai <tiwai@suse.de>

Documentation/sound/alsa/Procfile.txt

@@ -104,6 +104,11 @@ card*/pcm*/xrun_debug
 	When this value is greater than 1, the driver will show the
 	stack trace additionally.  This may help the debugging.
 
+	Since 2.6.30, this option also enables the hwptr check using
+	jiffies.  This detects spontaneous invalid pointer callback
+	values, but can be lead to too much corrections for a (mostly
+	buggy) hardware that doesn't give smooth pointer updates.
+
 card*/pcm*/sub*/info
 	The general information of this PCM sub-stream.
 

sound/core/pcm_lib.c

@@ -249,6 +249,11 @@ static int snd_pcm_update_hw_ptr_interrupt(struct snd_pcm_substream *substream)
 			new_hw_ptr = hw_base + pos;
 		}
 	}
+
+	/* Do jiffies check only in xrun_debug mode */
+	if (!xrun_debug(substream))
+		goto no_jiffies_check;
+
 	/* Skip the jiffies check for hardwares with BATCH flag.
 	 * Such hardware usually just increases the position at each IRQ,
 	 * thus it can't give any strange position.
@@ -336,7 +341,9 @@ int snd_pcm_update_hw_ptr(struct snd_pcm_substream *substream)
 			hw_base = 0;
 		new_hw_ptr = hw_base + pos;
 	}
-	if (((delta * HZ) / runtime->rate) > jdelta + HZ/100) {
+	/* Do jiffies check only in xrun_debug mode */
+	if (xrun_debug(substream) &&
+	    ((delta * HZ) / runtime->rate) > jdelta + HZ/100) {
 		hw_ptr_error(substream,
 			     "hw_ptr skipping! "
 			     "(pos=%ld, delta=%ld, period=%ld, jdelta=%lu/%lu)\n",