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

From: Jonathan Cameron
Date: Fri Feb 14 2020 - 10:10:23 EST

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In reply to: Olivier MOYSAN: "Re: [PATCH 4/4] iio: adc: stm32-dfsdm: add scale and offset support"
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On Fri, 14 Feb 2020 14:49:18 +0000
Olivier MOYSAN <olivier.moysan@xxxxxx> wrote:

> Hi Jonathan,
>
> On 2/14/20 2:11 PM, Jonathan Cameron wrote:
> > On Tue, 11 Feb 2020 15:19:01 +0000
> > Olivier MOYSAN <olivier.moysan@xxxxxx> wrote:
> >
> >> Hi Jonathan,
> >>
> >> On 2/8/20 5:18 PM, Jonathan Cameron wrote:
> >>> On Tue, 4 Feb 2020 11:10:08 +0100
> >>> Olivier Moysan <olivier.moysan@xxxxxx> wrote:
> >>>
> >>>> Add scale and offset attributes support to STM32 DFSDM.
> >>>>
> >>>> Signed-off-by: Olivier Moysan <olivier.moysan@xxxxxx>
> >>> 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.
> >>
> >> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ scaleÂ offset
> >> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ ^ÂÂÂÂÂ ^
> >> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ |ÂÂÂÂÂ |ÂÂÂÂÂÂ IIO ABI
> >> +-------------------------------------------------------------+
> >> ÂÂÂÂÂÂÂÂ +---------------+ÂÂÂÂÂÂÂÂÂ +-------------+
> >> ÂÂÂÂÂÂÂÂ |sd driverÂÂÂÂÂ |ÂÂÂÂÂÂÂÂÂ |DFSDM driver |
> >> ÂÂÂÂÂÂÂÂ +---------------+ÂÂÂÂÂÂÂÂÂ +-------------+
> >> +-------------------------------------------------------------+
> >> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ HW
> >> ÂÂÂÂÂÂÂÂ +---------------+ÂÂÂÂÂÂÂÂÂ +-------------+
> >> +------->+ sd-modulatorÂ +--------->+ DFSDM +-------->
> >> analogÂÂ +------+--------+ÂÂÂÂÂÂÂÂÂ +-------------+ output
> >> inputÂÂÂÂÂÂÂÂÂÂ ^
> >> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ | vref
> >> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ +
> >>
> >>
> >> 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.
> >
> >
> > ÂÂÂÂÂÂÂÂ scaleÂ offset raw
> > ÂÂÂÂÂÂÂÂÂÂ ^ÂÂÂÂÂ ^ ^
> > ÂÂÂÂÂÂÂÂÂÂ |ÂÂÂÂÂ |ÂÂÂÂ | ÂÂ IIO ABI
> > +-------------------------------------------------------------+
> > ÂÂÂÂÂÂÂÂ +---------------+ÂÂÂÂÂÂÂÂÂ +-------------+
> > ÂÂÂÂÂÂÂÂ |sd driverÂÂÂÂÂ |ÂÂÂÂÂÂÂÂÂ |DFSDM driver |
> > ÂÂÂÂÂÂÂÂ +---------------+ÂÂÂÂÂÂÂÂÂ +-------------+
> > +-------------------------------------------------------------+
> > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ HW
> > ÂÂÂÂÂÂÂÂ +---------------+ÂÂÂÂÂÂÂÂÂ +-------------+
> > +------->+ sd-modulatorÂ +--------->+ DFSDM +-------->
> > analogÂÂ +------+--------+ÂÂÂÂÂÂÂÂÂ +-------------+ output
> > inputÂÂÂÂÂÂÂÂÂÂ ^
> > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ | vref
> >> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ +
> >>
> 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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In reply to: Olivier MOYSAN: "Re: [PATCH 4/4] iio: adc: stm32-dfsdm: add scale and offset support"
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