Re: [PATCH v1 1/3] iio: adc: ad_sigma_delta: Add CS assert function

[Jonathan Cameron <jic23@kernel.org>]

On Sat, 21 Dec 2024 17:56:00 +0200
Alisa-Dariana Roman <alisadariana@gmail.com> wrote:

> Some sigma-delta ADCs, such as AD7191 and AD7780, have no registers and
> start conversion when CS is asserted. Add helper function to support
> this use case by allowing devices to assert CS without performing
> register operations.
Hi Alisa-Dariana,

I had a look at the ad7191 datasheet. Given this description,
I was expecting to see it do a pre pulse of the chip select to trigger
the acquisition.  However, what I see is a power down line (which is more
or less a chip select) but it just has a specified t1 delay before the
DOUT will change to the state for the first bit and the host
can start driving the clock.

That can be done by setting spi_device->cs_setup to whatever delay is
needed.  The text is spi_device docs are a little vague,
but I'd take it as t1 + t2 (maybe t3 to be safe).

That is going to be more reliable than trying to hold the cs across
messages / spi_sync() calls, particularly if the bus might not be
locked (which the code below suggests).

Jonathan


> 
> This function can be used by drivers through their set_mode callback.
> 
> Signed-off-by: Alisa-Dariana Roman <alisa.roman@analog.com>
> ---
>  drivers/iio/adc/ad_sigma_delta.c       | 24 ++++++++++++++++++++++++
>  include/linux/iio/adc/ad_sigma_delta.h |  1 +
>  2 files changed, 25 insertions(+)
> 
> diff --git a/drivers/iio/adc/ad_sigma_delta.c b/drivers/iio/adc/ad_sigma_delta.c
> index 0f355dac7813..c0f33d4baddf 100644
> --- a/drivers/iio/adc/ad_sigma_delta.c
> +++ b/drivers/iio/adc/ad_sigma_delta.c
> @@ -48,6 +48,30 @@ void ad_sd_set_comm(struct ad_sigma_delta *sigma_delta, uint8_t comm)
>  }
>  EXPORT_SYMBOL_NS_GPL(ad_sd_set_comm, "IIO_AD_SIGMA_DELTA");
>  
> +/**
> + * ad_sd_assert_cs() - Assert chip select line
> + *
> + * @sigma_delta: The sigma delta device
> + *
> + * Returns 0 on success, an error code otherwise.
> + **/
> +int ad_sd_assert_cs(struct ad_sigma_delta *sigma_delta)
> +{
> +	struct spi_transfer t = {
> +		.len = 0,
> +		.cs_change = sigma_delta->keep_cs_asserted,
> +	};
> +	struct spi_message m;
> +
> +	spi_message_init(&m);
> +	spi_message_add_tail(&t, &m);
> +
> +	if (sigma_delta->bus_locked)
> +		return spi_sync_locked(sigma_delta->spi, &m);
> +	return spi_sync(sigma_delta->spi, &m);
> +}
> +EXPORT_SYMBOL_NS_GPL(ad_sd_assert_cs, IIO_AD_SIGMA_DELTA);
> +
>  /**
>   * ad_sd_write_reg() - Write a register
>   *
> diff --git a/include/linux/iio/adc/ad_sigma_delta.h b/include/linux/iio/adc/ad_sigma_delta.h
> index 417073c52380..99ab56d04793 100644
> --- a/include/linux/iio/adc/ad_sigma_delta.h
> +++ b/include/linux/iio/adc/ad_sigma_delta.h
> @@ -178,6 +178,7 @@ static inline int ad_sigma_delta_postprocess_sample(struct ad_sigma_delta *sd,
>  }
>  
>  void ad_sd_set_comm(struct ad_sigma_delta *sigma_delta, uint8_t comm);
> +int ad_sd_assert_cs(struct ad_sigma_delta *sigma_delta);
>  int ad_sd_write_reg(struct ad_sigma_delta *sigma_delta, unsigned int reg,
>  	unsigned int size, unsigned int val);
>  int ad_sd_read_reg(struct ad_sigma_delta *sigma_delta, unsigned int reg,