iio: imu: inv_mpu6050: test whoami first and against all known values

SPI bus is never generating error during transfer, so to check if
a chip is correctly connected on a SPI bus we enforce whoami check
to be correct. In this way we can assure SPI probe is failing if
there is no chip connected.

Note that this is really papering over boards that claim the device
is there when it isn't.  Not a bad thing to cope with, but not
necessarily stable material.

Fixes: cec0154556 ("iio: inv_mpu6050: Check WHO_AM_I register on probe")
Signed-off-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/imu/inv_mpu6050/inv_mpu_core.c

@@ -777,10 +777,35 @@ static int inv_check_and_setup_chip(struct inv_mpu6050_state *st)
 {
 	int result;
 	unsigned int regval;
+	int i;
 
 	st->hw  = &hw_info[st->chip_type];
 	st->reg = hw_info[st->chip_type].reg;
 
+	/* check chip self-identification */
+	result = regmap_read(st->map, INV_MPU6050_REG_WHOAMI, &regval);
+	if (result)
+		return result;
+	if (regval != st->hw->whoami) {
+		/* check whoami against all possible values */
+		for (i = 0; i < INV_NUM_PARTS; ++i) {
+			if (regval == hw_info[i].whoami) {
+				dev_warn(regmap_get_device(st->map),
+					"whoami mismatch got %#02x (%s)"
+					"expected %#02hhx (%s)\n",
+					regval, hw_info[i].name,
+					st->hw->whoami, st->hw->name);
+				break;
+			}
+		}
+		if (i >= INV_NUM_PARTS) {
+			dev_err(regmap_get_device(st->map),
+				"invalid whoami %#02x expected %#02hhx (%s)\n",
+				regval, st->hw->whoami, st->hw->name);
+			return -ENODEV;
+		}
+	}
+
 	/* reset to make sure previous state are not there */
 	result = regmap_write(st->map, st->reg->pwr_mgmt_1,
 			      INV_MPU6050_BIT_H_RESET);
@@ -788,16 +813,6 @@ static int inv_check_and_setup_chip(struct inv_mpu6050_state *st)
 		return result;
 	msleep(INV_MPU6050_POWER_UP_TIME);
 
-	/* check chip self-identification */
-	result = regmap_read(st->map, INV_MPU6050_REG_WHOAMI, &regval);
-	if (result)
-		return result;
-	if (regval != st->hw->whoami) {
-		dev_warn(regmap_get_device(st->map),
-				"whoami mismatch got %#02x expected %#02hhx for %s\n",
-				regval, st->hw->whoami, st->hw->name);
-	}
-
 	/*
 	 * toggle power state. After reset, the sleep bit could be on
 	 * or off depending on the OTP settings. Toggling power would