[PATCH v3 3/3] iio: light: isl29018: support cover-glass gain compensation via DT

[Herman van Hazendonk <github.com@herrie.org>]

Boards that mount the ISL29018 behind tinted or coated cover glass
experience optical loss that effectively reduces the sensor's apparent
sensitivity.  The existing in_illuminance0_calibscale sysfs attribute
can correct for this at runtime, but firmware knows the loss factor at
design time and there is no way to seed it without a userspace helper.

Add support for an optional "isil,cover-comp-gain" device-tree property
that initialises calibscale at probe time.  If the property is present
but cannot be read, probe returns an error via dev_err_probe() so the
root cause is visible in the log.  If absent, calibscale defaults to 1
(unity gain, matching the previous behaviour).  Userspace can still
override the value at runtime through the sysfs attribute.

The approach follows the precedent set by the TSL2563 driver.

Signed-off-by: Herman van Hazendonk <github.com@herrie.org>
---
 drivers/iio/light/isl29018.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/drivers/iio/light/isl29018.c b/drivers/iio/light/isl29018.c
index f3312ad670d9..9daacb9b02bc 100644
--- a/drivers/iio/light/isl29018.c
+++ b/drivers/iio/light/isl29018.c
@@ -711,10 +711,13 @@ static int isl29018_probe(struct i2c_client *client)
 	struct iio_dev *indio_dev;
 	const void *ddata = NULL;
 	const char *name;
+	struct device *dev;
 	int dev_id;
 	int err;
 
-	indio_dev = devm_iio_device_alloc(&client->dev, sizeof(*chip));
+	dev = &client->dev;
+
+	indio_dev = devm_iio_device_alloc(dev, sizeof(*chip));
 	if (!indio_dev)
 		return -ENOMEM;
 
@@ -733,8 +736,16 @@ static int isl29018_probe(struct i2c_client *client)
 	mutex_init(&chip->lock);
 
 	chip->type = dev_id;
-	chip->calibscale = 1;
 	chip->ucalibscale = 0;
+	if (device_property_present(dev, "isil,cover-comp-gain")) {
+		err = device_property_read_u32(dev, "isil,cover-comp-gain",
+					       &chip->calibscale);
+		if (err)
+			return dev_err_probe(dev, err,
+					     "invalid isil,cover-comp-gain\n");
+	} else {
+		chip->calibscale = 1;
+	}
 	chip->int_time = ISL29018_INT_TIME_16;
 	chip->scale = isl29018_scales[chip->int_time][0];
 	chip->suspended = false;
-- 
2.43.0