media: ov5640: Handle delays when no reset_gpio set

Some module manufacturers [1][2] don't expose the RESETB and PWDN pins
of the sensor directly through the 15-pin FFC connector. Instead wiring
~PWDN gpio to the sensor pins with appropriate delays.

In such cases, reset_gpio will not be available to the driver, but it
will still be toggled when the sensor is powered on, and thus we should
still honor the wait time of >= 5ms + 1ms + 20ms (see figure 2-3 in [3])
before attempting any i/o operations over SCCB.

Also, rename the function to ov5640_powerup_sequence to better match the
datasheet (section 2.7).

[1] https://digilent.com/reference/_media/reference/add-ons/pcam-5c/pcam_5c_sch.pdf
[2] https://www.alinx.com/public/upload/file/AN5641_User_Manual.pdf
[3] https://cdn.sparkfun.com/datasheets/Sensors/LightImaging/OV5640_datasheet.pdf

Fixes: 19a81c14 ("[media] add Omnivision OV5640 sensor driver")
Reported-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Jai Luthra <j-luthra@ti.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

drivers/media/i2c/ov5640.c

@@ -2436,11 +2436,22 @@ static void ov5640_power(struct ov5640_dev *sensor, bool enable)
 	gpiod_set_value_cansleep(sensor->pwdn_gpio, enable ? 0 : 1);
 }
 
-static void ov5640_reset(struct ov5640_dev *sensor)
+/*
+ * From section 2.7 power up sequence:
+ * t0 + t1 + t2 >= 5ms	Delay from DOVDD stable to PWDN pull down
+ * t3 >= 1ms		Delay from PWDN pull down to RESETB pull up
+ * t4 >= 20ms		Delay from RESETB pull up to SCCB (i2c) stable
+ *
+ * Some modules don't expose RESETB/PWDN pins directly, instead providing a
+ * "PWUP" GPIO which is wired through appropriate delays and inverters to the
+ * pins.
+ *
+ * In such cases, this gpio should be mapped to pwdn_gpio in the driver, and we
+ * should still toggle the pwdn_gpio below with the appropriate delays, while
+ * the calls to reset_gpio will be ignored.
+ */
+static void ov5640_powerup_sequence(struct ov5640_dev *sensor)
 {
-	if (!sensor->reset_gpio)
-		return;
-
 	if (sensor->pwdn_gpio) {
 		gpiod_set_value_cansleep(sensor->reset_gpio, 0);
 
@@ -2489,8 +2500,7 @@ static int ov5640_set_power_on(struct ov5640_dev *sensor)
 		goto xclk_off;
 	}
 
-	ov5640_reset(sensor);
-	ov5640_power(sensor, true);
+	ov5640_powerup_sequence(sensor);
 
 	ret = ov5640_init_slave_id(sensor);
 	if (ret)