backlight: lp855x: Add enable regulator

The LP8556 datasheet describes an EN/VDDIO input, which serves "both as
a chip enable and as a power supply reference for PWM, SDA, and SCL
inputs." The LP8556 that I'm testing doesn't respond properly if I try
to talk I2C to it too quickly after enabling VDDIO, and the LP8555
datasheet mentions a t_RESPONSE delay of up to 1 millisecond.

Support this EN/VDDIO by adding a regulator property to the binding;
enabling this regulator at probe time; and sleeping for 1 to 2ms, if the
EN/VDDIO regulator was provided.

Signed-off-by: Brian Norris <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Acked-by: Milo Kim <[e-mail]>
Reviewed-by: Stephen Barber <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>

Documentation/devicetree/bindings/leds/backlight/lp855x.txt

···
   - rom-addr: Register address of ROM area to be updated (u8)
   - rom-val: Register value to be updated (u8)
   - power-supply: Regulator which controls the 3V rail
+  - enable-supply: Regulator which controls the EN/VDDIO input
 
 Example:
 
···
 	backlight@2c {
 		compatible = "ti,lp8557";
 		reg = <0x2c>;
+		enable-supply = <&backlight_vddio>;
 		power-supply = <&backlight_vdd>;
 
 		dev-ctrl = /bits/ 8 <0x41>;

drivers/video/backlight/lp855x_bl.c

···
 #include <linux/slab.h>
 #include <linux/i2c.h>
 #include <linux/backlight.h>
+#include <linux/delay.h>
 #include <linux/err.h>
 #include <linux/of.h>
 #include <linux/platform_data/lp855x.h>
···
 	struct lp855x_platform_data *pdata;
 	struct pwm_device *pwm;
 	struct regulator *supply;	/* regulator for VDD input */
+	struct regulator *enable;	/* regulator for EN/VDDIO input */
 };
 
 static int lp855x_write_byte(struct lp855x *lp, u8 reg, u8 data)
···
 		lp->supply = NULL;
 	}
 
+	lp->enable = devm_regulator_get_optional(lp->dev, "enable");
+	if (IS_ERR(lp->enable)) {
+		ret = PTR_ERR(lp->enable);
+		if (ret == -ENODEV) {
+			lp->enable = NULL;
+		} else {
+			if (ret != -EPROBE_DEFER)
+				dev_err(lp->dev, "error getting enable regulator: %d\n",
+					ret);
+			return ret;
+		}
+	}
+
 	if (lp->supply) {
 		ret = regulator_enable(lp->supply);
 		if (ret < 0) {
 			dev_err(&cl->dev, "failed to enable supply: %d\n", ret);
 			return ret;
 		}
+	}
+
+	if (lp->enable) {
+		ret = regulator_enable(lp->enable);
+		if (ret < 0) {
+			dev_err(lp->dev, "failed to enable vddio: %d\n", ret);
+			return ret;
+		}
+
+		/*
+		 * LP8555 datasheet says t_RESPONSE (time between VDDIO and
+		 * I2C) is 1ms.
+		 */
+		usleep_range(1000, 2000);
 	}
 
 	i2c_set_clientdata(cl, lp);