Re: [PATCH v4 2/5] iio: adc: ad7949: fix spi messages on non 14-bit controllers

[Liam Beguin]

On Sat Jul 31, 2021 at 10:52 AM EDT, Jonathan Cameron wrote:
> On Sat, 31 Jul 2021 15:29:21 +0100
> Jonathan Cameron <jic23@xxxxxxxxxx> wrote:
>
> > On Tue, 27 Jul 2021 19:29:03 -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.
> > > 
> > > Signed-off-by: Liam Beguin <lvb@xxxxxxxxxx>
> > > ---
> > >  drivers/iio/adc/ad7949.c | 62 ++++++++++++++++++++++++++++++++++------
> > >  1 file changed, 54 insertions(+), 8 deletions(-)
> > > 
> > > diff --git a/drivers/iio/adc/ad7949.c b/drivers/iio/adc/ad7949.c
> > > index 0b549b8bd7a9..f1702c54c8be 100644
> > > --- a/drivers/iio/adc/ad7949.c
> > > +++ b/drivers/iio/adc/ad7949.c
> > > @@ -67,6 +67,7 @@ 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 +78,7 @@ 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 +88,34 @@ 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;  
> > 
> > The define for this was removed in patch 1.  I'll fix that up whilst applying by
> > keeping it until this patch.  Please check build passes on intermediate points
> > during a patch series as otherwise we may break bisectability and that's really
> > annoying if you are bisecting!
> > 
> > Jonathan
> > 
> > >  	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:
> > > +		/* Here, type is big endian as it must be sent in two transfers */
> > > +		ad7949_adc->buffer = (u16)cpu_to_be16(ad7949_adc->cfg << 2);
>
> Gah, I wasn't thinking clearly when I suggested this. Sparse warns on
> the
> endian conversion
>
> One option is to resort to ignoring the fact we know it's aligned and
> use the put_unaligned_be16() and get_unaligned_be16 calls which sparse
> seems to be
> happy with. Alternative would be to just have a be16 buffer after the
> existing
> one in the iio_priv structure. Then you will have to change the various
> users
> of iio_priv()->buffer to point to the new value if we are doing 8 bit
> transfers.
>
> Whilst more invasive, this second option is the one I'd suggest.

Understood, I'll go with your suggestion.

Out of curiosity, other that being more explicit, is there another
we'd rather not use {get,put}_unaligned_be16()?

> Note that there will be no need to add an __cacheline_aligned marking to
> this
> new element because it will be in a cachline that is only used for DMA
> simply being
> after the other buffer element which is force to start on a new
> cacheline.

Noted, Thanks for taking the time to explaining this.

Liam

>
> Jonathan
>  
> > > +		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 +132,12 @@ 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,
> > >  		},
> > >  	};
> > >  
> > > @@ -157,7 +172,25 @@ 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;
> > > +		/* Shift-out padding bits */
> > > +		*val >>= 16 - ad7949_adc->resolution;
> > > +		break;
> > > +	case 14:
> > > +		*val = ad7949_adc->buffer & GENMASK(13, 0);
> > > +		break;
> > > +	case 8:
> > > +		/* Here, type is big endian as data was sent in two transfers */
> > > +		*val = be16_to_cpu(ad7949_adc->buffer);
> > > +		/* 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 +298,7 @@ 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 +325,18 @@ 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");  
> >