Subject: Re: [PATCH 4/4] iio: adc: stm32-dfsdm: add scale and offset support

From: Jonathan Cameron <[email-masked]>
Date: Fri, 14 Feb 2020 15:10:11 +0000

On Fri, 14 Feb 2020 14:49:18 +0000
Olivier MOYSAN <[email-masked]> wrote:

> Hi Jonathan,
> 
> On 2/14/20 2:11 PM, Jonathan Cameron wrote:
> > On Tue, 11 Feb 2020 15:19:01 +0000
> > Olivier MOYSAN <[email-masked]> wrote:
> >  
> >> Hi Jonathan,
> >>
> >> On 2/8/20 5:18 PM, Jonathan Cameron wrote:  
> >>> On Tue, 4 Feb 2020 11:10:08 +0100
> >>> Olivier Moysan <[email-masked]> wrote:
> >>>     
> >>>> Add scale and offset attributes support to STM32 DFSDM.
> >>>>
> >>>> Signed-off-by: Olivier Moysan <[email-masked]>  
> >>> Hmm. I can't remember this history of this but we've kind of
> >>> ended up backwards wrt to other consumer drivers.
> >>>
> >>> In some sense this is similar to the analog gyroscopes.  In those
> >>> the consumer driver is the gyroscope which is consuming the raw
> >>> readings from an ADC connected to the channel.  This results
> >>> in us getting readings reported by the gyroscope driver.
> >>>
> >>> Here we have a sigma delta convertor consuming the pulse train
> >>> from a sigma delta device.  So the channels are reported by
> >>> the sigma delta receiver, whereas i think the nearest equivalent
> >>> to the analog voltage outputing gyroscopes would have been if
> >>> we had reported the channel values at the sigma delta converter.  
> >> The DFSDM driver is currently used as a consumer of the sd modulator.
> >> The scale and offset values of the channels are already computed by
> >> the DFSDM driver, and provided by this driver to the IIO ABI.
> >> However, the DFSDM has no voltage reference, so it has to retrieve
> >> it from sd-modulator channels, for the scale factor computation.
> >>
> >>                                       scale  offset
> >>                                         ^      ^
> >>                                         |      |       IIO ABI
> >> +-------------------------------------------------------------+
> >>            +---------------+          +-------------+
> >>            |sd driver      |          |DFSDM driver |
> >>            +---------------+          +-------------+
> >> +-------------------------------------------------------------+
> >>                                                            HW
> >>            +---------------+          +-------------+
> >> +------->+ sd-modulator  +--------->+ DFSDM +-------->
> >> analog   +------+--------+          +-------------+ output
> >> input           ^
> >>                   | vref
> >>                   +
> >>
> >>
> >> Is it the topology your are expecting ?  
> > It's not the one we'd expect if we are aligning with similar cases
> > elsewhere in IIO.  For example, if we attach an analog accelerometer
> > to an ADC, we report the accel channels on the accelerometer not the
> > ADC.  The equivalent would be to see the DFSDM as providing a
> > conversion service to the SD device which is actually executing
> > the measurement and has the input channels.
> >
> >
> >           scale  offset  raw
> >             ^      ^      ^
> >             |      |      |                              IIO ABI
> >   +-------------------------------------------------------------+
> >             +---------------+          +-------------+
> >             |sd driver      |          |DFSDM driver |
> >             +---------------+          +-------------+
> >   +-------------------------------------------------------------+
> >                                                             HW
> >             +---------------+          +-------------+
> >   +------->+ sd-modulator  +--------->+ DFSDM +-------->
> >   analog   +------+--------+          +-------------+ output
> >   input           ^
> >                    | vref  
> >>                   +
> >>  
> Thanks for your clarification.
> ok, moving to this logic is a significant change.
> I need to evaluate further the impact on the dfsdm driver.

Understood!  If we can't do it without potentially breaking users then
such is life.

Jonathan

> 
> Regards
> Olivier
> >> If not, I probably missedsomething. Could you please clarify this point ?
> >>
> >> Regards
> >> Olivier  
> >>> This wasn't really an issue when the only values available were
> >>> raw, but if we are adding scale and offset, they are things that
> >>> belong to the ad1201 for example, not the upstream stm32-dfsdm unit.
> >>>
> >>> Thinking of it another way, we don't report an SPI ADC output in
> >>> the driver for the SPI master.
> >>>
> >>> Could we flip it around without breaking anything?
> >>>
> >>> Jonathan
> >>>     
> >>>> ---
> >>>>    drivers/iio/adc/stm32-dfsdm-adc.c | 105 +++++++++++++++++++++++++++++-
> >>>>    1 file changed, 102 insertions(+), 3 deletions(-)
> >>>>
> >>>> diff --git a/drivers/iio/adc/stm32-dfsdm-adc.c b/drivers/iio/adc/stm32-dfsdm-adc.c
> >>>> index 07b9dfdf8e76..b85fd3e90496 100644
> >>>> --- a/drivers/iio/adc/stm32-dfsdm-adc.c
> >>>> +++ b/drivers/iio/adc/stm32-dfsdm-adc.c
> >>>> @@ -10,6 +10,7 @@
> >>>>    #include <linux/dma-mapping.h>
> >>>>    #include <linux/iio/adc/stm32-dfsdm-adc.h>
> >>>>    #include <linux/iio/buffer.h>
> >>>> +#include <linux/iio/consumer.h>
> >>>>    #include <linux/iio/hw-consumer.h>
> >>>>    #include <linux/iio/sysfs.h>
> >>>>    #include <linux/iio/timer/stm32-lptim-trigger.h>
> >>>> @@ -67,6 +68,13 @@ struct stm32_dfsdm_dev_data {
> >>>>    	const struct regmap_config *regmap_cfg;
> >>>>    };
> >>>>    
> >>>> +struct stm32_dfsdm_sd_chan_info {
> >>>> +	int scale_val;
> >>>> +	int scale_val2;
> >>>> +	int offset;
> >>>> +	unsigned int differential;
> >>>> +};
> >>>> +
> >>>>    struct stm32_dfsdm_adc {
> >>>>    	struct stm32_dfsdm *dfsdm;
> >>>>    	const struct stm32_dfsdm_dev_data *dev_data;
> >>>> @@ -79,6 +87,7 @@ struct stm32_dfsdm_adc {
> >>>>    	struct iio_hw_consumer *hwc;
> >>>>    	struct completion completion;
> >>>>    	u32 *buffer;
> >>>> +	struct stm32_dfsdm_sd_chan_info *sd_chan;
> >>>>    
> >>>>    	/* Audio specific */
> >>>>    	unsigned int spi_freq;  /* SPI bus clock frequency */
> >>>> @@ -1271,7 +1280,10 @@ static int stm32_dfsdm_read_raw(struct iio_dev *indio_dev,
> >>>>    				int *val2, long mask)
> >>>>    {
> >>>>    	struct stm32_dfsdm_adc *adc = iio_priv(indio_dev);
> >>>> -	int ret;
> >>>> +	struct stm32_dfsdm_filter *fl = &adc->dfsdm->fl_list[adc->fl_id];
> >>>> +	struct stm32_dfsdm_filter_osr *flo = &fl->flo[fl->fast];
> >>>> +	u32 max = flo->max << (flo->lshift - chan->scan_type.shift);
> >>>> +	int ret, idx = chan->scan_index;
> >>>>    
> >>>>    	switch (mask) {
> >>>>    	case IIO_CHAN_INFO_RAW:
> >>>> @@ -1307,6 +1319,41 @@ static int stm32_dfsdm_read_raw(struct iio_dev *indio_dev,
> >>>>    		*val = adc->sample_freq;
> >>>>    
> >>>>    		return IIO_VAL_INT;
> >>>> +
> >>>> +	case IIO_CHAN_INFO_SCALE:
> >>>> +		/*
> >>>> +		 * Scale is expressed in mV.
> >>>> +		 * When fast mode is disabled, actual resolution may be lower
> >>>> +		 * than 2^n, where n=realbits-1.
> >>>> +		 * This leads to underestimating input voltage. To
> >>>> +		 * compensate this deviation, the voltage reference can be
> >>>> +		 * corrected with a factor = realbits resolution / actual max
> >>>> +		 */
> >>>> +		*val = div_u64((u64)adc->sd_chan[idx].scale_val *
> >>>> +			       (u64)BIT(DFSDM_DATA_RES - 1), max);
> >>>> +		*val2 = chan->scan_type.realbits;
> >>>> +		if (adc->sd_chan[idx].differential)
> >>>> +			*val *= 2;
> >>>> +
> >>>> +		return IIO_VAL_FRACTIONAL_LOG2;
> >>>> +
> >>>> +	case IIO_CHAN_INFO_OFFSET:
> >>>> +		/*
> >>>> +		 * DFSDM output data are in the range [-2^n,2^n-1],
> >>>> +		 * with n=realbits-1.
> >>>> +		 * - Differential modulator:
> >>>> +		 * Offset correspond to SD modulator offset.
> >>>> +		 * - Single ended modulator:
> >>>> +		 * Input is in [0V,Vref] range, where 0V corresponds to -2^n.
> >>>> +		 * Add 2^n to offset. (i.e. middle of input range)
> >>>> +		 * offset = offset(sd) * vref / res(sd) * max / vref.
> >>>> +		 */
> >>>> +		*val = div_u64((u64)max * adc->sd_chan[idx].offset,
> >>>> +			       BIT(adc->sd_chan[idx].scale_val2 - 1));
> >>>> +		if (!adc->sd_chan[idx].differential)
> >>>> +			*val += max;
> >>>> +
> >>>> +		return IIO_VAL_INT;
> >>>>    	}
> >>>>    
> >>>>    	return -EINVAL;
> >>>> @@ -1430,7 +1477,9 @@ static int stm32_dfsdm_adc_chan_init_one(struct iio_dev *indio_dev,
> >>>>    	 * IIO_CHAN_INFO_RAW: used to compute regular conversion
> >>>>    	 * IIO_CHAN_INFO_OVERSAMPLING_RATIO: used to set oversampling
> >>>>    	 */
> >>>> -	ch->info_mask_separate = BIT(IIO_CHAN_INFO_RAW);
> >>>> +	ch->info_mask_separate = BIT(IIO_CHAN_INFO_RAW) |
> >>>> +				 BIT(IIO_CHAN_INFO_SCALE) |
> >>>> +				 BIT(IIO_CHAN_INFO_OFFSET);
> >>>>    	ch->info_mask_shared_by_all = BIT(IIO_CHAN_INFO_OVERSAMPLING_RATIO) |
> >>>>    					BIT(IIO_CHAN_INFO_SAMP_FREQ);
> >>>>    
> >>>> @@ -1481,8 +1530,10 @@ static int stm32_dfsdm_adc_init(struct iio_dev *indio_dev)
> >>>>    {
> >>>>    	struct iio_chan_spec *ch;
> >>>>    	struct stm32_dfsdm_adc *adc = iio_priv(indio_dev);
> >>>> +	struct iio_channel *channels, *chan;
> >>>> +	struct stm32_dfsdm_sd_chan_info *sd_chan;
> >>>>    	int num_ch;
> >>>> -	int ret, chan_idx;
> >>>> +	int ret, chan_idx, val2;
> >>>>    
> >>>>    	adc->oversamp = DFSDM_DEFAULT_OVERSAMPLING;
> >>>>    	ret = stm32_dfsdm_compute_all_osrs(indio_dev, adc->oversamp);
> >>>> @@ -1506,6 +1557,22 @@ static int stm32_dfsdm_adc_init(struct iio_dev *indio_dev)
> >>>>    	if (!ch)
> >>>>    		return -ENOMEM;
> >>>>    
> >>>> +	/* Get SD modulator channels */
> >>>> +	channels = iio_channel_get_all(&indio_dev->dev);
> >>>> +	if (IS_ERR(channels)) {
> >>>> +		dev_err(&indio_dev->dev, "Failed to get channel %ld\n",
> >>>> +			PTR_ERR(channels));
> >>>> +		return PTR_ERR(channels);
> >>>> +	}
> >>>> +	chan = &channels[0];
> >>>> +
> >>>> +	adc->sd_chan = devm_kzalloc(&indio_dev->dev,
> >>>> +				    sizeof(*adc->sd_chan) * num_ch, GFP_KERNEL);
> >>>> +	if (!adc->sd_chan)
> >>>> +		return -ENOMEM;
> >>>> +
> >>>> +	sd_chan = adc->sd_chan;
> >>>> +
> >>>>    	for (chan_idx = 0; chan_idx < num_ch; chan_idx++) {
> >>>>    		ch[chan_idx].scan_index = chan_idx;
> >>>>    		ret = stm32_dfsdm_adc_chan_init_one(indio_dev, &ch[chan_idx]);
> >>>> @@ -1513,6 +1580,38 @@ static int stm32_dfsdm_adc_init(struct iio_dev *indio_dev)
> >>>>    			dev_err(&indio_dev->dev, "Channels init failed\n");
> >>>>    			return ret;
> >>>>    		}
> >>>> +
> >>>> +		if (!chan->indio_dev)
> >>>> +			return -EINVAL;
> >>>> +
> >>>> +		ret = iio_read_channel_scale(chan, &sd_chan->scale_val,
> >>>> +					     &sd_chan->scale_val2);
> >>>> +		if (ret < 0) {
> >>>> +			dev_err(&indio_dev->dev,
> >>>> +				"Failed to get channel %d scale\n", chan_idx);
> >>>> +			return ret;
> >>>> +		}
> >>>> +
> >>>> +		if (iio_channel_has_info(chan->channel, IIO_CHAN_INFO_OFFSET)) {
> >>>> +			ret = iio_read_channel_offset(chan, &sd_chan->offset,
> >>>> +						      &val2);
> >>>> +			if (ret < 0) {
> >>>> +				dev_err(&indio_dev->dev,
> >>>> +					"Failed to get channel %d offset\n",
> >>>> +					chan_idx);
> >>>> +				return ret;
> >>>> +			}
> >>>> +		}
> >>>> +
> >>>> +		sd_chan->differential = chan->channel->differential;
> >>>> +
> >>>> +		dev_dbg(&indio_dev->dev, "Channel %d %s scale ref=%d offset=%d",
> >>>> +			chan_idx, chan->channel->differential ?
> >>>> +			"differential" : "single-ended",
> >>>> +			sd_chan->scale_val, sd_chan->offset);
> >>>> +
> >>>> +		chan++;
> >>>> +		sd_chan++;
> >>>>    	}
> >>>>    
> >>>>    	indio_dev->num_channels = num_ch;  

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