drivers: adc: sam0: Adjust resolution with the oversampling factor

The old SAM0 adc drivers forces you to put 12 bits when using oversampling.
The adc_raw_to_millivolts_dt function is not working properly because if
we uses an oversampling of 8, we get 16 bits of resolution.
The function uses directly the resolution in the DT,
 so the voltage will not be correct.
To counter this, i forced the user to put the right resolution for his
oversampling factor.
Fixes: #74607
Signed-off-by: Robin Carrupt <[email protected]>

drivers/adc/adc_sam0.c

@@ -343,26 +343,52 @@ static int start_read(const struct device *dev,
 	switch (sequence->resolution) {
 	case 8:
 		if (sequence->oversampling) {
-			LOG_ERR("Oversampling requires 12 bit resolution");
+			LOG_ERR("Oversampling requires minimum 13 bit resolution");
 			return -EINVAL;
 		}
-
 		ADC_RESSEL(adc) = ADC_RESSEL_8BIT;
 		break;
 	case 10:
 		if (sequence->oversampling) {
-			LOG_ERR("Oversampling requires 12 bit resolution");
+			LOG_ERR("Oversampling requires minimum 13 bit resolution");
 			return -EINVAL;
 		}
-
 		ADC_RESSEL(adc) = ADC_RESSEL_10BIT;
 		break;
 	case 12:
 		if (sequence->oversampling) {
-			ADC_RESSEL(adc) = ADC_RESSEL_16BIT;
-		} else {
-			ADC_RESSEL(adc) = ADC_RESSEL_12BIT;
+			LOG_ERR("Oversampling requires minimum 13 bit resolution");
+			return -EINVAL;
+		}
+		ADC_RESSEL(adc) = ADC_RESSEL_12BIT;
+		break;
+	case 13:
+		if (sequence->oversampling != 2U) {
+			LOG_ERR("13 bit resolution requires an oversampling of 2");
+			return -EINVAL;
+		}
+		ADC_RESSEL(adc) = ADC_RESSEL_16BIT;
+		break;
+	case 14:
+		if (sequence->oversampling != 4U) {
+			LOG_ERR("14 bit resolution requires an oversampling of 4");
+			return -EINVAL;
+		}
+		ADC_RESSEL(adc) = ADC_RESSEL_16BIT;
+		break;
+	case 15:
+		if (sequence->oversampling != 6U) {
+			LOG_ERR("15 bit resolution requires an oversampling of 6");
+			return -EINVAL;
+		}
+		ADC_RESSEL(adc) = ADC_RESSEL_16BIT;
+		break;
+	case 16:
+		if (sequence->oversampling != 8U) {
+			LOG_ERR("16 bit resolution requires an oversampling of 8");
+			return -EINVAL;
 		}
+		ADC_RESSEL(adc) = ADC_RESSEL_16BIT;
 		break;
 	default:
 		LOG_ERR("ADC resolution value %d is not valid",