Re: [PATCH v6 3/4] iio: adc: ltc2378: Enable high-speed data capture

[David Lechner]

On 7/9/26 3:50 PM, Marcelo Schmitt wrote:
> Make use of SPI transfer offloading to speed up data capture, enabling data
> acquisition at faster sample rates (up to 2 MSPS).
> 

...

> +static int ltc2378_prepare_offload_message(struct device *dev,
> +					   struct ltc2378_state *st)
> +{
> +	unsigned int resolution = st->info->offload_chan.scan_type.realbits;
> +
> +	st->offload_xfer.bits_per_word = resolution;
> +	st->offload_xfer.len = spi_bpw_to_bytes(resolution);
> +	st->offload_xfer.offload_flags = SPI_OFFLOAD_XFER_RX_STREAM;
> +
> +	/* Initialize message with offload */
> +	spi_message_init_with_transfers(&st->offload_msg, &st->offload_xfer, 1);
> +	st->offload_msg.offload = st->offload;
> +
> +	return devm_spi_optimize_message(dev, st->spi, &st->offload_msg);
> +}

Would be more logical to move this function after buffer stuff.

> +
> +static int ltc2378_offload_buffer_postenable(struct iio_dev *indio_dev)
> +{
> +	struct ltc2378_state *st = iio_priv(indio_dev);
> +	int ret;
> +
> +	ret = pwm_set_waveform_might_sleep(st->cnv_trigger, &st->cnv_wf, true);
> +	if (ret)
> +		return ret;
> +
> +	ret = spi_offload_trigger_enable(st->offload, st->offload_trigger,
> +					 &st->offload_trigger_config);
> +	if (ret)
> +		goto out_pwm_disable;
> +
> +	return 0;
> +
> +out_pwm_disable:
> +	pwm_disable(st->cnv_trigger);
> +	return ret;
> +}
> +
> +static int ltc2378_offload_buffer_predisable(struct iio_dev *indio_dev)
> +{
> +	struct ltc2378_state *st = iio_priv(indio_dev);
> +
> +	spi_offload_trigger_disable(st->offload, st->offload_trigger);
> +	pwm_disable(st->cnv_trigger);
> +
> +	return 0;
> +}
> +
> +static const struct iio_buffer_setup_ops ltc2378_offload_buffer_ops = {
> +	.postenable = &ltc2378_offload_buffer_postenable,
> +	.predisable = &ltc2378_offload_buffer_predisable,
> +};
> +

... to here or even below somewhere.

> +static int ltc2378_spi_offload_setup(struct iio_dev *indio_dev,
> +				     struct ltc2378_state *st)
> +{
> +	struct device *dev = &st->spi->dev;
> +	struct dma_chan *rx_dma;
> +
> +	indio_dev->setup_ops = &ltc2378_offload_buffer_ops;
> +
> +	st->offload_trigger = devm_spi_offload_trigger_get(dev, st->offload,
> +							   SPI_OFFLOAD_TRIGGER_PERIODIC);
> +	if (IS_ERR(st->offload_trigger))
> +		return dev_err_probe(dev, PTR_ERR(st->offload_trigger),
> +				     "failed to get offload trigger\n");
> +
> +	st->offload_trigger_config.type = SPI_OFFLOAD_TRIGGER_PERIODIC;
> +
> +	rx_dma = devm_spi_offload_rx_stream_request_dma_chan(dev, st->offload);
> +	if (IS_ERR(rx_dma))
> +		return dev_err_probe(dev, PTR_ERR(rx_dma), "failed to get offload RX DMA\n");
> +
> +	return devm_iio_dmaengine_buffer_setup_with_handle(dev, indio_dev, rx_dma,
> +							   IIO_BUFFER_DIRECTION_IN);
> +}
> +

...

> @@ -340,8 +672,53 @@ static int ltc2378_probe(struct spi_device *spi)
>  		return dev_err_probe(dev, PTR_ERR(st->cnv_gpio),
>  				     "failed to get CNV GPIO");
>  
> -	indio_dev->channels = &st->info->chan;
> -	indio_dev->num_channels = 1;
> +	st->offload = devm_spi_offload_get(dev, spi, &ltc2378_offload_config);
> +	ret = PTR_ERR_OR_ZERO(st->offload);
> +	/* Fall back to low speed usage when no SPI offload is available. */
> +	if (ret == -ENODEV) {
> +		indio_dev->info = &ltc2378_iio_info;
> +		indio_dev->channels = &st->info->chan;
> +		indio_dev->num_channels = 1;
> +	} else if (ret) {
> +		return dev_err_probe(dev, ret, "failed to get offload\n");
> +	} else {
> +		indio_dev->info = &ltc2378_offload_iio_info;
> +		indio_dev->channels = &st->info->offload_chan;
> +		indio_dev->num_channels = 1;
> +		ret = ltc2378_spi_offload_setup(indio_dev, st);
> +		if (ret)
> +			return dev_err_probe(dev, ret,
> +					     "failed to setup SPI offload\n");
> +
> +		ret = ltc2378_pwm_get(st);
> +		if (ret)
> +			return dev_err_probe(dev, ret, "failed to get PWM\n");
> +
> +		st->sample_freq_range[0] = 1; /* min */
> +		st->sample_freq_range[1] = 1; /* step */
> +		st->sample_freq_range[2] = st->info->max_sample_rate_Hz; /* max */
> +
> +		/*
> +		 * Start with a slower sampling rate so there is some room for
> +		 * adjusting the sample averaging and the sampling frequency
> +		 * without hitting the maximum conversion rate.
> +		 */
> +		ret = ltc2378_update_conversion_rate(st, st->info->max_sample_rate_Hz >> 4);
> +		if (ret)
> +			return dev_err_probe(dev, ret,
> +					     "failed to set offload samp freq\n");
> +
> +		ret = ltc2378_prepare_offload_message(&spi->dev, st);
> +		if (ret)
> +			return dev_err_probe(dev, ret, "failed to optimize SPI message\n");
> +
> +		/*
> +		 * Set single-read transfer bits_per_word so the SPI subsystem
> +		 * rearanges data to CPU endianness, enabling us to reuse

s/rearanges/rearranges/

> +		 * offload_chan specifications for single-shot reads.
> +		 */
> +		st->xfer.bits_per_word = st->info->offload_chan.scan_type.realbits;
> +	}
>  
>  	st->xfer.rx_buf = &st->scan.data;
>  	st->xfer.len = spi_bpw_to_bytes(indio_dev->channels[0].scan_type.realbits);
> @@ -412,3 +789,4 @@ module_spi_driver(ltc2378_driver);
>  MODULE_AUTHOR("Marcelo Schmitt <marcelo.schmitt@xxxxxxxxxx>");
>  MODULE_DESCRIPTION("Analog Devices LTC2378 ADC series driver");
>  MODULE_LICENSE("GPL");
> +MODULE_IMPORT_NS("IIO_DMAENGINE_BUFFER");