Re: [PATCH RFC v4 15/15] iio: adc: ad4695: Add support for SPI offload

[Jonathan Cameron <jic23@kernel.org>]

On Wed, 23 Oct 2024 15:59:22 -0500
David Lechner <dlechner@baylibre.com> wrote:

> Add support for SPI offload to the ad4695 driver. SPI offload allows
> sampling data at the max sample rate (500kSPS or 1MSPS).
> 
> This is developed and tested against the ADI example FPGA design for
> this family of ADCs [1].
> 
> [1]: http://analogdevicesinc.github.io/hdl/projects/ad469x_fmc/index.html
> 
> Signed-off-by: David Lechner <dlechner@baylibre.com>
A few questions inline. In general looks ok, but it's complex code and I'm
too snowed under for a very close look at the whole thing for a least a few weeks.

Jonathan

> ---
>  drivers/iio/adc/Kconfig  |   1 +
>  drivers/iio/adc/ad4695.c | 470 +++++++++++++++++++++++++++++++++++++++++++----
>  2 files changed, 440 insertions(+), 31 deletions(-)
> 
> diff --git a/drivers/iio/adc/Kconfig b/drivers/iio/adc/Kconfig
> index 92dfb495a8ce..f76a3f62a9ad 100644
> --- a/drivers/iio/adc/Kconfig
> +++ b/drivers/iio/adc/Kconfig
> @@ -53,6 +53,7 @@ config AD4695
>  	depends on SPI
>  	select REGMAP_SPI
>  	select IIO_BUFFER
> +	select IIO_BUFFER_DMAENGINE
>  	select IIO_TRIGGERED_BUFFER
>  	help
>  	  Say yes here to build support for Analog Devices AD4695 and similar

> +static int ad4695_offload_buffer_postenable(struct iio_dev *indio_dev)
> +{
> +	struct ad4695_state *st = iio_priv(indio_dev);
> +	struct spi_offload_trigger_config config = {
> +		.type = SPI_OFFLOAD_TRIGGER_DATA_READY,
> +	};
> +	struct spi_transfer *xfer = &st->buf_read_xfer[0];
> +	struct pwm_state state;
> +	u8 temp_chan_bit = st->chip_info->num_voltage_inputs;
> +	u8 num_slots = 0;
> +	u8 temp_en = 0;
> +	unsigned int bit;
> +	int ret;
> +
> +	iio_for_each_active_channel(indio_dev, bit) {
> +		if (bit == temp_chan_bit) {
> +			temp_en = 1;
> +			continue;
> +		}
> +
> +		ret = regmap_write(st->regmap, AD4695_REG_AS_SLOT(num_slots),
> +				   FIELD_PREP(AD4695_REG_AS_SLOT_INX, bit));
> +		if (ret)
> +			return ret;
> +
> +		num_slots++;
> +	}
> +
> +	/*
> +	 * For non-offload, we could discard data to work around this
> +	 * restriction, but with offload, that is not possible.
> +	 */
> +	if (num_slots < 2) {
> +		dev_err(&st->spi->dev,
> +			"At least two voltage channels must be enabled.\n");
> +		return -EINVAL;
> +	}
> +
> +	ret = regmap_update_bits(st->regmap, AD4695_REG_TEMP_CTRL,
> +				 AD4695_REG_TEMP_CTRL_TEMP_EN,
> +				 FIELD_PREP(AD4695_REG_TEMP_CTRL_TEMP_EN,
> +					    temp_en));
> +	if (ret)
> +		return ret;
> +
> +	/* Each BUSY event means just one sample for one channel is ready. */
> +	memset(xfer, 0, sizeof(*xfer));
> +	xfer->offload_flags = SPI_OFFLOAD_XFER_RX_STREAM;
> +	xfer->bits_per_word = 16;
> +	xfer->len = 2;
> +
> +	spi_message_init_with_transfers(&st->buf_read_msg, xfer, 1);
> +	st->buf_read_msg.offload = st->offload;
> +
> +	st->spi->max_speed_hz = st->spi_max_speed_hz;
> +	ret = spi_optimize_message(st->spi, &st->buf_read_msg);
> +	st->spi->max_speed_hz = AD4695_REG_ACCESS_SCLK_HZ;
> +	if (ret)
> +		return ret;
> +
> +	/*
> +	 * NB: technically, this is part the SPI offload trigger enable, but it
> +	 * doesn't work to call it from the offload trigger enable callback
> +	 * due to issues with ordering with respect to entering/exiting
> +	 * conversion mode.
Give some detail on the operations order.

> +	 */
> +	ret = regmap_set_bits(st->regmap, AD4695_REG_GP_MODE,
> +			      AD4695_REG_GP_MODE_BUSY_GP_EN);
> +	if (ret)
> +		goto err_unoptimize_message;
> +
> +	ret = spi_offload_trigger_enable(st->offload, st->offload_trigger,
> +					 &config);
> +	if (ret)
> +		goto err_disable_busy_output;
> +
> +	ret = ad4695_enter_advanced_sequencer_mode(st, num_slots);
> +	if (ret)
> +		goto err_offload_trigger_disable;
> +
> +	guard(mutex)(&st->cnv_pwm_lock);
> +	pwm_get_state(st->cnv_pwm, &state);
> +	/*
> +	 * PWM subsystem generally rounds down, so requesting 2x minimum high
> +	 * time ensures that we meet the minimum high time in any case.
> +	 */
> +	state.duty_cycle = AD4695_T_CNVH_NS * 2;
> +	ret = pwm_apply_might_sleep(st->cnv_pwm, &state);
> +	if (ret)
> +		goto err_offload_exit_conversion_mode;
> +
> +	return 0;
> +
> +err_offload_exit_conversion_mode:
> +	/* have to unwind in a different order to avoid triggering offload */

Needs more details here.

> +	spi_offload_trigger_disable(st->offload, st->offload_trigger);
> +	ad4695_cnv_manual_trigger(st);
> +	ad4695_exit_conversion_mode(st);
> +	goto err_disable_busy_output;
> +
> +err_offload_trigger_disable:
> +	spi_offload_trigger_disable(st->offload, st->offload_trigger);
> +
> +err_disable_busy_output:
> +	regmap_clear_bits(st->regmap, AD4695_REG_GP_MODE,
> +			  AD4695_REG_GP_MODE_BUSY_GP_EN);
> +
> +err_unoptimize_message:
> +	spi_unoptimize_message(&st->buf_read_msg);
> +
> +	return ret;
> +}

> +
>  static int ad4695_write_raw(struct iio_dev *indio_dev,
>  			    struct iio_chan_spec const *chan,
>  			    int val, int val2, long mask)
> @@ -779,6 +992,17 @@ static int ad4695_write_raw(struct iio_dev *indio_dev,
>  			default:
>  				return -EINVAL;
>  			}
> +		case IIO_CHAN_INFO_SAMP_FREQ: {
> +			struct pwm_state state;
> +
> +			if (val <= 0)
> +				return -EINVAL;
> +
> +			guard(mutex)(&st->cnv_pwm_lock);
> +			pwm_get_state(st->cnv_pwm, &state);

What limits this to rates the ADC can cope with?

> +			state.period = DIV_ROUND_UP_ULL(NSEC_PER_SEC, val);
> +			return pwm_apply_might_sleep(st->cnv_pwm, &state);
> +		}
>  		default:
>  			return -EINVAL;
>  		}

>  static int ad4695_probe(struct spi_device *spi)
>  {
>  	struct device *dev = &spi->dev;
>  	struct ad4695_state *st;
>  	struct iio_dev *indio_dev;
> -	struct gpio_desc *cnv_gpio;
>  	bool use_internal_ldo_supply;
>  	bool use_internal_ref_buffer;
>  	int ret;
>  
> -	cnv_gpio = devm_gpiod_get_optional(dev, "cnv", GPIOD_OUT_LOW);
> -	if (IS_ERR(cnv_gpio))
> -		return dev_err_probe(dev, PTR_ERR(cnv_gpio),
> -				     "Failed to get CNV GPIO\n");
> -
> -	/* Driver currently requires CNV pin to be connected to SPI CS */
> -	if (cnv_gpio)
> -		return dev_err_probe(dev, -ENODEV,
> -				     "CNV GPIO is not supported\n");
> -
>  	indio_dev = devm_iio_device_alloc(dev, sizeof(*st));
>  	if (!indio_dev)
>  		return -ENOMEM;
> @@ -1002,8 +1374,13 @@ static int ad4695_probe(struct spi_device *spi)
>  		return -EINVAL;
>  
>  	/* Registers cannot be read at the max allowable speed */
> +	st->spi_max_speed_hz = spi->max_speed_hz;
>  	spi->max_speed_hz = AD4695_REG_ACCESS_SCLK_HZ;
>  
> +	ret = devm_add_action_or_reset(dev, ad4695_restore_spi_max_speed_hz, st);

Why do you need to put it back in devm? What happens after this but without
a driver restart that uses that faster rate?

> +	if (ret)
> +		return ret;
> +
>  	st->regmap = devm_regmap_init_spi(spi, &ad4695_regmap_config);
>  	if (IS_ERR(st->regmap))
>  		return dev_err_probe(dev, PTR_ERR(st->regmap),
> @@ -1014,6 +1391,11 @@ static int ad4695_probe(struct spi_device *spi)
>  		return dev_err_probe(dev, PTR_ERR(st->regmap16),
>  				     "Failed to initialize regmap16\n");
>  
> +	st->cnv_gpio = devm_gpiod_get_optional(dev, "cnv", GPIOD_OUT_LOW);
> +	if (IS_ERR(st->cnv_gpio))
> +		return dev_err_probe(dev, PTR_ERR(st->cnv_gpio),
> +				     "Failed to get CNV GPIO\n");
> +
>  	ret = devm_regulator_bulk_get_enable(dev,
>  					     ARRAY_SIZE(ad4695_power_supplies),
>  					     ad4695_power_supplies);
> @@ -1139,14 +1521,39 @@ static int ad4695_probe(struct spi_device *spi)
>  	indio_dev->info = &ad4695_info;
>  	indio_dev->modes = INDIO_DIRECT_MODE;
>  	indio_dev->channels = st->iio_chan;
> -	indio_dev->num_channels = st->chip_info->num_voltage_inputs + 2;
>  
> -	ret = devm_iio_triggered_buffer_setup(dev, indio_dev,
> -					      iio_pollfunc_store_time,
> -					      ad4695_trigger_handler,
> -					      &ad4695_buffer_setup_ops);
> -	if (ret)
> -		return ret;
> +	static const struct spi_offload_config ad4695_offload_config = {
> +		.capability_flags = SPI_OFFLOAD_CAP_TRIGGER
> +				  | SPI_OFFLOAD_CAP_RX_STREAM_DMA,
> +	};
> +
> +	st->offload = devm_spi_offload_get(dev, spi, &ad4695_offload_config);
> +	ret = PTR_ERR_OR_ZERO(st->offload);
> +	if (ret && ret != -ENODEV)
> +		return dev_err_probe(dev, ret, "failed to get SPI offload\n");
> +
> +	if (ret == -ENODEV) {
> +		/* If no SPI offload, fall back to low speed usage. */
> +		dev_info(dev, "SPI offload not available\n");

As previous. Too noisy for a normal thing that might happen.
Maybe if we can't figure it out from anything userspace an see we could add
a print on the 'offload is enabled' side of things.

> +
> +		/* Driver currently requires CNV pin to be connected to SPI CS */
> +		if (st->cnv_gpio)
> +			return dev_err_probe(dev, -EINVAL,
> +					     "CNV GPIO is not supported\n");
> +
> +		indio_dev->num_channels = st->chip_info->num_voltage_inputs + 2;
> +
> +		ret = devm_iio_triggered_buffer_setup(dev, indio_dev,
> +						      iio_pollfunc_store_time,
> +						      ad4695_trigger_handler,
> +						      &ad4695_buffer_setup_ops);
> +		if (ret)
> +			return ret;
> +	} else {
> +		ret = ad4695_probe_spi_offload(indio_dev, st);
> +		if (ret)
> +			return ret;
> +	}