ALSA: pcm: support for period wakeup disabling

This patch allows to disable period interrupts which are
not needed when the application relies on a system timer
to wake-up and refill the ring buffer. The behavior of
the driver is left unchanged, and interrupts are only
disabled if the application requests this configuration.
The behavior in case of underruns is slightly different,
instead of being detected during the period interrupts the
underruns are detected when the application calls
snd_pcm_update_avail, which in turns forces a refresh of the
hw pointer and shows the buffer is empty.

More specifically this patch makes a lot of sense when
PulseAudio relies on timer-based scheduling to access audio
devices such as HDAudio or Intel SST. Disabling interrupts
removes two unwanted wake-ups due to period elapsed events
in low-power playback modes. It also simplifies PulseAudio
voice modules used for speech calls.

To quote Lennart "This patch looks very interesting and
desirable. This is something have long been waiting for."

Support for this in hardware drivers is optional.

Signed-off-by: Pierre-Louis Bossart <[e-mail]>
Signed-off-by: Clemens Ladisch <[e-mail]>
Signed-off-by: Takashi Iwai <[e-mail]>

include/sound/asound.h

···
 #define SNDRV_PCM_INFO_HALF_DUPLEX	0x00100000	/* only half duplex */
 #define SNDRV_PCM_INFO_JOINT_DUPLEX	0x00200000	/* playback and capture stream are somewhat correlated */
 #define SNDRV_PCM_INFO_SYNC_START	0x00400000	/* pcm support some kind of sync go */
+#define SNDRV_PCM_INFO_NO_PERIOD_WAKEUP	0x00800000	/* period wakeup can be disabled */
 #define SNDRV_PCM_INFO_FIFO_IN_FRAMES	0x80000000	/* internal kernel flag - FIFO size is in frames */
 
 typedef int __bitwise snd_pcm_state_t;
···
 #define	SNDRV_PCM_HW_PARAM_LAST_INTERVAL	SNDRV_PCM_HW_PARAM_TICK_TIME
 
 #define SNDRV_PCM_HW_PARAMS_NORESAMPLE	(1<<0)	/* avoid rate resampling */
+#define SNDRV_PCM_HW_PARAMS_EXPORT_BUFFER	(1<<1)	/* export buffer */
+#define SNDRV_PCM_HW_PARAMS_NO_PERIOD_WAKEUP	(1<<2)	/* disable period wakeups */
 
 struct snd_interval {
 	unsigned int min, max;

include/sound/pcm.h

···
 	unsigned int info;
 	unsigned int rate_num;
 	unsigned int rate_den;
+	unsigned int no_period_wakeup: 1;
 
 	/* -- SW params -- */
 	int tstamp_mode;		/* mmap timestamp is updated */

sound/core/pcm_lib.c

···
 			   (unsigned long)new_hw_ptr,
 			   (unsigned long)runtime->hw_ptr_base);
 	}
+
+	/* without period interrupts, there are no regular pointer updates */
+	if (runtime->no_period_wakeup)
+		goto no_delta_check;
+
 	/* something must be really wrong */
 	if (delta >= runtime->buffer_size + runtime->period_size) {
 		hw_ptr_error(substream,
···
 			     (long)old_hw_ptr);
 	}
 
+ no_delta_check:
 	if (runtime->status->hw_ptr == new_hw_ptr)
 		return 0;
 

sound/core/pcm_native.c

···
 	runtime->info = params->info;
 	runtime->rate_num = params->rate_num;
 	runtime->rate_den = params->rate_den;
+	runtime->no_period_wakeup =
+			(params->info & SNDRV_PCM_INFO_NO_PERIOD_WAKEUP) &&
+			(params->flags & SNDRV_PCM_HW_PARAMS_NO_PERIOD_WAKEUP);
 
 	bits = snd_pcm_format_physical_width(runtime->format);
 	runtime->sample_bits = bits;