[ALSA] rtctimer: add option to make RTC timer the default sequencer timer

ALSA Core,ALSA sequencer
Add an option to make the RTC timer the default sequencer timer.  This
becomes necessary for precise MIDI timing when the system timer runs at
less than 1000 Hz.
Signed-off-by: NClemens Ladisch <clemens@ladisch.de>

sound/core/Kconfig

@@ -99,6 +99,18 @@ config SND_RTCTIMER
 	  To compile this driver as a module, choose M here: the module
 	  will be called snd-rtctimer.
 
+config SND_SEQ_RTCTIMER_DEFAULT
+	bool "Use RTC as default sequencer timer"
+	depends on SND_RTCTIMER && SND_SEQUENCER
+	default y
+	help
+	  Say Y here to use the RTC timer as the default sequencer
+	  timer.  This is strongly recommended because it ensures
+	  precise MIDI timing even when the system timer runs at less
+	  than 1000 Hz.
+
+	  If in doubt, say Y.
+
 config SND_VERBOSE_PRINTK
 	bool "Verbose printk"
 	depends on SND

sound/core/seq/seq.c

@@ -43,7 +43,13 @@ int seq_client_load[64] = {[0 ... 63] = -1};
 int seq_default_timer_class = SNDRV_TIMER_CLASS_GLOBAL;
 int seq_default_timer_sclass = SNDRV_TIMER_SCLASS_NONE;
 int seq_default_timer_card = -1;
-int seq_default_timer_device = SNDRV_TIMER_GLOBAL_SYSTEM;
+int seq_default_timer_device =
+#ifdef CONFIG_SND_SEQ_RTCTIMER_DEFAULT
+	SNDRV_TIMER_GLOBAL_RTC
+#else
+	SNDRV_TIMER_GLOBAL_SYSTEM
+#endif
+	;
 int seq_default_timer_subdevice = 0;
 int seq_default_timer_resolution = 0;	/* Hz */