iio: adc: qcom-vadc-common: add reverse scaling for PMIC5 Gen2 ADC_TM

Add reverse scaling function for PMIC5 Gen2 ADC_TM, to convert
temperature to raw ADC code, for setting thresholds for
thermistor channels.
Signed-off-by: Jishnu Prakash <quic_jprakash@quicinc.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/1648991869-20899-3-git-send-email-quic_jprakash@quicinc.comSigned-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

drivers/iio/adc/qcom-vadc-common.c

@@ -677,6 +677,17 @@ u16 qcom_adc_tm5_temp_volt_scale(unsigned int prescale_ratio,
 }
 EXPORT_SYMBOL(qcom_adc_tm5_temp_volt_scale);
 
+u16 qcom_adc_tm5_gen2_temp_res_scale(int temp)
+{
+	int64_t resistance;
+
+	resistance = qcom_vadc_map_temp_voltage(adcmap7_100k,
+		ARRAY_SIZE(adcmap7_100k), temp);
+
+	return div64_s64(resistance * RATIO_MAX_ADC7, resistance + R_PU_100K);
+}
+EXPORT_SYMBOL(qcom_adc_tm5_gen2_temp_res_scale);
+
 int qcom_adc5_hw_scale(enum vadc_scale_fn_type scaletype,
 		    unsigned int prescale_ratio,
 		    const struct adc5_data *data,

include/linux/iio/adc/qcom-vadc-common.h

@@ -152,6 +152,8 @@ int qcom_adc5_hw_scale(enum vadc_scale_fn_type scaletype,
 u16 qcom_adc_tm5_temp_volt_scale(unsigned int prescale_ratio,
 				 u32 full_scale_code_volt, int temp);
 
+u16 qcom_adc_tm5_gen2_temp_res_scale(int temp);
+
 int qcom_adc5_prescaling_from_dt(u32 num, u32 den);
 
 int qcom_adc5_hw_settle_time_from_dt(u32 value, const unsigned int *hw_settle);