iio: hrtimer: Allow sub Hz granularity

Allow setting frequency below 1Hz or sub 1Hz precision.
Useful for slow sensors like ALS.

Test frequency is set properly:
modprobe iio-trig-hrtimer && \
mkdir /sys/kernel/config/iio/triggers/hrtimer/t1 && \
cd /sys/bus/iio/devices/triggerX ;
for i in 1 .1 .01 .001 ; do
  echo $i > sampling_frequency
  cat sampling_frequency
done

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20210226014733.2108544-1-gwendal@chromium.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/iio/iio_configfs.rst

@@ -99,3 +99,4 @@ Each trigger can have one or more attributes specific to the trigger type.
 
 "hrtimer" trigger type doesn't have any configurable attribute from /config dir.
 It does introduce the sampling_frequency attribute to trigger directory.
+That attribute sets the polling frequency in Hz, with mHz precision.

drivers/iio/trigger/iio-trig-hrtimer.c

@@ -16,13 +16,16 @@
 #include <linux/iio/trigger.h>
 #include <linux/iio/sw_trigger.h>
 
+/* Defined locally, not in time64.h yet. */
+#define PSEC_PER_SEC   1000000000000LL
+
 /* default sampling frequency - 100Hz */
 #define HRTIMER_DEFAULT_SAMPLING_FREQUENCY 100
 
 struct iio_hrtimer_info {
 	struct iio_sw_trigger swt;
 	struct hrtimer timer;
-	unsigned long sampling_frequency;
+	int sampling_frequency[2];
 	ktime_t period;
 };
 
@@ -38,7 +41,9 @@ ssize_t iio_hrtimer_show_sampling_frequency(struct device *dev,
 	struct iio_trigger *trig = to_iio_trigger(dev);
 	struct iio_hrtimer_info *info = iio_trigger_get_drvdata(trig);
 
-	return snprintf(buf, PAGE_SIZE, "%lu\n", info->sampling_frequency);
+	return iio_format_value(buf, IIO_VAL_INT_PLUS_MICRO,
+			ARRAY_SIZE(info->sampling_frequency),
+			info->sampling_frequency);
 }
 
 static
@@ -48,18 +53,26 @@ ssize_t iio_hrtimer_store_sampling_frequency(struct device *dev,
 {
 	struct iio_trigger *trig = to_iio_trigger(dev);
 	struct iio_hrtimer_info *info = iio_trigger_get_drvdata(trig);
-	unsigned long val;
-	int ret;
+	unsigned long long val;
+	u64 period;
+	int integer, fract, ret;
 
-	ret = kstrtoul(buf, 10, &val);
+	ret = iio_str_to_fixpoint(buf, 100, &integer, &fract);
 	if (ret)
 		return ret;
+	if (integer < 0 || fract < 0)
+		return -ERANGE;
 
-	if (!val || val > NSEC_PER_SEC)
+	val = fract + 1000 * integer;  /* mHz */
+
+	if (!val || val > UINT_MAX)
 		return -EINVAL;
 
-	info->sampling_frequency = val;
-	info->period = NSEC_PER_SEC / val;
+	info->sampling_frequency[0] = integer;  /* Hz */
+	info->sampling_frequency[1] = fract * 1000;  /* uHz */
+	period = PSEC_PER_SEC;
+	do_div(period, val);
+	info->period = period;  /* nS */
 
 	return len;
 }
@@ -135,8 +148,8 @@ static struct iio_sw_trigger *iio_trig_hrtimer_probe(const char *name)
 	hrtimer_init(&trig_info->timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL_HARD);
 	trig_info->timer.function = iio_hrtimer_trig_handler;
 
-	trig_info->sampling_frequency = HRTIMER_DEFAULT_SAMPLING_FREQUENCY;
-	trig_info->period = NSEC_PER_SEC / trig_info->sampling_frequency;
+	trig_info->sampling_frequency[0] = HRTIMER_DEFAULT_SAMPLING_FREQUENCY;
+	trig_info->period = NSEC_PER_SEC / trig_info->sampling_frequency[0];
 
 	ret = iio_trigger_register(trig_info->swt.trigger);
 	if (ret)