media: mt9p031: Increase post-reset delay

The MT9P006 sensor driver sporadically fails to probe because the sensor
responds with a NACK condition to I2C address on the bus during an attempt
to read the sensor MT9P031_CHIP_VERSION register in mt9p031_registered().

Neither the MT9P006 nor MT9P031 datasheets are clear on reset signal timing.
Older MT9M034 [1] datasheet provides those timing figures in Appendix-A and
indicates it is necessary to wait 850000 EXTCLK cycles before starting any
I2C communication.

Add such a delay, which does make the sporadic I2C NACK go away, so it is
likely similar constraint applies to this sensor.

[1] https://www.onsemi.com/pdf/datasheet/mt9m034-d.pdfSigned-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

drivers/media/i2c/mt9p031.c

@@ -307,6 +307,7 @@ static inline int mt9p031_pll_disable(struct mt9p031 *mt9p031)
 
 static int mt9p031_power_on(struct mt9p031 *mt9p031)
 {
+	unsigned long rate, delay;
 	int ret;
 
 	/* Ensure RESET_BAR is active */
@@ -334,7 +335,12 @@ static int mt9p031_power_on(struct mt9p031 *mt9p031)
 	/* Now RESET_BAR must be high */
 	if (mt9p031->reset) {
 		gpiod_set_value(mt9p031->reset, 0);
-		usleep_range(1000, 2000);
+		/* Wait 850000 EXTCLK cycles before de-asserting reset. */
+		rate = clk_get_rate(mt9p031->clk);
+		if (!rate)
+			rate = 6000000;	/* Slowest supported clock, 6 MHz */
+		delay = DIV_ROUND_UP(850000 * 1000, rate);
+		msleep(delay);
 	}
 
 	return 0;