linux-kernel - Re: [PATCH v1 13/15] iio: adc: ad7768-1: add multiple scan types to support 16-bits mode

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Message-ID: <111f571d-1d88-42f7-b9a5-4b1cb328e26b@baylibre.com>
Date: Tue, 7 Jan 2025 17:49:03 -0600
From: David Lechner <dlechner@...libre.com>
To: Jonathan Santos <Jonathan.Santos@...log.com>, linux-iio@...r.kernel.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: lars@...afoo.de, Michael.Hennerich@...log.com, jic23@...nel.org,
 robh@...nel.org, krzk+dt@...nel.org, conor+dt@...nel.org,
 marcelo.schmitt1@...il.com
Subject: Re: [PATCH v1 13/15] iio: adc: ad7768-1: add multiple scan types to
 support 16-bits mode

On 1/7/25 9:26 AM, Jonathan Santos wrote:
> When the device is configured to Sinc5 filter and decimation x8,
> output data is reduced to 16-bits in order to support 1 MHz of
> sampling frequency due to clock limitation.

We aren't going to get a 1 MHz sample rate without SPI offload support so maybe
we should save this patch until then?

In this patch, we are still reading 24-bits per sample, so we aren't really
getting any benefit. It is probably fine for now to leave it as 24-bit even if
the last 8 bits are all 0 or just noise.

Also, the datasheet says:

	this path allows viewing of wider bandwidth; however, it is quantization
	noise limited so that output data is reduced to 16 bits

So this doesn't actually seem related to higher sample rates. There is a CONVLEN
bit in the INTERFACE_FORMAT register that globally reduces the output size to
16-bit, which I suspect would be what we will need for achieving the highest
sample rate when we add SPI offload support.

> 
> Use multiple scan types feature to enable the driver to switch
> scan type in runtime, making possible to support both 24-bit and
> 16-bit resolution.
> 
> Signed-off-by: Jonathan Santos <Jonathan.Santos@...log.com>
> ---
>  drivers/iio/adc/ad7768-1.c | 65 ++++++++++++++++++++++++++++++++------
>  1 file changed, 56 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/iio/adc/ad7768-1.c b/drivers/iio/adc/ad7768-1.c
> index 9741a6d47942..5e4e7d387f9a 100644
> --- a/drivers/iio/adc/ad7768-1.c
> +++ b/drivers/iio/adc/ad7768-1.c
> @@ -134,6 +134,11 @@ struct ad7768_clk_configuration {
>  	enum ad7768_pwrmode pwrmode;
>  };
>  
> +enum ad7768_scan_type {
> +	AD7768_SCAN_TYPE_NORMAL,
> +	AD7768_SCAN_TYPE_HIGH_SPEED,
> +};
> +
>  static const char * const ad7768_vcm_modes[] = {
>  	"(AVDD1-AVSS)/2",
>  	"2V5",
> @@ -145,6 +150,10 @@ static const char * const ad7768_vcm_modes[] = {
>  	"OFF",
>  };
>  
> +static const int ad7768_mclk_div_rates[4] = {
> +	16, 8, 4, 2,
> +};
> +
>  static const struct ad7768_clk_configuration ad7768_clk_config[] = {
>  	{ AD7768_MCLK_DIV_2, AD7768_DEC_RATE_8, 16,  AD7768_FAST_MODE },
>  	{ AD7768_MCLK_DIV_2, AD7768_DEC_RATE_16, 32,  AD7768_FAST_MODE },
> @@ -159,6 +168,21 @@ static const struct ad7768_clk_configuration ad7768_clk_config[] = {
>  	{ AD7768_MCLK_DIV_16, AD7768_DEC_RATE_1024, 16384, AD7768_ECO_MODE },
>  };
>  
> +static const struct iio_scan_type ad7768_scan_type[] = {
> +	[AD7768_SCAN_TYPE_NORMAL] = {
> +		.sign = 's',
> +		.realbits = 24,
> +		.storagebits = 32,

What happened to .shift = 8, ? If there is a reason for removing it, please add
that to the commit description.

> +		.endianness = IIO_BE,
> +	},
> +	[AD7768_SCAN_TYPE_HIGH_SPEED] = {
> +		.sign = 's',
> +		.realbits = 16,
> +		.storagebits = 32,

I guess it doesn't matter much since we are reading one sample at a time, but
I would expect storagebits to be 16 instead of 32. Or if it really needs to be
32, does it need shift = 16?

> +		.endianness = IIO_BE,
> +	},
> +};
> +
>  static int ad7768_get_vcm(struct iio_dev *dev, const struct iio_chan_spec *chan);
>  static int ad7768_set_vcm(struct iio_dev *dev, const struct iio_chan_spec *chan,
>  			  unsigned int mode);

...

> @@ -308,6 +329,15 @@ static int ad7768_scan_direct(struct iio_dev *indio_dev)
>  	ret = ad7768_spi_reg_read(st, AD7768_REG_ADC_DATA, &readval, 3);
>  	if (ret < 0)
>  		return ret;
> +
> +	/*
> +	 * When the decimation rate is set to x8, the ADC data precision is reduced
> +	 * from 24 bits to 16 bits. Since the AD7768_REG_ADC_DATA register provides
> +	 * 24-bit data, the precision is reduced by right-shifting the read value
> +	 * by 8 bits.
> +	 */
> +	if (st->dec_rate == 8)
> +		readval = readval >> 8;

Why not change size of ad7768_spi_reg_read() instead of reading 3 bytes and
throwing one away?

>  	/*
>  	 * Any SPI configuration of the AD7768-1 can only be
>  	 * performed in continuous conversion mode.