drm/bridge: sii902x: Retry status read after DDI I2C

The following happens when connection a DVI output driven
from the SiI9022 using a DVI-to-VGA adapter plug:

i2c i2c-0: sendbytes: NAK bailout.
i2c i2c-0: sendbytes: NAK bailout.

Then no picture. Apparently the I2C engine inside the SiI9022
is not smart enough to try to fall back to DDC I2C. Or the
vendor have not integrated the electronics properly. I don't
know which one it is.

After this, the I2C bus seems stalled and the first attempt to
read the status register fails, and the code returns with
negative return value, and the display fails to initialized.

Instead, retry status readout five times and continue even
if this fails.

Tested on the ARM Versatile Express with a DVI-to-VGA
connector, it now gives picture.

Introduce a helper struct device *dev variable to make
the code more readable.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Liviu Dudau <Liviu.Dudau@arm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180305101702.13441-1-linus.walleij@linaro.org

drivers/gpu/drm/bridge/sii902x.c

@@ -137,7 +137,9 @@ static int sii902x_get_modes(struct drm_connector *connector)
 	struct sii902x *sii902x = connector_to_sii902x(connector);
 	struct regmap *regmap = sii902x->regmap;
 	u32 bus_format = MEDIA_BUS_FMT_RGB888_1X24;
+	struct device *dev = &sii902x->i2c->dev;
 	unsigned long timeout;
+	unsigned int retries;
 	unsigned int status;
 	struct edid *edid;
 	int num = 0;
@@ -159,7 +161,7 @@ static int sii902x_get_modes(struct drm_connector *connector)
 		 time_before(jiffies, timeout));
 
 	if (!(status & SII902X_SYS_CTRL_DDC_BUS_GRTD)) {
-		dev_err(&sii902x->i2c->dev, "failed to acquire the i2c bus\n");
+		dev_err(dev, "failed to acquire the i2c bus\n");
 		return -ETIMEDOUT;
 	}
 
@@ -179,9 +181,19 @@ static int sii902x_get_modes(struct drm_connector *connector)
 	if (ret)
 		return ret;
 
-	ret = regmap_read(regmap, SII902X_SYS_CTRL_DATA, &status);
+	/*
+	 * Sometimes the I2C bus can stall after failure to use the
+	 * EDID channel. Retry a few times to see if things clear
+	 * up, else continue anyway.
+	 */
+	retries = 5;
+	do {
+		ret = regmap_read(regmap, SII902X_SYS_CTRL_DATA,
+				  &status);
+		retries--;
+	} while (ret && retries);
 	if (ret)
-		return ret;
+		dev_err(dev, "failed to read status (%d)\n", ret);
 
 	ret = regmap_update_bits(regmap, SII902X_SYS_CTRL_DATA,
 				 SII902X_SYS_CTRL_DDC_BUS_REQ |
@@ -201,7 +213,7 @@ static int sii902x_get_modes(struct drm_connector *connector)
 
 	if (status & (SII902X_SYS_CTRL_DDC_BUS_REQ |
 		      SII902X_SYS_CTRL_DDC_BUS_GRTD)) {
-		dev_err(&sii902x->i2c->dev, "failed to release the i2c bus\n");
+		dev_err(dev, "failed to release the i2c bus\n");
 		return -ETIMEDOUT;
 	}