drm/panel: otm8009a: Add support for the optional power-supply

Add support for the optional power-supply.

Note: A "dummy regulator" is returned by devm_regulator_get()
if the optional regulator is not present in the device tree,
simplifying the source code when enabling/disabling the regulator.
Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Reviewed-by: Yannick Fertré <yannick.fertre@st.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180205094532.23547-3-philippe.cornu@st.com

drivers/gpu/drm/panel/panel-orisetech-otm8009a.c

@@ -11,6 +11,7 @@
 #include <drm/drm_panel.h>
 #include <linux/backlight.h>
 #include <linux/gpio/consumer.h>
+#include <linux/regulator/consumer.h>
 #include <video/mipi_display.h>
 
 #define DRV_NAME "orisetech_otm8009a"
@@ -62,6 +63,7 @@ struct otm8009a {
 	struct drm_panel panel;
 	struct backlight_device *bl_dev;
 	struct gpio_desc *reset_gpio;
+	struct regulator *supply;
 	bool prepared;
 	bool enabled;
 };
@@ -279,6 +281,8 @@ static int otm8009a_unprepare(struct drm_panel *panel)
 		msleep(20);
 	}
 
+	regulator_disable(ctx->supply);
+
 	ctx->prepared = false;
 
 	return 0;
@@ -292,6 +296,12 @@ static int otm8009a_prepare(struct drm_panel *panel)
 	if (ctx->prepared)
 		return 0;
 
+	ret = regulator_enable(ctx->supply);
+	if (ret < 0) {
+		DRM_ERROR("failed to enable supply: %d\n", ret);
+		return ret;
+	}
+
 	if (ctx->reset_gpio) {
 		gpiod_set_value_cansleep(ctx->reset_gpio, 0);
 		gpiod_set_value_cansleep(ctx->reset_gpio, 1);
@@ -414,6 +424,13 @@ static int otm8009a_probe(struct mipi_dsi_device *dsi)
 		return PTR_ERR(ctx->reset_gpio);
 	}
 
+	ctx->supply = devm_regulator_get(dev, "power");
+	if (IS_ERR(ctx->supply)) {
+		ret = PTR_ERR(ctx->supply);
+		dev_err(dev, "failed to request regulator: %d\n", ret);
+		return ret;
+	}
+
 	mipi_dsi_set_drvdata(dsi, ctx);
 
 	ctx->dev = dev;