Re: [PATCH v8 5/8] iio: adc: ad4030: Add SPI offload support

[Andy Shevchenko]

On Fri, Feb 06, 2026 at 04:01:33PM -0300, Marcelo Schmitt wrote:
> AD4030 and similar ADCs can capture data at sample rates up to 2 mega
> samples per second (MSPS). Not all SPI controllers are able to achieve such
> high throughputs and even when the controller is fast enough to run
> transfers at the required speed, it may be costly to the CPU to handle
> transfer data at such high sample rates. Add SPI offload support for AD4030
> and similar ADCs to enable data capture at maximum sample rates.

...

> +static int ad4030_update_conversion_rate(struct ad4030_state *st,
> +					 unsigned int freq_hz, unsigned int avg_log2)
> +{
> +	struct spi_offload_trigger_config *config = &st->offload_trigger_config;
> +	unsigned int offload_period_ns, cnv_rate_hz;
> +	struct pwm_waveform cnv_wf = { };
> +	u64 target = AD4030_TCNVH_NS;
> +	u64 offload_offset_ns;
> +	int ret;
> +
> +	/*
> +	 * When averaging/oversampling over N samples, we fire the offload
> +	 * trigger once at every N pulses of the CNV signal. Conversely, the CNV
> +	 * signal needs to be N times faster than the offload trigger. Take that
> +	 * into account to correctly re-evaluate both the PWM waveform connected
> +	 * to CNV and the SPI offload trigger.
> +	 */
> +	cnv_rate_hz = freq_hz << avg_log2;
> +
> +	cnv_wf.period_length_ns = DIV_ROUND_CLOSEST(NSEC_PER_SEC, cnv_rate_hz);

See below.

> +	/*
> +	 * The datasheet lists a minimum time of 9.8 ns, but no maximum. If the
> +	 * rounded PWM's value is less than 10, increase the target value by 10
> +	 * and attempt to round the waveform again, until the value is at least
> +	 * 10 ns. Use a separate variable to represent the target in case the
> +	 * rounding is severe enough to keep putting the first few results under
> +	 * the minimum 10ns condition checked by the while loop.
> +	 */
> +	do {
> +		cnv_wf.duty_length_ns = target;
> +		ret = pwm_round_waveform_might_sleep(st->cnv_trigger, &cnv_wf);
> +		if (ret)
> +			return ret;
> +		target += AD4030_TCNVH_NS;
> +	} while (cnv_wf.duty_length_ns < AD4030_TCNVH_NS);

Does the above have a side-effect on period_length_ns? If not, the below check
should be moved up, otherwise here should be a short comment explaining the
side-effect(s).

> +	if (!in_range(cnv_wf.period_length_ns, AD4030_TCYC_NS, INT_MAX))
> +		return -EINVAL;

> +	offload_period_ns = DIV_ROUND_CLOSEST(NSEC_PER_SEC, freq_hz);
> +
> +	config->periodic.frequency_hz = DIV_ROUND_UP(HZ_PER_GHZ, offload_period_ns);
> +
> +	/*
> +	 * The hardware does the capture on zone 2 (when SPI trigger PWM
> +	 * is used). This means that the SPI trigger signal should happen at
> +	 * tsync + tquiet_con_delay being tsync the conversion signal period
> +	 * and tquiet_con_delay 9.8ns. Hence set the PWM phase accordingly.
> +	 *
> +	 * The PWM waveform API only supports nanosecond resolution right now,
> +	 * so round this setting to the closest available value.
> +	 */
> +	offload_offset_ns = AD4030_TQUIET_CNV_DELAY_NS;
> +	do {
> +		config->periodic.offset_ns = offload_offset_ns;
> +		ret = spi_offload_trigger_validate(st->offload_trigger, config);
> +		if (ret)
> +			return ret;
> +		offload_offset_ns += AD4030_TQUIET_CNV_DELAY_NS;
> +	} while (config->periodic.offset_ns < AD4030_TQUIET_CNV_DELAY_NS);
> +
> +	st->cnv_wf = cnv_wf;
> +
> +	return 0;
> +}

-- 
With Best Regards,
Andy Shevchenko