backlight: ktd253: Stabilize backlight

Remove interrupt disablement during backlight setting. It is
way to dangerous and makes platforms instable by having it
miss vblank IRQs leading to the graphics derailing.

The code is using ndelay() which is not available on
platforms such as ARM and will result in 32 * udelay(1)
which is substantial.

Add some code to detect if an interrupt occurs during the
tight loop and in that case just redo it from the top.

Fixes: 5317f37e ("backlight: Add Kinetic KTD253 backlight driver")
Cc: Stephan Gerhold <stephan@gerhold.net>
Reported-by: newbyte@disroot.org
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/video/backlight/ktd253-backlight.c

@@ -25,6 +25,7 @@
 
 #define KTD253_T_LOW_NS (200 + 10) /* Additional 10ns as safety factor */
 #define KTD253_T_HIGH_NS (200 + 10) /* Additional 10ns as safety factor */
+#define KTD253_T_OFF_CRIT_NS 100000 /* 100 us, now it doesn't look good */
 #define KTD253_T_OFF_MS 3
 
 struct ktd253_backlight {
@@ -34,13 +35,50 @@ struct ktd253_backlight {
 	u16 ratio;
 };
 
+static void ktd253_backlight_set_max_ratio(struct ktd253_backlight *ktd253)
+{
+	gpiod_set_value_cansleep(ktd253->gpiod, 1);
+	ndelay(KTD253_T_HIGH_NS);
+	/* We always fall back to this when we power on */
+}
+
+static int ktd253_backlight_stepdown(struct ktd253_backlight *ktd253)
+{
+	/*
+	 * These GPIO operations absolutely can NOT sleep so no _cansleep
+	 * suffixes, and no using GPIO expanders on slow buses for this!
+	 *
+	 * The maximum number of cycles of the loop is 32  so the time taken
+	 * should nominally be:
+	 * (T_LOW_NS + T_HIGH_NS + loop_time) * 32
+	 *
+	 * Architectures do not always support ndelay() and we will get a few us
+	 * instead. If we get to a critical time limit an interrupt has likely
+	 * occured in the low part of the loop and we need to restart from the
+	 * top so we have the backlight in a known state.
+	 */
+	u64 ns;
+
+	ns = ktime_get_ns();
+	gpiod_set_value(ktd253->gpiod, 0);
+	ndelay(KTD253_T_LOW_NS);
+	gpiod_set_value(ktd253->gpiod, 1);
+	ns = ktime_get_ns() - ns;
+	if (ns >= KTD253_T_OFF_CRIT_NS) {
+		dev_err(ktd253->dev, "PCM on backlight took too long (%llu ns)\n", ns);
+		return -EAGAIN;
+	}
+	ndelay(KTD253_T_HIGH_NS);
+	return 0;
+}
+
 static int ktd253_backlight_update_status(struct backlight_device *bl)
 {
 	struct ktd253_backlight *ktd253 = bl_get_data(bl);
 	int brightness = backlight_get_brightness(bl);
 	u16 target_ratio;
 	u16 current_ratio = ktd253->ratio;
-	unsigned long flags;
+	int ret;
 
 	dev_dbg(ktd253->dev, "new brightness/ratio: %d/32\n", brightness);
 
@@ -62,37 +100,34 @@ static int ktd253_backlight_update_status(struct backlight_device *bl)
 	}
 
 	if (current_ratio == 0) {
-		gpiod_set_value_cansleep(ktd253->gpiod, 1);
-		ndelay(KTD253_T_HIGH_NS);
-		/* We always fall back to this when we power on */
+		ktd253_backlight_set_max_ratio(ktd253);
 		current_ratio = KTD253_MAX_RATIO;
 	}
 
-	/*
-	 * WARNING:
-	 * The loop to set the correct current level is performed
-	 * with interrupts disabled as it is timing critical.
-	 * The maximum number of cycles of the loop is 32
-	 * so the time taken will be (T_LOW_NS + T_HIGH_NS + loop_time) * 32,
-	 */
-	local_irq_save(flags);
 	while (current_ratio != target_ratio) {
 		/*
 		 * These GPIO operations absolutely can NOT sleep so no
 		 * _cansleep suffixes, and no using GPIO expanders on
 		 * slow buses for this!
 		 */
-		gpiod_set_value(ktd253->gpiod, 0);
-		ndelay(KTD253_T_LOW_NS);
-		gpiod_set_value(ktd253->gpiod, 1);
-		ndelay(KTD253_T_HIGH_NS);
-		/* After 1/32 we loop back to 32/32 */
-		if (current_ratio == KTD253_MIN_RATIO)
+		ret = ktd253_backlight_stepdown(ktd253);
+		if (ret == -EAGAIN) {
+			/*
+			 * Something disturbed the backlight setting code when
+			 * running so we need to bring the PWM back to a known
+			 * state. This shouldn't happen too much.
+			 */
+			gpiod_set_value_cansleep(ktd253->gpiod, 0);
+			msleep(KTD253_T_OFF_MS);
+			ktd253_backlight_set_max_ratio(ktd253);
+			current_ratio = KTD253_MAX_RATIO;
+		} else if (current_ratio == KTD253_MIN_RATIO) {
+			/* After 1/32 we loop back to 32/32 */
 			current_ratio = KTD253_MAX_RATIO;
-		else
+		} else {
 			current_ratio--;
+		}
 	}
-	local_irq_restore(flags);
 	ktd253->ratio = current_ratio;
 
 	dev_dbg(ktd253->dev, "new ratio set to %d/32\n", target_ratio);