drm/panel: simple: Support panels with HPD where HPD isn't connected

Some eDP panels that are designed to be always connected to a board
use their HPD signal to signal that they've finished powering on and
they're ready to be talked to.

However, for various reasons it's possible that the HPD signal from
the panel isn't actually hooked up.  In the case where the HPD isn't
hooked up you can look at the timing diagram on the panel datasheet
and insert a delay for the maximum amount of time that the HPD might
take to come up.

Let's add support in simple-panel for this concept.

At the moment we will co-opt the existing "prepare" delay to keep
track of the delay and we'll use a boolean to specify that a given
panel should only apply the delay if the "no-hpd" property was
specified.
Reviewed-by: Sean Paul <sean@poorly.run>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20181025222134.174583-2-dianders@chromium.org

drivers/gpu/drm/panel/panel-simple.c

@@ -56,6 +56,8 @@ struct panel_desc {
 	/**
 	 * @prepare: the time (in milliseconds) that it takes for the panel to
 	 *           become ready and start receiving video data
+	 * @hpd_absent_delay: Add this to the prepare delay if we know Hot
+	 *                    Plug Detect isn't used.
 	 * @enable: the time (in milliseconds) that it takes for the panel to
 	 *          display the first valid frame after starting to receive
 	 *          video data
@@ -66,6 +68,7 @@ struct panel_desc {
 	 */
 	struct {
 		unsigned int prepare;
+		unsigned int hpd_absent_delay;
 		unsigned int enable;
 		unsigned int disable;
 		unsigned int unprepare;
@@ -79,6 +82,7 @@ struct panel_simple {
 	struct drm_panel base;
 	bool prepared;
 	bool enabled;
+	bool no_hpd;
 
 	const struct panel_desc *desc;
 
@@ -202,6 +206,7 @@ static int panel_simple_unprepare(struct drm_panel *panel)
 static int panel_simple_prepare(struct drm_panel *panel)
 {
 	struct panel_simple *p = to_panel_simple(panel);
+	unsigned int delay;
 	int err;
 
 	if (p->prepared)
@@ -215,8 +220,11 @@ static int panel_simple_prepare(struct drm_panel *panel)
 
 	gpiod_set_value_cansleep(p->enable_gpio, 1);
 
-	if (p->desc->delay.prepare)
-		msleep(p->desc->delay.prepare);
+	delay = p->desc->delay.prepare;
+	if (p->no_hpd)
+		delay += p->desc->delay.hpd_absent_delay;
+	if (delay)
+		msleep(delay);
 
 	p->prepared = true;
 
@@ -305,6 +313,8 @@ static int panel_simple_probe(struct device *dev, const struct panel_desc *desc)
 	panel->prepared = false;
 	panel->desc = desc;
 
+	panel->no_hpd = of_property_read_bool(dev->of_node, "no-hpd");
+
 	panel->supply = devm_regulator_get(dev, "power");
 	if (IS_ERR(panel->supply))
 		return PTR_ERR(panel->supply);