drm/bridge: it6505: Add caching for EDID

Add caching when EDID is read, and invalidate the cache until the
bridge detects HPD low or sink count changes on HPD_IRQ.

It takes 1.2s for IT6505 bridge to read a 3-block EDID, and skipping
one EDID read would be a notable difference on user experience.
Signed-off-by: Pin-yen Lin <treapking@chromium.org>
Reviewed-by: Robert Foss <robert.foss@linaro.org>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20221115112720.911158-1-treapking@chromium.org

drivers/gpu/drm/bridge/ite-it6505.c

@@ -456,6 +456,8 @@ struct it6505 {
 
 	/* it6505 driver hold option */
 	bool enable_drv_hold;
+
+	struct edid *cached_edid;
 };
 
 struct it6505_step_train_para {
@@ -2243,6 +2245,12 @@ static void it6505_plugged_status_to_codec(struct it6505 *it6505)
 				   status == connector_status_connected);
 }
 
+static void it6505_remove_edid(struct it6505 *it6505)
+{
+	kfree(it6505->cached_edid);
+	it6505->cached_edid = NULL;
+}
+
 static int it6505_process_hpd_irq(struct it6505 *it6505)
 {
 	struct device *dev = &it6505->client->dev;
@@ -2269,6 +2277,7 @@ static int it6505_process_hpd_irq(struct it6505 *it6505)
 		it6505_reset_logic(it6505);
 		it6505_int_mask_enable(it6505);
 		it6505_init(it6505);
+		it6505_remove_edid(it6505);
 		return 0;
 	}
 
@@ -2352,6 +2361,7 @@ static void it6505_irq_hpd(struct it6505 *it6505)
 			it6505_video_reset(it6505);
 	} else {
 		memset(it6505->dpcd, 0, sizeof(it6505->dpcd));
+		it6505_remove_edid(it6505);
 
 		if (it6505->hdcp_desired)
 			it6505_stop_hdcp(it6505);
@@ -3011,16 +3021,18 @@ static struct edid *it6505_bridge_get_edid(struct drm_bridge *bridge,
 {
 	struct it6505 *it6505 = bridge_to_it6505(bridge);
 	struct device *dev = &it6505->client->dev;
-	struct edid *edid;
 
-	edid = drm_do_get_edid(connector, it6505_get_edid_block, it6505);
+	if (!it6505->cached_edid) {
+		it6505->cached_edid = drm_do_get_edid(connector, it6505_get_edid_block,
+						      it6505);
 
-	if (!edid) {
-		DRM_DEV_DEBUG_DRIVER(dev, "failed to get edid!");
-		return NULL;
+		if (!it6505->cached_edid) {
+			DRM_DEV_DEBUG_DRIVER(dev, "failed to get edid!");
+			return NULL;
+		}
 	}
 
-	return edid;
+	return drm_edid_duplicate(it6505->cached_edid);
 }
 
 static const struct drm_bridge_funcs it6505_bridge_funcs = {
@@ -3360,6 +3372,7 @@ static void it6505_i2c_remove(struct i2c_client *client)
 	drm_dp_aux_unregister(&it6505->aux);
 	it6505_debugfs_remove(it6505);
 	it6505_poweroff(it6505);
+	it6505_remove_edid(it6505);
 }
 
 static const struct i2c_device_id it6505_id[] = {