Re: [PATCH 12/13] iio: offload: add new PWM triggered DMA buffer driver

[Nuno Sá]

On Wed, 2024-01-10 at 13:49 -0600, David Lechner wrote:
> This adds a new driver for handling SPI offloading using a PWM as the
> trigger and DMA for the received data. This will be used by ADCs in
> conjunction with SPI controllers with offloading support to be able
> to sample at high rates without CPU intervention.
> 
> Signed-off-by: David Lechner <dlechner@xxxxxxxxxxxx>
> ---
>  drivers/iio/Kconfig                                |   1 +
>  drivers/iio/Makefile                               |   1 +
>  .../iio/buffer/industrialio-hw-triggered-buffer.c  |   1 +
>  drivers/iio/offload/Kconfig                        |  21 ++
>  drivers/iio/offload/Makefile                       |   2 +
>  drivers/iio/offload/iio-pwm-triggered-dma-buffer.c | 212
> +++++++++++++++++++++
>  6 files changed, 238 insertions(+)
> 
> diff --git a/drivers/iio/Kconfig b/drivers/iio/Kconfig
> index 52eb46ef84c1..56738282d82f 100644
> --- a/drivers/iio/Kconfig
> +++ b/drivers/iio/Kconfig
> @@ -90,6 +90,7 @@ source "drivers/iio/imu/Kconfig"
>  source "drivers/iio/light/Kconfig"
>  source "drivers/iio/magnetometer/Kconfig"
>  source "drivers/iio/multiplexer/Kconfig"
> +source "drivers/iio/offload/Kconfig"
>  source "drivers/iio/orientation/Kconfig"
>  source "drivers/iio/test/Kconfig"
>  if IIO_TRIGGER
> diff --git a/drivers/iio/Makefile b/drivers/iio/Makefile
> index 9622347a1c1b..20acf5e1a4a7 100644
> --- a/drivers/iio/Makefile
> +++ b/drivers/iio/Makefile
> @@ -34,6 +34,7 @@ obj-y += imu/
>  obj-y += light/
>  obj-y += magnetometer/
>  obj-y += multiplexer/
> +obj-y += offload/
>  obj-y += orientation/
>  obj-y += position/
>  obj-y += potentiometer/
> diff --git a/drivers/iio/buffer/industrialio-hw-triggered-buffer.c
> b/drivers/iio/buffer/industrialio-hw-triggered-buffer.c
> index 7a8a71066b0e..a2fae6059616 100644
> --- a/drivers/iio/buffer/industrialio-hw-triggered-buffer.c
> +++ b/drivers/iio/buffer/industrialio-hw-triggered-buffer.c
> @@ -86,6 +86,7 @@ static int iio_hw_trigger_buffer_probe(struct
> auxiliary_device *adev,
>  }
>  
>  static const struct auxiliary_device_id iio_hw_trigger_buffer_id_table[] = {
> +	{ .name = "pwm-triggered-dma-buffer.triggered-buffer" },
>  	{ }
>  };
>  MODULE_DEVICE_TABLE(auxiliary, iio_hw_trigger_buffer_id_table);
> diff --git a/drivers/iio/offload/Kconfig b/drivers/iio/offload/Kconfig
> new file mode 100644
> index 000000000000..760c0cfe0e9c
> --- /dev/null
> +++ b/drivers/iio/offload/Kconfig
> @@ -0,0 +1,21 @@
> +# SPDX-License-Identifier: GPL-2.0-only
> +#
> +# SPI offload handlers for Industrial I/O
> +#
> +# When adding new entries keep the list in alphabetical order
> +
> +menu "SPI offload handlers"
> +
> +config IIO_PWM_TRIGGERED_DMA_BUFFER
> +	tristate "PWM trigger and DMA buffer connected to SPI offload"
> +	select AUXILIARY_BUS
> +	select IIO_BUFFER_DMAENGINE
> +	help
> +	  Provides a periodic hardware trigger via a PWM connected to the
> +	  trigger input of a SPI offload and a hardware buffer implemented
> +	  via DMA connected to the data output stream the a SPI offload.
> +
> +	  To compile this driver as a module, choose M here: the
> +	  module will be called "iio-pwm-triggered-dma-buffer".
> +
> +endmenu
> diff --git a/drivers/iio/offload/Makefile b/drivers/iio/offload/Makefile
> new file mode 100644
> index 000000000000..7300ce82f066
> --- /dev/null
> +++ b/drivers/iio/offload/Makefile
> @@ -0,0 +1,2 @@
> +
> +obj-$(CONFIG_IIO_PWM_TRIGGERED_DMA_BUFFER) := iio-pwm-triggered-dma-buffer.o
> diff --git a/drivers/iio/offload/iio-pwm-triggered-dma-buffer.c
> b/drivers/iio/offload/iio-pwm-triggered-dma-buffer.c
> new file mode 100644
> index 000000000000..970ea82316f6
> --- /dev/null
> +++ b/drivers/iio/offload/iio-pwm-triggered-dma-buffer.c
> @@ -0,0 +1,212 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Platform driver for a PWM trigger and DMA buffer connected to a SPI
> + * controller offload instance implementing the iio-hw-triggered-buffer
> + * interface.
> + *
> + * Copyright (C) 2023 Analog Devices, Inc.
> + * Copyright (C) 2023 BayLibre, SAS
> + */
> +
> +#include <linux/auxiliary_bus.h>
> +#include <linux/pwm.h>
> +#include <linux/module.h>
> +#include <linux/mod_devicetable.h>
> +#include <linux/platform_device.h>
> +#include <linux/iio/buffer.h>
> +#include <linux/iio/hw_triggered_buffer_impl.h>
> +#include <linux/iio/iio.h>
> +#include <linux/iio/trigger.h>
> +#include <linux/iio/sysfs.h>
> +#include <linux/iio/buffer-dmaengine.h>
> +#include <linux/sysfs.h>
> +
> +struct iio_pwm_triggered_dma_buffer {
> +	struct iio_hw_triggered_buffer_device hw;
> +	struct pwm_device *pwm;
> +};
> +
> +static const struct iio_trigger_ops iio_pwm_triggered_dma_buffer_ops;
> +
> +static int iio_pwm_triggered_dma_buffer_set_state(struct iio_trigger *trig,
> bool state)
> +{
> +	struct iio_pwm_triggered_dma_buffer *st =
> iio_trigger_get_drvdata(trig);
> +
> +	if (state)
> +		return pwm_enable(st->pwm);
> +
> +	pwm_disable(st->pwm);
> +
> +	return 0;
> +}
> +
> +static int iio_pwm_triggered_dma_buffer_validate_device(struct iio_trigger
> *trig,
> +							struct iio_dev
> *indio_dev)
> +{
> +	/* Don't allow assigning trigger via sysfs. */
> +	return -EINVAL;
> +}
> +
> +static const struct iio_trigger_ops iio_pwm_triggered_dma_buffer_ops = {
> +	.set_trigger_state = iio_pwm_triggered_dma_buffer_set_state,
> +	.validate_device = iio_pwm_triggered_dma_buffer_validate_device,
> +};
> +
> +static u32 axi_spi_engine_offload_pwm_trigger_get_rate(struct iio_trigger
> *trig)
> +{
> +	struct iio_pwm_triggered_dma_buffer *st =
> iio_trigger_get_drvdata(trig);
> +	u64 period_ns = pwm_get_period(st->pwm);
> +
> +	if (period_ns)
> +		return DIV_ROUND_CLOSEST_ULL(NSEC_PER_SEC, period_ns);
> +
> +	return 0;
> +}
> +
> +static int
> +axi_spi_engine_offload_set_samp_freq(struct iio_pwm_triggered_dma_buffer *st,
> +				     u32 requested_hz)
> +{
> +	int period_ns;
> +
> +	if (requested_hz == 0)
> +		return -EINVAL;
> +
> +	period_ns = DIV_ROUND_UP(NSEC_PER_SEC, requested_hz);
> +
> +	/*
> +	 * FIXME: We really just need a clock, not a PWM. The current duty
> cycle
> +	 * value is a hack to work around the edge vs. level offload trigger
> +	 * issue in the ADI AXI SPI Engine firmware.
> +	 */
> +	return pwm_config(st->pwm, 10, period_ns);
> +}
> +
> +static ssize_t sampling_frequency_show(struct device *dev,
> +				       struct device_attribute *attr, char
> *buf)
> +{
> +	struct iio_trigger *trig = to_iio_trigger(dev);
> +
> +	return sysfs_emit(buf, "%u\n",
> +			  axi_spi_engine_offload_pwm_trigger_get_rate(trig));
> +}
> +
> +static ssize_t sampling_frequency_store(struct device *dev,
> +					struct device_attribute *attr,
> +					const char *buf, size_t len)
> +{
> +	struct iio_trigger *trig = to_iio_trigger(dev);
> +	struct iio_pwm_triggered_dma_buffer *st =
> iio_trigger_get_drvdata(trig);
> +	int ret;
> +	u32 val;
> +
> +	ret = kstrtou32(buf, 10, &val);
> +	if (ret)
> +		return ret;
> +
> +	ret = axi_spi_engine_offload_set_samp_freq(st, val);
> +	if (ret)
> +		return ret;
> +
> +	return len;
> +}
> +

This is also something that made me wonder... In the end of the day, the
frequency at which we want to sample the data depends on the converter device.
This completely detached interface makes it more easy for the user to screw up
things, right?

Things might even become more complicated in usecases where we have an
additional pwm (on top of the data fetch trigger one) for triggering conversions
in the converter and we might need to properly control the phase between those
two signals. So, how would we handle it in the current form? We have one pwm
belonging to the offload engine and one belonging to the converter but they need
to cope together...

I'm aware you have some alternative ideas for not using pwms but the series is
using pwms... And the above usecase is real and it's sitting out of tree waiting
for the offload stuff to get merged so I can upstream that driver :)

- Nuno Sá