> From: Jonathan Cameron <jic23@xxxxxxxxxx>
> Sent: Sunday, January 24, 2021 2:43 PM
> To: Sa, Nuno <Nuno.Sa@xxxxxxxxxx>
> Cc: devicetree@xxxxxxxxxxxxxxx; linux-iio@xxxxxxxxxxxxxxx; Rob
> Herring <robh+dt@xxxxxxxxxx>; Peter Meerwald-Stadler
> <pmeerw@xxxxxxxxxx>; Lars-Peter Clausen <lars@xxxxxxxxxx>;
> Hennerich, Michael <Michael.Hennerich@xxxxxxxxxx>; Ardelean,
> Alexandru <alexandru.Ardelean@xxxxxxxxxx>
> 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@xxxxxxxxxx> 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@xxxxxxxxxx>
>
> 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;
> > }
