[PATCH v6] iio: hrtimer: Allow sub Hz granularity

[Gwendal Grignou <gwendal@chromium.org>]

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>
---
Changes since v5:
- Properly support do_div on 32bit architecture: quotient must be u64, dividend
  u32.
- Use PSEC_PER_SEC from
  https://patchwork.kernel.org/project/linux-iio/patch/20210112153709.1074-1-andriy.shevchenko@linux.intel.com/

Changes since v4:
- Use do_div() properly.

Changes since v3:
- Fix rebasing issue.

Changes since v2:
- Add do_div to allow divide by a u64 on 32bit machines.

Changes since v1:
- Added documentation.

 Documentation/iio/iio_configfs.rst     |  1 +
 drivers/iio/trigger/iio-trig-hrtimer.c | 28 +++++++++++++++++---------
 2 files changed, 19 insertions(+), 10 deletions(-)

diff --git a/Documentation/iio/iio_configfs.rst b/Documentation/iio/iio_configfs.rst
index ecbfdb3afef7e..807589ef2bea0 100644
--- a/Documentation/iio/iio_configfs.rst
+++ b/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.
diff --git a/drivers/iio/trigger/iio-trig-hrtimer.c b/drivers/iio/trigger/iio-trig-hrtimer.c
index 58c1c30d5612b..b560194b4f1f3 100644
--- a/drivers/iio/trigger/iio-trig-hrtimer.c
+++ b/drivers/iio/trigger/iio-trig-hrtimer.c
@@ -22,7 +22,7 @@
 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 +38,9 @@ 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 +50,24 @@ 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 (!val || val > NSEC_PER_SEC)
+	val = fract + 1000 * integer;
+
+	if (!val || val > UINT_MAX)
 		return -EINVAL;
 
-	info->sampling_frequency = val;
-	info->period = NSEC_PER_SEC / val;
+	info->sampling_frequency[0] = integer;
+	info->sampling_frequency[1] = fract * 1000;
+	period = PSEC_PER_SEC;
+	do_div(period, val);
+	info->period = period;
 
 	return len;
 }
@@ -135,8 +143,8 @@ 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)
-- 
2.30.0.296.g2bfb1c46d8-goog