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

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

On Sat Jul 17, 2021 at 1:23 PM EDT, Jonathan Cameron wrote:
> 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);

Hi Jonathan,

Apologies for the delay on this.

>
> 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);

That's much nicer, I'll rework the patch using this.

Thanks,
Liam

>
> > +		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");