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

[Olivier MOYSAN <olivier.moysan@st.com>]

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@st.com> 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@st.com> wrote:
>>>   
>>>> Add scale and offset attributes support to STM32 DFSDM.
>>>>
>>>> Signed-off-by: Olivier Moysan <olivier.moysan@st.com>
>>> 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.

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;