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

[Tzung-Bi Shih]

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.

plugged_cb and codec_dev are also in danger of race condition.  Instead
of using mutex to protect them:
- Checks NULLs first.
- Uses WRITE_ONCE() to prevent store tearing (i.e. write to plugged_cb
  after codec_dev).
- Uses codec_dev as a signal to report HDMI jack status.

Fixes: 5d3c64477392 ("drm/mediatek: support HDMI jack status reporting")

Signed-off-by: Tzung-Bi Shih <tzungbi at google.com>
---
Previous discussion: https://patchwork.kernel.org/patch/11367625/
Previous attempt: https://patchwork.kernel.org/patch/11378413/

 drivers/gpu/drm/mediatek/mtk_hdmi.c | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/mediatek/mtk_hdmi.c b/drivers/gpu/drm/mediatek/mtk_hdmi.c
index 03aeb73005ef..b1e5d0c538fa 100644
--- a/drivers/gpu/drm/mediatek/mtk_hdmi.c
+++ b/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;
 
-	if (hdmi->plugged_cb && hdmi->codec_dev)
+	mutex_lock(&hdmi->update_plugged_status_lock);
+	connected = mtk_cec_hpd_high(hdmi->cec_dev);
+	if (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 +1674,12 @@ static int mtk_hdmi_audio_hook_plugged_cb(struct device *dev, void *data,
 {
 	struct mtk_hdmi *hdmi = data;
 
-	hdmi->plugged_cb = fn;
-	hdmi->codec_dev = codec_dev;
+	if (!fn || !codec_dev)
+		return -EINVAL;
+
+	/* Use WRITE_ONCE() to prevent store tearing. */
+	WRITE_ONCE(hdmi->plugged_cb, fn);
+	WRITE_ONCE(hdmi->codec_dev, codec_dev);
 	mtk_hdmi_update_plugged_status(hdmi);
 
 	return 0;
@@ -1729,6 +1738,7 @@ 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);
-- 
2.25.0.225.g125e21ebc7-goog