Re: [PATCH v9 4/6] iio: adc: ad4691: add SPI offload support

[Jonathan Cameron <jic23@kernel.org>]

On Thu, 30 Apr 2026 13:16:46 +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.

Probably good to call out that oversampling hasn't been introduced
to the driver yet. This confused Sashiko ;)

> 
> Kconfig gains a dependency on IIO_BUFFER_DMAENGINE.
> 
> Signed-off-by: Radu Sabau <radu.sabau@analog.com>
One minor thing inline.

>  static int ad4691_reg_read(void *context, unsigned int reg, unsigned int *val)
>  {
>  	struct spi_device *spi = context;
> @@ -712,6 +791,7 @@ static const struct iio_buffer_setup_ops ad4691_manual_buffer_setup_ops = {
>  static int ad4691_cnv_burst_buffer_preenable(struct iio_dev *indio_dev)
>  {
>  	struct ad4691_state *st = iio_priv(indio_dev);
> +	unsigned int acc_mask;
>  	unsigned int k, i;
>  	int ret;
>  
> @@ -758,9 +838,9 @@ static int ad4691_cnv_burst_buffer_preenable(struct iio_dev *indio_dev)
>  	if (ret)
>  		goto err_unoptimize;
>  
> -	ret = regmap_write(st->regmap, AD4691_ACC_MASK_REG,
> -			   ~bitmap_read(indio_dev->active_scan_mask, 0,
> -				iio_get_masklength(indio_dev)) & GENMASK(15, 0));
> +	acc_mask = ~bitmap_read(indio_dev->active_scan_mask, 0,
> +				iio_get_masklength(indio_dev)) & GENMASK(15, 0);

Not obvious to me why this change is here.  If you want it, push back to the
original patch that introduced this code.

> +	ret = regmap_write(st->regmap, AD4691_ACC_MASK_REG, acc_mask);
>  	if (ret)
>  		goto err_unoptimize;
>  
> @@ -803,6 +883,209 @@ static const struct iio_buffer_setup_ops ad4691_cnv_burst_buffer_setup_ops = {
>  	.postdisable = &ad4691_cnv_burst_buffer_postdisable,
>  };