leds: Add support for hardware accelerated LED flashing

Extends the leds subsystem with a blink_set() callback function which can
be optionally implemented by a LED driver. If implemented, the driver can use
the hardware acceleration for blinking a LED.
Signed-off-by: Márton Németh <nm127@freemail.hu>
Signed-off-by: Richard Purdie <rpurdie@rpsys.net>

Documentation/leds-class.txt

@@ -48,6 +48,26 @@ above leaves scope for further attributes should they be needed. If sections
 of the name don't apply, just leave that section blank.
 
 
+Hardware accelerated blink of LEDs
+==================================
+
+Some LEDs can be programmed to blink without any CPU interaction. To
+support this feature, a LED driver can optionally implement the
+blink_set() function (see <linux/leds.h>). If implemeted, triggers can
+attempt to use it before falling back to software timers. The blink_set()
+function should return 0 if the blink setting is supported, or -EINVAL
+otherwise, which means that LED blinking will be handled by software.
+
+The blink_set() function should choose a user friendly blinking
+value if it is called with *delay_on==0 && *delay_off==0 parameters. In
+this case the driver should give back the chosen value through delay_on
+and delay_off parameters to the leds subsystem.
+
+Any call to the brightness_set() callback function should cancel the
+previously programmed hardware blinking function so setting the brightness
+to 0 can also cancel the blinking of the LED.
+
+
 Known Issues
 ============
 
@@ -56,10 +76,6 @@ would cause nightmare dependency issues. I see this as a minor issue
 compared to the benefits the simple trigger functionality brings. The
 rest of the LED subsystem can be modular.
 
-Some leds can be programmed to flash in hardware. As this isn't a generic
-LED device property, this should be exported as a device specific sysfs
-attribute rather than part of the class if this functionality is required.
-
 
 Future Development
 ==================

drivers/leds/Kconfig

@@ -152,7 +152,11 @@ config LEDS_TRIGGER_TIMER
 	depends on LEDS_TRIGGERS
 	help
 	  This allows LEDs to be controlled by a programmable timer
-	  via sysfs. If unsure, say Y.
+	  via sysfs. Some LED hardware can be programmed to start
+	  blinking the LED without any further software interaction.
+	  For more details read Documentation/leds-class.txt.
+
+	  If unsure, say Y.
 
 config LEDS_TRIGGER_IDE_DISK
 	bool "LED IDE Disk Trigger"

drivers/leds/ledtrig-timer.c

@@ -77,8 +77,21 @@ static ssize_t led_delay_on_store(struct device *dev,
 		count++;
 
 	if (count == size) {
-		timer_data->delay_on = state;
-		mod_timer(&timer_data->timer, jiffies + 1);
+		if (timer_data->delay_on != state) {
+			/* the new value differs from the previous */
+			timer_data->delay_on = state;
+
+			/* deactivate previous settings */
+			del_timer_sync(&timer_data->timer);
+
+			/* try to activate hardware acceleration, if any */
+			if (!led_cdev->blink_set ||
+			    led_cdev->blink_set(led_cdev,
+				&timer_data->delay_on, &timer_data->delay_off)) {
+				/* no hardware acceleration, blink via timer */
+				mod_timer(&timer_data->timer, jiffies + 1);
+			}
+		}
 		ret = count;
 	}
 
@@ -110,8 +123,21 @@ static ssize_t led_delay_off_store(struct device *dev,
 		count++;
 
 	if (count == size) {
-		timer_data->delay_off = state;
-		mod_timer(&timer_data->timer, jiffies + 1);
+		if (timer_data->delay_off != state) {
+			/* the new value differs from the previous */
+			timer_data->delay_off = state;
+
+			/* deactivate previous settings */
+			del_timer_sync(&timer_data->timer);
+
+			/* try to activate hardware acceleration, if any */
+			if (!led_cdev->blink_set ||
+			    led_cdev->blink_set(led_cdev,
+				&timer_data->delay_on, &timer_data->delay_off)) {
+				/* no hardware acceleration, blink via timer */
+				mod_timer(&timer_data->timer, jiffies + 1);
+			}
+		}
 		ret = count;
 	}
 
@@ -143,6 +169,13 @@ static void timer_trig_activate(struct led_classdev *led_cdev)
 	if (rc)
 		goto err_out_delayon;
 
+	/* If there is hardware support for blinking, start one
+	 * user friendly blink rate chosen by the driver.
+	 */
+	if (led_cdev->blink_set)
+		led_cdev->blink_set(led_cdev,
+			&timer_data->delay_on, &timer_data->delay_off);
+
 	return;
 
 err_out_delayon:

include/linux/leds.h

@@ -38,6 +38,11 @@ struct led_classdev {
 	void		(*brightness_set)(struct led_classdev *led_cdev,
 					  enum led_brightness brightness);
 
+	/* Activate hardware accelerated blink */
+	int		(*blink_set)(struct led_classdev *led_cdev,
+				     unsigned long *delay_on,
+				     unsigned long *delay_off);
+
 	struct device		*dev;
 	struct list_head	 node;			/* LED Device list */
 	char			*default_trigger;	/* Trigger to use */