drm/panel: panel-simple: Cache the EDID as long as we retain power

It doesn't make sense to go out to the bus and read the EDID over and
over again. Let's cache it and throw away the cache when we turn power
off from the panel. Autosuspend means that even if there are several
calls to read the EDID before we officially turn the power on then we
should get good use out of this cache.
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210423095743.v5.18.If050957eaa85cf45b10bcf61e6f7fa61c9750ebf@changeid

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

@@ -189,6 +189,8 @@ struct panel_simple {
 	struct gpio_desc *enable_gpio;
 	struct gpio_desc *hpd_gpio;
 
+	struct edid *edid;
+
 	struct drm_display_mode override_mode;
 
 	enum drm_panel_orientation orientation;
@@ -345,6 +347,9 @@ static int panel_simple_suspend(struct device *dev)
 	regulator_disable(p->supply);
 	p->unprepared_time = ktime_get();
 
+	kfree(p->edid);
+	p->edid = NULL;
+
 	return 0;
 }
 
@@ -510,15 +515,13 @@ static int panel_simple_get_modes(struct drm_panel *panel,
 
 	/* probe EDID if a DDC bus is available */
 	if (p->ddc) {
-		struct edid *edid;
-
 		pm_runtime_get_sync(panel->dev);
 
-		edid = drm_get_edid(connector, p->ddc);
-		if (edid) {
-			num += drm_add_edid_modes(connector, edid);
-			kfree(edid);
-		}
+		if (!p->edid)
+			p->edid = drm_get_edid(connector, p->ddc);
+
+		if (p->edid)
+			num += drm_add_edid_modes(connector, p->edid);
 
 		pm_runtime_mark_last_busy(panel->dev);
 		pm_runtime_put_autosuspend(panel->dev);