RE: [PATCH v1 2/4] iio: adc: ad7949: fix spi messages on non 14-bit controllers

["Liam Beguin" <liambeguin@gmail.com>]

On Fri Jul 9, 2021 at 4:19 AM EDT, Sa, Nuno wrote:
>
>
> > -----Original Message-----
> > From: Liam Beguin <liambeguin@gmail.com>
> > Sent: Friday, July 9, 2021 1:56 AM
> > To: liambeguin@gmail.com; lars@metafoo.de; Hennerich, Michael
> > <Michael.Hennerich@analog.com>; jic23@kernel.org; charles-
> > antoine.couret@essensium.com
> > Cc: linux-kernel@vger.kernel.org; linux-iio@vger.kernel.org;
> > devicetree@vger.kernel.org; robh+dt@kernel.org
> > Subject: [PATCH v1 2/4] iio: adc: ad7949: fix spi messages on non 14-bit
> > controllers
> > 
> > [External]
> > 
> > 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>
> > ---
> >  drivers/iio/adc/ad7949.c | 67
> > ++++++++++++++++++++++++++++++++++------
> >  1 file changed, 58 insertions(+), 9 deletions(-)
> > 
> > diff --git a/drivers/iio/adc/ad7949.c b/drivers/iio/adc/ad7949.c
> > index 93aacf4f680b..bbc6b56330a3 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 +69,59 @@ 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;
> >  };
> > 
> > +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)
> > +		ad7949_adc->bits_per_word = ad7949_adc-
> > >resolution;
> > +	else if (bpw == SPI_BPW_MASK(16))
> > +		ad7949_adc->bits_per_word = 16;
> > +	else
> > +		ad7949_adc->bits_per_word = 8;
> > +}
> > +
> >  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:
> > +		/* 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;
> > +	}
>
> Honestly I didn't went through the driver but just a question... Are we
> sure that 'ad7949_adc->resolution' will have something valid (8, 14,
> 16)?
> A default statement is always a nice to have :).
>  

Thanks for pointing that out. You're right that based on this driver
there's no guaranty that `ad7949_adc->resolution` will have a valid
value. I'll add a default to 8-bit.

Thanks,
Liam

> >  	spi_message_init_with_transfers(&msg, tx, 1);
> >  	ret = spi_sync(ad7949_adc->spi, &msg);
> > 
> > @@ -105,14 +138,12 @@ 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 +178,24 @@ 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 */
> > +		if (ad7949_adc->resolution == 14)
> > +			*val = *val >> 2;
> > +		break;
> > +	case 14:
> > +		*val = ad7949_adc->buffer & GENMASK(13, 0);
> > +		break;
> > +	case 8:
> > +		/* Convert byte array to u16, MSB first */
> > +		*val = (ad7949_adc->buf8[0] << 8) | ad7949_adc-
> > >buf8[1];
> > +		/* Shift-out padding bits */
> > +		if (ad7949_adc->resolution == 14)
> > +			*val = *val >> 2;
> > +		break;
> > +	}
> > 
> >  	return 0;
> >  }
> > @@ -280,6 +328,7 @@ 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)) {
> > --
> > 2.30.1.489.g328c10930387