drm/panel-edp: Fix "prepare_to_enable" if panel doesn't handle HPD

While cleaning up the descriptions of the delay for eDP panels I
realized that we'd have a bug if any panels need the
"prepare_to_enable" but HPD handling isn't happening in the panel
driver. Let's put in a stopgap to at least make us not violate
timings. This is not perfectly optimal but trying to do better is
hard. At the moment only 2 panels specify this delay and only 30 ms is
at stake. These panels are also currently hooked up with "hpd-gpios"
so effectively this "fix" is just a theoretical fix and won't actually
do anything for any devices currently supported in mainline.
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210914132020.v5.13.Ia8288d36df4b12770af59ae3ff73ef7e08fb4e2e@changeid

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

@@ -508,12 +508,32 @@ static int panel_edp_prepare(struct drm_panel *panel)
 static int panel_edp_enable(struct drm_panel *panel)
 {
 	struct panel_edp *p = to_panel_edp(panel);
+	unsigned int delay;
 
 	if (p->enabled)
 		return 0;
 
-	if (p->desc->delay.enable)
-		msleep(p->desc->delay.enable);
+	delay = p->desc->delay.enable;
+
+	/*
+	 * If there is a "prepare_to_enable" delay then that's supposed to be
+	 * the delay from HPD going high until we can turn the backlight on.
+	 * However, we can only count this if HPD is handled by the panel
+	 * driver, not if it goes to a dedicated pin on the controller.
+	 * If we aren't handling the HPD pin ourselves then the best we
+	 * can do is assume that HPD went high immediately before we were
+	 * called (and link training took zero time).
+	 *
+	 * NOTE: if we ever end up in this "if" statement then we're
+	 * guaranteed that the panel_edp_wait() call below will do no delay.
+	 * It already handles that case, though, so we don't need any special
+	 * code for it.
+	 */
+	if (p->desc->delay.prepare_to_enable && !p->hpd_gpio && !p->no_hpd)
+		delay = max(delay, p->desc->delay.prepare_to_enable);
+
+	if (delay)
+		msleep(delay);
 
 	panel_edp_wait(p->prepared_time, p->desc->delay.prepare_to_enable);