media: mt9p031: Fix corrupted frame after restarting stream

To prevent corrupted frames after starting and stopping the sensor its
datasheet specifies a specific pause sequence to follow:

Stopping:
	Set Pause_Restart Bit -> Set Restart Bit -> Set Chip_Enable Off

Restarting:
	Set Chip_Enable On -> Clear Pause_Restart Bit

The Restart Bit is cleared automatically and must not be cleared
manually as this would cause undefined behavior.
Signed-off-by: Dirk Bender <d.bender@phytec.de>
Signed-off-by: Stefan Riedmueller <s.riedmueller@phytec.de>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

drivers/media/i2c/mt9p031.c

@@ -79,7 +79,9 @@
 #define		MT9P031_PIXEL_CLOCK_INVERT		(1 << 15)
 #define		MT9P031_PIXEL_CLOCK_SHIFT(n)		((n) << 8)
 #define		MT9P031_PIXEL_CLOCK_DIVIDE(n)		((n) << 0)
-#define MT9P031_FRAME_RESTART				0x0b
+#define MT9P031_RESTART					0x0b
+#define		MT9P031_FRAME_PAUSE_RESTART		(1 << 1)
+#define		MT9P031_FRAME_RESTART			(1 << 0)
 #define MT9P031_SHUTTER_DELAY				0x0c
 #define MT9P031_RST					0x0d
 #define		MT9P031_RST_ENABLE			1
@@ -456,9 +458,23 @@ static int mt9p031_set_params(struct mt9p031 *mt9p031)
 static int mt9p031_s_stream(struct v4l2_subdev *subdev, int enable)
 {
 	struct mt9p031 *mt9p031 = to_mt9p031(subdev);
+	struct i2c_client *client = v4l2_get_subdevdata(subdev);
+	int val;
 	int ret;
 
 	if (!enable) {
+		/* enable pause restart */
+		val = MT9P031_FRAME_PAUSE_RESTART;
+		ret = mt9p031_write(client, MT9P031_RESTART, val);
+		if (ret < 0)
+			return ret;
+
+		/* enable restart + keep pause restart set */
+		val |= MT9P031_FRAME_RESTART;
+		ret = mt9p031_write(client, MT9P031_RESTART, val);
+		if (ret < 0)
+			return ret;
+
 		/* Stop sensor readout */
 		ret = mt9p031_set_output_control(mt9p031,
 						 MT9P031_OUTPUT_CONTROL_CEN, 0);
@@ -478,6 +494,16 @@ static int mt9p031_s_stream(struct v4l2_subdev *subdev, int enable)
 	if (ret < 0)
 		return ret;
 
+	/*
+	 * - clear pause restart
+	 * - don't clear restart as clearing restart manually can cause
+	 *   undefined behavior
+	 */
+	val = MT9P031_FRAME_RESTART;
+	ret = mt9p031_write(client, MT9P031_RESTART, val);
+	if (ret < 0)
+		return ret;
+
 	return mt9p031_pll_enable(mt9p031);
 }