> +static int rzn1_adc_read_raw_ch(struct rzn1_adc *rzn1_adc, unsigned int chan, int *val)
> +{
> +	u32 *adc1_data, *adc2_data;
> +	int adc1_ch, adc2_ch;
> +	u32 adc_data;
> +	int ret;
> +
> +	if (chan < 8) {
> +		/* chan 0..7 used to get ADC1 ch 0..7 */
> +		adc1_ch = chan;
> +		adc1_data = &adc_data;
> +		adc2_ch = -1;
> +		adc2_data = NULL;
> +	} else if (chan < 16) {
> +		/* chan 8..15 used to get ADC2 ch 0..7 */
> +		adc1_ch = -1;
> +		adc1_data = NULL;
> +		adc2_ch = chan - 8;
> +		adc2_data = &adc_data;
> +	} else {
> +		return -EINVAL;
> +	}

How about putting part of the logic into the setup function? So, here
only:

	if (chan >= 16)
		return -EINVAL

> +
> +	ret = pm_runtime_resume_and_get(rzn1_adc->dev);
> +	if (ret < 0)
> +		return ret;
> +
> +	mutex_lock(&rzn1_adc->lock);
> +
> +	rzn1_adc_vc_setup_conversion(rzn1_adc, chan, adc1_ch, adc2_ch);

	rzn1_adc_vc_setup_conversion(rzn1_adc, chan);

And in that function:

> +static void rzn1_adc_vc_setup_conversion(struct rzn1_adc *rzn1_adc, u32 ch,
> +					 int adc1_ch, int adc2_ch)
> +{
> +	u32 vc = 0;
> +
> +	if (adc1_ch != -1)
> +		vc |= RZN1_ADC_VC_ADC1_ENABLE | RZN1_ADC_VC_ADC1_CHANNEL_SEL(adc1_ch);
> +
> +	if (adc2_ch != -1)
> +		vc |= RZN1_ADC_VC_ADC2_ENABLE | RZN1_ADC_VC_ADC2_CHANNEL_SEL(adc2_ch);
> +
> +	writel(vc, rzn1_adc->regs + RZN1_ADC_VC_REG(ch));
> +}

	if (ch < 8)
		vc |= RZN1_ADC_VC_ADC1_ENABLE | RZN1_ADC_VC_ADC1_CHANNEL_SEL(ch);
	else
		vc |= RZN1_ADC_VC_ADC2_ENABLE | RZN1_ADC_VC_ADC2_CHANNEL_SEL(ch - 8);

And a similar simplification for rzn1_adc_vc_wait_conversion().

Should work and the code is even more readable, I'd say. And has less
lines.