Re: [PATCH v2 2/4] iio: adc: ad7949: fix spi messages on non 14-bit controllers

[Jonathan Cameron]

On Fri,  9 Jul 2021 11:58:54 -0400
Liam Beguin <liambeguin@xxxxxxxxx> wrote:

> From: Liam Beguin <lvb@xxxxxxxxxx>
> 
> 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.

Hmm. I assumed spi controllers would all support 16 bits, but a quick
grep confirms otherwise...  ah well!

> 
> Signed-off-by: Liam Beguin <lvb@xxxxxxxxxx>
> ---
>  drivers/iio/adc/ad7949.c | 69 ++++++++++++++++++++++++++++++++++------
>  1 file changed, 60 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/iio/adc/ad7949.c b/drivers/iio/adc/ad7949.c
> index 93aacf4f680b..770112ac820f 100644
> --- a/drivers/iio/adc/ad7949.c
> +++ b/drivers/iio/adc/ad7949.c
> @@ -11,6 +11,7 @@
>  #include <linux/module.h>
>  #include <linux/regulator/consumer.h>
>  #include <linux/spi/spi.h>
> +#include <linux/bitfield.h>
>  
>  #define AD7949_MASK_TOTAL		GENMASK(13, 0)
>  #define AD7949_CFG_REG_SIZE_BITS	14
> @@ -57,6 +58,7 @@ 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
> @@ -67,28 +69,60 @@ 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;
> +	union {
> +		__be16 buffer;
> +		u8 buf8[2];
> +	} ____cacheline_aligned;

This union made me a little nervous given issues with them under c spec, but
seems that you only use buffer or buf8, there is no mixing. I think I'd
rather you didn't have the union though as it makes it look like there might be.

Given we are doing cacheline spacing anyway for dma safety it take no more
real space to just have

__be16 buffer ____cacheline_aligned;
buf8[2];



>  };
>  
> +static void ad7949_set_bits_per_word(struct ad7949_adc_chip *ad7949_adc)
> +{
> +	u32 adc_mask = SPI_BPW_MASK(ad7949_adc->resolution);
> +	u32 bpw = adc_mask & ad7949_adc->spi->controller->bits_per_word_mask;
> +
> +	if (bpw == adc_mask)

if (bpw) is sufficient I think.

> +		ad7949_adc->bits_per_word = ad7949_adc->resolution;
> +	else if (bpw == SPI_BPW_MASK(16))

I'm missing something here...  If the resolution is 14, but the SPI controller
supports 8 and 16 I'd expect us to hit here .  adc_mask will not have the 16bit
bit set though so bpw == 0..

else if (ad7949_adc->spi->controller_bits_per_word_mask & SPI_BPW_MASK(16))

should do the right thing.

> +		ad7949_adc->bits_per_word = 16;
> +	else
> +		ad7949_adc->bits_per_word = 8;

I'd like a comment here on why this is the right default.  Took me a while
to think through it.

> +}
> +
>  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;
>  	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:
> +	default:

Default here should be an error. Imagine a 12 bit device is added.  Then
the above set_bits_per_word() might return either 12 or 8.  If it returns
12 then this is not the right thing to do. I'd argue we want to know about
that rather than papering over it.  Possibly we want to not let anything other
than 14 bits through in set_bits_per_word() so we print an error early...

> +		/* Pack 14-bit value into 2 bytes, MSB first */
> +		ad7949_adc->buf8[0] = FIELD_GET(GENMASK(13, 6), ad7949_adc->cfg);
> +		ad7949_adc->buf8[1] = FIELD_GET(GENMASK(5, 0), ad7949_adc->cfg);
> +		ad7949_adc->buf8[1] = ad7949_adc->buf8[1] << 2;
> +		break;
> +	}
> +
>  	spi_message_init_with_transfers(&msg, tx, 1);
>  	ret = spi_sync(ad7949_adc->spi, &msg);
>  
> @@ -105,14 +139,12 @@ 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);
>  	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,
>  		},
>  	};
>  
> @@ -147,7 +179,25 @@ 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;

buffer is __be16 and *val is an int so this isn't valid.

Now curious bit is I'm assuming original code worked... Given big endian
platforms are rare, that makes me suspect something odd is going on.
 
> +		/* Shift-out padding bits */
> +		if (ad7949_adc->resolution == 14)
> +			*val = *val >> 2;

		*val >>= 16 - ad7949_adc->resolution;

No one likes conditionals if we don't need them!


> +		break;
> +	case 14:
> +		*val = ad7949_adc->buffer & GENMASK(13, 0);
> +		break;
> +	case 8:
> +	default:
> +		/* Convert byte array to u16, MSB first */
> +		*val = (ad7949_adc->buf8[0] << 8) | ad7949_adc->buf8[1];

Use unaligned_get_be16()

> +		/* Shift-out padding bits */
> +		if (ad7949_adc->resolution == 14)
> +			*val = *val >> 2;

As above make this unconditional.

> +		break;
> +	}
>  
>  	return 0;
>  }
> @@ -280,6 +330,7 @@ static int ad7949_spi_probe(struct spi_device *spi)
>  	spec = &ad7949_adc_spec[spi_get_device_id(spi)->driver_data];
>  	indio_dev->num_channels = spec->num_channels;
>  	ad7949_adc->resolution = spec->resolution;
> +	ad7949_set_bits_per_word(ad7949_adc);
>  
>  	ad7949_adc->vref = devm_regulator_get(dev, "vref");
>  	if (IS_ERR(ad7949_adc->vref)) {