ALSA: pcm_lib - add possibility to log last 10 DMA ring buffer positions

In some debug cases, it might be usefull to see previous ring buffer
positions to determine position problems from the lowlevel drivers.

Signed-off-by: Jaroslav Kysela <perex@perex.cz>

include/sound/pcm.h

@@ -262,6 +262,8 @@ struct snd_pcm_hw_constraint_list {
 	unsigned int mask;
 };
 
+struct snd_pcm_hwptr_log;
+
 struct snd_pcm_runtime {
 	/* -- Status -- */
 	struct snd_pcm_substream *trigger_master;
@@ -340,6 +342,10 @@ struct snd_pcm_runtime {
 	/* -- OSS things -- */
 	struct snd_pcm_oss_runtime oss;
 #endif
+
+#ifdef CONFIG_SND_PCM_XRUN_DEBUG
+	struct snd_pcm_hwptr_log *hwptr_log;
+#endif
 };
 
 struct snd_pcm_group {		/* keep linked substreams */