[PATCH v5 13/15] drm/panel-edp: Fix "prepare_to_enable" if panel doesn't handle HPD

[Douglas Anderson <dianders@chromium.org>]

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>
---

(no changes since v4)

Changes in v4:
- panel-simple-edp => panel-edp

Changes in v3:
- Fix "prepare_to_enable" patch new for v3.

 drivers/gpu/drm/panel/panel-edp.c | 24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/panel/panel-edp.c b/drivers/gpu/drm/panel/panel-edp.c
index 80a37a4e5750..a83ef9905ea7 100644
--- a/drivers/gpu/drm/panel/panel-edp.c
+++ b/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);
 
-- 
2.33.0.309.g3052b89438-goog