Re: [PATCH v8 4/6] iio: adc: ad4691: add SPI offload support

[Jonathan Cameron <jic23@kernel.org>]

On Thu, 16 Apr 2026 12:18:49 +0300
Radu Sabau via B4 Relay <devnull+radu.sabau.analog.com@kernel.org> wrote:

> From: Radu Sabau <radu.sabau@analog.com>
> 
> Add SPI offload support to enable DMA-based, CPU-independent data
> acquisition using the SPI Engine offload framework.
> 
> When an SPI offload is available (devm_spi_offload_get() succeeds),
> the driver registers a DMA engine IIO buffer and uses dedicated buffer
> setup operations. If no offload is available the existing software
> triggered buffer path is used unchanged.
> 
> Both CNV Burst Mode and Manual Mode support offload, but use different
> trigger mechanisms:
> 
> CNV Burst Mode: the SPI Engine is triggered by the ADC's DATA_READY
> signal on the GP pin specified by the trigger-source consumer reference
> in the device tree (one cell = GP pin number 0-3). For this mode the
> driver acts as both an SPI offload consumer (DMA RX stream, message
> optimization) and a trigger source provider: it registers the
> GP/DATA_READY output via devm_spi_offload_trigger_register() so the
> offload framework can match the '#trigger-source-cells' phandle and
> automatically fire the SPI Engine DMA transfer at end-of-conversion.
> 
> Manual Mode: the SPI Engine is triggered by a periodic trigger at
> the configured sampling frequency. The pre-built SPI message uses
> the pipelined CNV-on-CS protocol: N+1 16-bit transfers are issued
> for N active channels (the first result is discarded as garbage from
> the pipeline flush) and the remaining N results are captured by DMA.
> 
> All offload transfers use 16-bit frames (bits_per_word=16, len=2).
> The channel scan_type (storagebits=16, shift=0, IIO_BE) is shared
> between the software triggered-buffer and offload paths; no separate
> scan_type or channel array is needed for the offload case. The
> ad4691_manual_channels[] array introduced in the triggered-buffer
> commit is reused here: it hides the IIO_CHAN_INFO_OVERSAMPLING_RATIO
> attribute, which is not applicable in Manual Mode.
> 
> Kconfig gains a dependency on IIO_BUFFER_DMAENGINE.
> 
> Signed-off-by: Radu Sabau <radu.sabau@analog.com>

Another case where you would ideally have pushed the refactor
back to where the original code was introduced. It might have looked
slightly over the top there, but it would have been correct and reduce
churn in this later patch.

> @@ -825,12 +1111,25 @@ static ssize_t sampling_frequency_store(struct device *dev,
>  	if (ret)
>  		return ret;
>  
> -	ret = iio_device_claim_direct(indio_dev);
> -	if (ret)
> -		return ret;
> +	IIO_DEV_ACQUIRE_DIRECT_MODE(indio_dev, claim);
> +	if (IIO_DEV_ACQUIRE_FAILED(claim))
> +		return -EBUSY;
Ideally this would have been pushed back into the patch that introduced
the code to reduce the churn here.  Fix that up if you end up doing a v9.

> +
> +	if (st->manual_mode && st->offload) {
> +		struct spi_offload_trigger_config config = {
> +			.type = SPI_OFFLOAD_TRIGGER_PERIODIC,
> +			.periodic = { .frequency_hz = freq },
> +		};
> +
> +		ret = spi_offload_trigger_validate(st->offload->trigger, &config);
> +		if (ret)
> +			return ret;
> +
> +		st->offload->trigger_hz = config.periodic.frequency_hz;
> +		return len;
> +	}
>  
>  	ret = ad4691_set_pwm_freq(st, freq);
> -	iio_device_release_direct(indio_dev);
>  	if (ret)
>  		return ret;