drm/mediatek: fix race condition for HDMI jack status reporting

hdmi_conn_detect and mtk_hdmi_audio_hook_plugged_cb would be called
by different threads.

Imaging the following calling sequence:
           Thread A                            Thread B
--------------------------------------------------------------------
mtk_hdmi_audio_hook_plugged_cb()
mtk_cec_hpd_high() -> disconnected
                                     hdmi_conn_detect()
                                     mtk_cec_hpd_high() -> connected
                                     plugged_cb(connected)
plugged_cb(disconnected)

The latest disconnected is false reported.  Makes mtk_cec_hpd_high
and plugged_cb atomic to fix.

Also uses the same lock to protect read/write of plugged_cb and codec_dev.

Fixes: 5d3c6447 ("drm/mediatek: support HDMI jack status reporting")
Signed-off-by: Tzung-Bi Shih <tzungbi@google.com>
Acked-by: CK Hu <ck.hu@mediatek.com>
Link: https://lore.kernel.org/r/20200217105513.2.I477092c2f104fd589133436c3ae4590e6fc6323b@changeidSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/gpu/drm/mediatek/mtk_hdmi.c

@@ -12,6 +12,7 @@
 #include <linux/io.h>
 #include <linux/kernel.h>
 #include <linux/mfd/syscon.h>
+#include <linux/mutex.h>
 #include <linux/of_platform.h>
 #include <linux/of.h>
 #include <linux/of_gpio.h>
@@ -171,6 +172,7 @@ struct mtk_hdmi {
 	bool enabled;
 	hdmi_codec_plugged_cb plugged_cb;
 	struct device *codec_dev;
+	struct mutex update_plugged_status_lock;
 };
 
 static inline struct mtk_hdmi *hdmi_ctx_from_bridge(struct drm_bridge *b)
@@ -1199,10 +1201,13 @@ static void mtk_hdmi_clk_disable_audio(struct mtk_hdmi *hdmi)
 static enum drm_connector_status
 mtk_hdmi_update_plugged_status(struct mtk_hdmi *hdmi)
 {
-	bool connected = mtk_cec_hpd_high(hdmi->cec_dev);
+	bool connected;
 
+	mutex_lock(&hdmi->update_plugged_status_lock);
+	connected = mtk_cec_hpd_high(hdmi->cec_dev);
 	if (hdmi->plugged_cb && hdmi->codec_dev)
 		hdmi->plugged_cb(hdmi->codec_dev, connected);
+	mutex_unlock(&hdmi->update_plugged_status_lock);
 
 	return connected ?
 	       connector_status_connected : connector_status_disconnected;
@@ -1669,8 +1674,11 @@ static int mtk_hdmi_audio_hook_plugged_cb(struct device *dev, void *data,
 {
 	struct mtk_hdmi *hdmi = data;
 
+	mutex_lock(&hdmi->update_plugged_status_lock);
 	hdmi->plugged_cb = fn;
 	hdmi->codec_dev = codec_dev;
+	mutex_unlock(&hdmi->update_plugged_status_lock);
+
 	mtk_hdmi_update_plugged_status(hdmi);
 
 	return 0;
@@ -1729,6 +1737,7 @@ static int mtk_drm_hdmi_probe(struct platform_device *pdev)
 		return ret;
 	}
 
+	mutex_init(&hdmi->update_plugged_status_lock);
 	platform_set_drvdata(pdev, hdmi);
 
 	ret = mtk_hdmi_output_init(hdmi);