[PATCH 4/5] iio:adxl372: Add sampling frequency support

[PATCH 4/5] iio:adxl372: Add sampling frequency support

[Stefan Popa]

This patch adds the option for the user to select the sampling frequency.
Also, the user can read the available frequencies and read the currently
set frequency via the read_raw function. The frequency can be set via the
write_raw function.

When the frequency is set, the bandwidth is also checked and ensured
that it is constrained to at most half of the sampling frequency.

Signed-off-by: Stefan Popa <stefan.popa@analog.com>
---
 drivers/iio/accel/adxl372.c | 60 ++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 59 insertions(+), 1 deletion(-)

diff --git a/drivers/iio/accel/adxl372.c b/drivers/iio/accel/adxl372.c
index 645902d..498c740 100644
--- a/drivers/iio/accel/adxl372.c
+++ b/drivers/iio/accel/adxl372.c
@@ -205,7 +205,8 @@ static const int adxl372_samp_freq_tbl[5] = {
 	.modified = 1,							\
 	.channel2 = IIO_MOD_##axis,					\
 	.info_mask_separate = BIT(IIO_CHAN_INFO_RAW),			\
-	.info_mask_shared_by_type = BIT(IIO_CHAN_INFO_SCALE),		\
+	.info_mask_shared_by_type = BIT(IIO_CHAN_INFO_SCALE) |		\
+				    BIT(IIO_CHAN_INFO_SAMP_FREQ),	\
 	.scan_index = index,						\
 	.scan_type = {							\
 		.sign = 's',						\
@@ -291,6 +292,19 @@ static int adxl372_set_odr(struct adxl372_state *st,
 	return ret;
 }
 
+static int adxl372_find_closest_match(const int *array,
+				      unsigned int size, int val)
+{
+	int i;
+
+	for (i = 0; i < size; i++) {
+		if (val <= array[i])
+			return i;
+	}
+
+	return size - 1;
+}
+
 static int adxl372_set_bandwidth(struct adxl372_state *st,
 				 enum adxl372_bandwidth bw)
 {
@@ -568,6 +582,37 @@ static int adxl372_read_raw(struct iio_dev *indio_dev,
 		*val = 0;
 		*val2 = ADXL372_USCALE;
 		return IIO_VAL_INT_PLUS_MICRO;
+	case IIO_CHAN_INFO_SAMP_FREQ:
+		*val = adxl372_samp_freq_tbl[st->odr];
+		return IIO_VAL_INT;
+	}
+
+	return -EINVAL;
+}
+
+static int adxl372_write_raw(struct iio_dev *indio_dev,
+			     struct iio_chan_spec const *chan,
+			     int val, int val2, long info)
+{
+	struct adxl372_state *st = iio_priv(indio_dev);
+	int odr_index, ret;
+
+	switch (info) {
+	case IIO_CHAN_INFO_SAMP_FREQ:
+		odr_index = adxl372_find_closest_match(adxl372_samp_freq_tbl,
+					ARRAY_SIZE(adxl372_samp_freq_tbl),
+					val);
+		ret = adxl372_set_odr(st, odr_index);
+		if (ret < 0)
+			return ret;
+		/*
+		 * The maximum bandwidth is constrained to at most half of
+		 * the ODR to ensure that the Nyquist criteria is not violated
+		 */
+		if (st->bw > odr_index)
+			ret = adxl372_set_bandwidth(st, odr_index);
+
+		return ret;
 	default:
 		return -EINVAL;
 	}
@@ -722,8 +767,21 @@ static const struct iio_trigger_ops adxl372_trigger_ops = {
 	.set_trigger_state = adxl372_dready_trig_set_state,
 };
 
+static IIO_CONST_ATTR_SAMP_FREQ_AVAIL("400 800 1600 3200 6400");
+
+static struct attribute *adxl372_attributes[] = {
+	&iio_const_attr_sampling_frequency_available.dev_attr.attr,
+	NULL,
+};
+
+static const struct attribute_group adxl372_attrs_group = {
+	.attrs = adxl372_attributes,
+};
+
 static const struct iio_info adxl372_info = {
+	.attrs = &adxl372_attrs_group,
 	.read_raw = adxl372_read_raw,
+	.write_raw = adxl372_write_raw,
 	.debugfs_reg_access = &adxl372_reg_access,
 	.hwfifo_set_watermark = adxl372_set_watermark,
 };
-- 
2.7.4

Re: [PATCH 4/5] iio:adxl372: Add sampling frequency support

[Andy Shevchenko]

On Thu, Jul 12, 2018 at 6:36 PM, Stefan Popa <stefan.popa@analog.com> wrote:
> This patch adds the option for the user to select the sampling frequency.
> Also, the user can read the available frequencies and read the currently
> set frequency via the read_raw function. The frequency can be set via the
> write_raw function.
>
> When the frequency is set, the bandwidth is also checked and ensured
> that it is constrained to at most half of the sampling frequency.

> +static int adxl372_find_closest_match(const int *array,
> +                                     unsigned int size, int val)
> +{
> +       int i;
> +
> +       for (i = 0; i < size; i++) {
> +               if (val <= array[i])
> +                       return i;
> +       }
> +
> +       return size - 1;
> +}

Perhaps it's time to extend bsearch with something called
bsearch_closest().

-- 
With Best Regards,
Andy Shevchenko

Re: [PATCH 4/5] iio:adxl372: Add sampling frequency support

[Lars-Peter Clausen]

On 07/12/2018 08:25 PM, Andy Shevchenko wrote:
> On Thu, Jul 12, 2018 at 6:36 PM, Stefan Popa <stefan.popa@analog.com> wrote:
>> This patch adds the option for the user to select the sampling frequency.
>> Also, the user can read the available frequencies and read the currently
>> set frequency via the read_raw function. The frequency can be set via the
>> write_raw function.
>>
>> When the frequency is set, the bandwidth is also checked and ensured
>> that it is constrained to at most half of the sampling frequency.
> 
>> +static int adxl372_find_closest_match(const int *array,
>> +                                     unsigned int size, int val)
>> +{
>> +       int i;
>> +
>> +       for (i = 0; i < size; i++) {
>> +               if (val <= array[i])
>> +                       return i;
>> +       }
>> +
>> +       return size - 1;
>> +}
> 
> Perhaps it's time to extend bsearch with something called
> bsearch_closest().
> 

While it makes sense to have a helper function for this the generic bsearch
is not a good solution for small arrays. The generated code is probably
larger and the function call overhead of calling the cmp function completely
outweighs the savings from doing a binary search. bsearch() is good if your
array has 100s or 1000s of entries. If it only has 5, not so much.