Re: [PATCH v3 2/4] iio: adc: ad7949: fix spi messages on non 14-bit controllers

[Jonathan Cameron <jic23@kernel.org>]

On Tue, 13 Jul 2021 00:34:23 -0400
Liam Beguin <liambeguin@gmail.com> wrote:

> From: Liam Beguin <lvb@xiphos.com>
> 
> This driver supports devices with 14-bit and 16-bit sample sizes.
> This is not always handled properly by spi controllers and can fail. To
> work around this limitation, pad samples to 16-bit and split the sample
> into two 8-bit messages in the event that only 8-bit messages are
> supported by the controller.
> 
> Signed-off-by: Liam Beguin <lvb@xiphos.com>

A small tidy up below that will make this even neater...

> ---
>  drivers/iio/adc/ad7949.c | 68 +++++++++++++++++++++++++++++++++++-----
>  1 file changed, 60 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/iio/adc/ad7949.c b/drivers/iio/adc/ad7949.c
> index 0b549b8bd7a9..65f78751225b 100644
> --- a/drivers/iio/adc/ad7949.c
> +++ b/drivers/iio/adc/ad7949.c
> @@ -12,6 +12,7 @@
>  #include <linux/regulator/consumer.h>
>  #include <linux/spi/spi.h>
>  #include <linux/bitfield.h>
> +#include <asm/unaligned.h>
>  
>  #define AD7949_MASK_TOTAL		GENMASK(13, 0)
>  
> @@ -67,6 +68,7 @@ static const struct ad7949_adc_spec ad7949_adc_spec[] = {
>   * @indio_dev: reference to iio structure
>   * @spi: reference to spi structure
>   * @resolution: resolution of the chip
> + * @bits_per_word: number of bits per SPI word
>   * @cfg: copy of the configuration register
>   * @current_channel: current channel in use
>   * @buffer: buffer to send / receive data to / from device
> @@ -77,6 +79,7 @@ struct ad7949_adc_chip {
>  	struct iio_dev *indio_dev;
>  	struct spi_device *spi;
>  	u8 resolution;
> +	u8 bits_per_word;
>  	u16 cfg;
>  	unsigned int current_channel;
>  	u16 buffer ____cacheline_aligned;
> @@ -86,19 +89,37 @@ static int ad7949_spi_write_cfg(struct ad7949_adc_chip *ad7949_adc, u16 val,
>  				u16 mask)
>  {
>  	int ret;
> -	int bits_per_word = ad7949_adc->resolution;
> -	int shift = bits_per_word - AD7949_CFG_REG_SIZE_BITS;
> +	u8 buf8[2];
>  	struct spi_message msg;
>  	struct spi_transfer tx[] = {
>  		{
>  			.tx_buf = &ad7949_adc->buffer,
>  			.len = 2,
> -			.bits_per_word = bits_per_word,
> +			.bits_per_word = ad7949_adc->bits_per_word,
>  		},
>  	};
>  
> +	ad7949_adc->buffer = 0;
>  	ad7949_adc->cfg = (val & mask) | (ad7949_adc->cfg & ~mask);
> -	ad7949_adc->buffer = ad7949_adc->cfg << shift;
> +
> +	switch (ad7949_adc->bits_per_word) {
> +	case 16:
> +		ad7949_adc->buffer = ad7949_adc->cfg << 2;
> +		break;
> +	case 14:
> +		ad7949_adc->buffer = ad7949_adc->cfg;
> +		break;
> +	case 8:
> +		/* Pack 14-bit value into 2 bytes, MSB first */
> +		buf8[0] = FIELD_GET(GENMASK(13, 6), ad7949_adc->cfg);
> +		buf8[1] = FIELD_GET(GENMASK(5, 0), ad7949_adc->cfg) << 2;
> +		memcpy(&ad7949_adc->buffer, buf8, 2);

Can probably tidy this up given the form - all we actually need to do is ensure the
value ends up big endian.

		/* Type is only big endian for this case as must be done as two transfers */
		ad7949_adc->buffer = (u16)cpu_to_be16(ad7959_adc->cfg << 2);

> +		break;
> +	default:
> +		dev_err(&ad7949_adc->indio_dev->dev, "unsupported BPW\n");
> +		return -EINVAL;
> +	}
> +
>  	spi_message_init_with_transfers(&msg, tx, 1);
>  	ret = spi_sync(ad7949_adc->spi, &msg);
>  
> @@ -115,14 +136,13 @@ static int ad7949_spi_read_channel(struct ad7949_adc_chip *ad7949_adc, int *val,
>  {
>  	int ret;
>  	int i;
> -	int bits_per_word = ad7949_adc->resolution;
> -	int mask = GENMASK(ad7949_adc->resolution - 1, 0);
> +	u8 buf8[2];

As below, not needed that I can see.

>  	struct spi_message msg;
>  	struct spi_transfer tx[] = {
>  		{
>  			.rx_buf = &ad7949_adc->buffer,
>  			.len = 2,
> -			.bits_per_word = bits_per_word,
> +			.bits_per_word = ad7949_adc->bits_per_word,
>  		},
>  	};
>  
> @@ -157,7 +177,26 @@ static int ad7949_spi_read_channel(struct ad7949_adc_chip *ad7949_adc, int *val,
>  
>  	ad7949_adc->current_channel = channel;
>  
> -	*val = ad7949_adc->buffer & mask;
> +	switch (ad7949_adc->bits_per_word) {
> +	case 16:
> +		*val = ad7949_adc->buffer;
> +		/* Shift-out padding bits */
> +		*val >>= 16 - ad7949_adc->resolution;
> +		break;
> +	case 14:
> +		*val = ad7949_adc->buffer & GENMASK(13, 0);
> +		break;
> +	case 8:
> +		memcpy(buf8, &ad7949_adc->buffer, 2);
> +		/* Convert byte array to u16, MSB first */
> +		*val = get_unaligned_be16(buf8);

Why the local copy first?

		*val = get_unaligned_be16(ad7949_adc->buffer);

should get what you want, it doesn't care about the type.
Even better, that buffer is aligned, so we can do
be16_to_cpu() safely I think...

> +		/* Shift-out padding bits */
> +		*val >>= 16 - ad7949_adc->resolution;
> +		break;
> +	default:
> +		dev_err(&ad7949_adc->indio_dev->dev, "unsupported BPW\n");
> +		return -EINVAL;
> +	}
>  
>  	return 0;
>  }
> @@ -265,6 +304,7 @@ static int ad7949_spi_init(struct ad7949_adc_chip *ad7949_adc)
>  
>  static int ad7949_spi_probe(struct spi_device *spi)
>  {
> +	u32 spi_ctrl_mask = spi->controller->bits_per_word_mask;
>  	struct device *dev = &spi->dev;
>  	const struct ad7949_adc_spec *spec;
>  	struct ad7949_adc_chip *ad7949_adc;
> @@ -291,6 +331,18 @@ static int ad7949_spi_probe(struct spi_device *spi)
>  	indio_dev->num_channels = spec->num_channels;
>  	ad7949_adc->resolution = spec->resolution;
>  
> +	/* Set SPI bits per word */
> +	if (spi_ctrl_mask & SPI_BPW_MASK(ad7949_adc->resolution)) {
> +		ad7949_adc->bits_per_word = ad7949_adc->resolution;
> +	} else if (spi_ctrl_mask == SPI_BPW_MASK(16)) {
> +		ad7949_adc->bits_per_word = 16;
> +	} else if (spi_ctrl_mask == SPI_BPW_MASK(8)) {
> +		ad7949_adc->bits_per_word = 8;
> +	} else {
> +		dev_err(dev, "unable to find common BPW with spi controller\n");
> +		return -EINVAL;
> +	}
> +
>  	ad7949_adc->vref = devm_regulator_get(dev, "vref");
>  	if (IS_ERR(ad7949_adc->vref)) {
>  		dev_err(dev, "fail to request regulator\n");