RE: [PATCH 2/4] iio: adis16480: fix pps mode sampling frequency math

["Sa, Nuno" <Nuno.Sa@analog.com>]

> From: Jonathan Cameron <jic23@kernel.org>
> Sent: Sunday, January 24, 2021 2:43 PM
> To: Sa, Nuno <Nuno.Sa@analog.com>
> Cc: devicetree@vger.kernel.org; linux-iio@vger.kernel.org; Rob
> Herring <robh+dt@kernel.org>; Peter Meerwald-Stadler
> <pmeerw@pmeerw.net>; Lars-Peter Clausen <lars@metafoo.de>;
> Hennerich, Michael <Michael.Hennerich@analog.com>; Ardelean,
> Alexandru <alexandru.Ardelean@analog.com>
> Subject: Re: [PATCH 2/4] iio: adis16480: fix pps mode sampling
> frequency math
> 
> [External]
> 
> On Thu, 21 Jan 2021 12:49:52 +0100
> Nuno Sá <nuno.sa@analog.com> wrote:
> 
> > When using PPS mode, the input clock needs to be scaled so that we
> have
> > an IMU sample rate between (optimally) 4000 and 4250. After this,
> we can
> > use the decimation filter to lower the sampling rate in order to get
> what
> > the user wants. Optimally, the user sample rate is a multiple of both
> the
> > IMU sample rate and the input clock. Hence, calculating the
> sync_scale
> > dynamically gives us better chances of achieving a perfect/integer
> value
> > for DEC_RATE. The math here is:
> >  1. lcm of the input clock and the desired output rate.
> >  2. get the highest multiple of the previous result lower than the adis
> >     max rate.
> >  3. The last result becomes the IMU sample rate. Use that to calculate
> >     SYNC_SCALE and DEC_RATE (to get the user output rate).
> >
> > Fixes: 326e2357553d3 ("iio: imu: adis16480: Add support for external
> clock")
> > Signed-off-by: Nuno Sá <nuno.sa@analog.com>
> 
> As this is a fix, please move the refactor of the lock to after this patch.
> We don't really want to need to backport that patch in order to apply
> this
> to older kernels.

Hmm, completely missed this... will do that.

- Nuno Sá

> I'll reply to the cover letter as to what might make sense to do for
> the case where we are potentially running the sensor too slow.
> 
> Otherwise, patch looks fine to me.
> 
> Jonathan

> > ---
> >  drivers/iio/imu/adis16480.c | 120 ++++++++++++++++++++++++++-
> ---------
> >  1 file changed, 86 insertions(+), 34 deletions(-)
> >
> > diff --git a/drivers/iio/imu/adis16480.c b/drivers/iio/imu/adis16480.c
> > index dfe86c589325..7620822f3350 100644
> > --- a/drivers/iio/imu/adis16480.c
> > +++ b/drivers/iio/imu/adis16480.c
> > @@ -17,6 +17,7 @@
> >  #include <linux/slab.h>
> >  #include <linux/sysfs.h>
> >  #include <linux/module.h>
> > +#include <linux/lcm.h>
> >
> >  #include <linux/iio/iio.h>
> >  #include <linux/iio/sysfs.h>
> > @@ -312,7 +313,8 @@ static int adis16480_debugfs_init(struct
> iio_dev *indio_dev)
> >  static int adis16480_set_freq(struct iio_dev *indio_dev, int val, int
> val2)
> >  {
> >  	struct adis16480 *st = iio_priv(indio_dev);
> > -	unsigned int t, reg;
> > +	unsigned int t, sample_rate = st->clk_freq;
> > +	int ret;
> >
> >  	if (val < 0 || val2 < 0)
> >  		return -EINVAL;
> > @@ -321,28 +323,63 @@ static int adis16480_set_freq(struct iio_dev
> *indio_dev, int val, int val2)
> >  	if (t == 0)
> >  		return -EINVAL;
> >
> > +	adis_dev_lock(&st->adis);
> >  	/*
> > -	 * When using PPS mode, the rate of data collection is equal to
> the
> > -	 * product of the external clock frequency and the scale factor
> in the
> > -	 * SYNC_SCALE register.
> > -	 * When using sync mode, or internal clock, the output data
> rate is
> > -	 * equal with  the clock frequency divided by DEC_RATE + 1.
> > +	 * When using PPS mode, the input clock needs to be scaled so
> that we have an IMU
> > +	 * sample rate between (optimally) 4000 and 4250. After this,
> we can use the
> > +	 * decimation filter to lower the sampling rate in order to get
> what the user wants.
> > +	 * Optimally, the user sample rate is a multiple of both the IMU
> sample rate and
> > +	 * the input clock. Hence, calculating the sync_scale dynamically
> gives us better
> > +	 * chances of achieving a perfect/integer value for DEC_RATE.
> The math here is:
> > +	 *	1. lcm of the input clock and the desired output rate.
> > +	 *	2. get the highest multiple of the previous result lower
> than the adis max rate.
> > +	 *	3. The last result becomes the IMU sample rate. Use
> that to calculate SYNC_SCALE
> > +	 *	   and DEC_RATE (to get the user output rate)
> >  	 */
> >  	if (st->clk_mode == ADIS16480_CLK_PPS) {
> > -		t = t / st->clk_freq;
> > -		reg = ADIS16495_REG_SYNC_SCALE;
> > -	} else {
> > -		t = st->clk_freq / t;
> > -		reg = ADIS16480_REG_DEC_RATE;
> > +		unsigned long scaled_rate = lcm(st->clk_freq, t);
> > +		int sync_scale;
> > +		struct device *dev = &st->adis.spi->dev;
> > +
> > +		/*
> > +		 * If lcm is bigger than the IMU maximum sampling rate
> there's no perfect
> > +		 * solution. In this case, we get the highest multiple of
> the input clock
> > +		 * lower that the IMU max sample rate.
> > +		 */
> > +		if (scaled_rate > st->chip_info->int_clk)
> > +			scaled_rate = st->chip_info->int_clk / st-
> >clk_freq * st->clk_freq;
> > +		else
> > +			scaled_rate = st->chip_info->int_clk /
> scaled_rate * scaled_rate;
> > +
> > +		/*
> > +		 * This is not an hard requirement but it's not advised to
> run the IMU
> > +		 * with a sample rate lower than 4000Hz due to possible
> undersampling
> > +		 * issues so we will log a warning here. We could even
> force the rate
> > +		 * to 4000 but some users might really want this...
> > +		 */
> > +		if (scaled_rate < 4000000)
> > +			dev_warn(dev, "Possible undersampling issues
> due to sampling rate=%lu < 4000\n",
> > +				 scaled_rate / 1000);
> > +
> > +		sync_scale = scaled_rate / st->clk_freq;
> > +		ret = __adis_write_reg_16(&st->adis,
> ADIS16495_REG_SYNC_SCALE, sync_scale);
> > +		if (ret)
> > +			goto error;
> > +
> > +		sample_rate = scaled_rate;
> >  	}
> >
> > +	t = DIV_ROUND_CLOSEST(sample_rate, t);
> > +	if (t)
> > +		t--;
> > +
> >  	if (t > st->chip_info->max_dec_rate)
> >  		t = st->chip_info->max_dec_rate;
> >
> > -	if ((t != 0) && (st->clk_mode != ADIS16480_CLK_PPS))
> > -		t--;
> > -
> > -	return adis_write_reg_16(&st->adis, reg, t);
> > +	ret = __adis_write_reg_16(&st->adis,
> ADIS16480_REG_DEC_RATE, t);
> > +error:
> > +	adis_dev_unlock(&st->adis);
> > +	return ret;
> >  }
> >
> >  static int adis16480_get_freq(struct iio_dev *indio_dev, int *val, int
> *val2)
> > @@ -350,34 +387,35 @@ static int adis16480_get_freq(struct iio_dev
> *indio_dev, int *val, int *val2)
> >  	struct adis16480 *st = iio_priv(indio_dev);
> >  	uint16_t t;
> >  	int ret;
> > -	unsigned int freq;
> > -	unsigned int reg;
> > +	unsigned int freq, sample_rate = st->clk_freq;
> >
> > -	if (st->clk_mode == ADIS16480_CLK_PPS)
> > -		reg = ADIS16495_REG_SYNC_SCALE;
> > -	else
> > -		reg = ADIS16480_REG_DEC_RATE;
> > +	adis_dev_lock(&st->adis);
> >
> > -	ret = adis_read_reg_16(&st->adis, reg, &t);
> > +	if (st->clk_mode == ADIS16480_CLK_PPS) {
> > +		u16 sync_scale;
> > +
> > +		ret = __adis_read_reg_16(&st->adis,
> ADIS16495_REG_SYNC_SCALE, &sync_scale);
> > +		if (ret)
> > +			goto error;
> > +
> > +		sample_rate = st->clk_freq * sync_scale;
> > +	}
> > +
> > +	ret = __adis_read_reg_16(&st->adis,
> ADIS16480_REG_DEC_RATE, &t);
> >  	if (ret)
> > -		return ret;
> > +		goto error;
> >
> > -	/*
> > -	 * When using PPS mode, the rate of data collection is equal to
> the
> > -	 * product of the external clock frequency and the scale factor
> in the
> > -	 * SYNC_SCALE register.
> > -	 * When using sync mode, or internal clock, the output data
> rate is
> > -	 * equal with  the clock frequency divided by DEC_RATE + 1.
> > -	 */
> > -	if (st->clk_mode == ADIS16480_CLK_PPS)
> > -		freq = st->clk_freq * t;
> > -	else
> > -		freq = st->clk_freq / (t + 1);
> > +	adis_dev_unlock(&st->adis);
> > +
> > +	freq = DIV_ROUND_CLOSEST(sample_rate, (t + 1));
> >
> >  	*val = freq / 1000;
> >  	*val2 = (freq % 1000) * 1000;
> >
> >  	return IIO_VAL_INT_PLUS_MICRO;
> > +error:
> > +	adis_dev_unlock(&st->adis);
> > +	return ret;
> >  }
> >
> >  enum {
> > @@ -1278,6 +1316,20 @@ static int adis16480_probe(struct
> spi_device *spi)
> >
> >  		st->clk_freq = clk_get_rate(st->ext_clk);
> >  		st->clk_freq *= 1000; /* micro */
> > +		if (st->clk_mode == ADIS16480_CLK_PPS) {
> > +			u16 sync_scale;
> > +
> > +			/*
> > +			 * In PPS mode, the IMU sample rate is the
> clk_freq * sync_scale. Hence,
> > +			 * default the IMU sample rate to the highest
> multiple of the input clock
> > +			 * lower than the IMU max sample rate. The
> internal sample rate is the
> > +			 * max...
> > +			 */
> > +			sync_scale = st->chip_info->int_clk / st-
> >clk_freq;
> > +			ret = __adis_write_reg_16(&st->adis,
> ADIS16495_REG_SYNC_SCALE, sync_scale);
> > +			if (ret)
> > +				return ret;
> > +		}
> >  	} else {
> >  		st->clk_freq = st->chip_info->int_clk;
> >  	}