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Message-ID: <CAE3SzaTFgLsLCw-bqSTygaanCpHKHGRj7ssGim84WjB-DxZPVA@mail.gmail.com>
Date: Fri, 25 Jul 2025 00:24:22 +0530
From: Akshay Jindal <akshayaj.lkd@...il.com>
To: anshulusr@...il.com, jic23@...nel.org, dlechner@...libre.com,
nuno.sa@...log.com, andy@...nel.org
Cc: shuah@...nel.org, linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] iio: light: ltr390: Add conditional data freshness
check with sysfs control
On Tue, Jul 22, 2025 at 1:25 AM Akshay Jindal <akshayaj.lkd@...il.com> wrote:
>
> Add logic to check the sensor’s data freshness status bit before reading
> data, and skip reads if the data is stale.
>
> Introduce a new boolean sysfs attribute, `data_fresh_check_enable`, to allow
> users to enable or disable this behavior at runtime:
>
> - 1: Enable data freshness check (default)
> - 0: Disable data freshness check
>
> This provides flexibility to choose between ensuring fresh data and allowing
> faster reads when occasional staleness is acceptable.
>
> Document the new attribute under Documentation/ABI/testing/sysfs-bus-iio.
>
> Signed-off-by: Akshay Jindal <akshayaj.lkd@...il.com>
> ---
> -> Tested on Raspberrypi 4B. Follow for more details.
>
> akshayajpi@...pberrypi:~ $ uname -r
> 6.12.35-v8+
> akshayajpi@...pberrypi:~ $ uname -a
> Linux raspberrypi 6.12.35-v8+ #5 SMP PREEMPT Tue Jul 15 17:38:06 IST 2025 aarch64 GNU/Linux
>
> -> Sensor Detection, overlaying of device tree and Driver loading
> akshayajpi@...pberrypi:~ $ i2cdetect -y 1
> 0 1 2 3 4 5 6 7 8 9 a b c d e f
> 00: -- -- -- -- -- -- -- --
> 10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
> 20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
> 30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
> 40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
> 50: -- -- -- 53 -- -- -- -- -- -- -- -- -- -- -- --
> 60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
> 70: -- -- -- -- -- -- -- --
> akshayajpi@...pberrypi:~ $ sudo dtoverlay i2c-sensor ltr390
> akshayajpi@...pberrypi:~ $ lsmod|grep ltr390
> ltr390 16384 0
> industrialio 110592 1 ltr390
> regmap_i2c 12288 1 ltr390
>
> -> Sysfs Attribute Creation validation
> akshayajpi@...pberrypi:~ $ ls -ltrh /sys/bus/iio/devices/iio\:device0/
> total 0
> -rw-r--r-- 1 root root 4.0K Jul 21 23:52 uevent
> -r--r--r-- 1 root root 4.0K Jul 21 23:52 name
> -r--r--r-- 1 root root 4.0K Jul 21 23:52 waiting_for_supplier
> lrwxrwxrwx 1 root root 0 Jul 21 23:52 subsystem -> ../../../../../../../bus/iio
> -r--r--r-- 1 root root 4.0K Jul 21 23:52 scale_available
> -r--r--r-- 1 root root 4.0K Jul 21 23:52 sampling_frequency_available
> -rw-r--r-- 1 root root 4.0K Jul 21 23:52 sampling_frequency
> drwxr-xr-x 2 root root 0 Jul 21 23:52 power
> lrwxrwxrwx 1 root root 0 Jul 21 23:52 of_node -> ../../../../../../../firmware/devicetree/base/soc/i2c@...04000/ltr390@53
> -rw-r--r-- 1 root root 4.0K Jul 21 23:52 in_uvindex_scale
> -rw-r--r-- 1 root root 4.0K Jul 21 23:52 in_uvindex_raw
> -r--r--r-- 1 root root 4.0K Jul 21 23:52 integration_time_available
> -rw-r--r-- 1 root root 4.0K Jul 21 23:52 integration_time
> -rw-r--r-- 1 root root 4.0K Jul 21 23:52 in_illuminance_scale
> -rw-r--r-- 1 root root 4.0K Jul 21 23:52 in_illuminance_raw
> drwxr-xr-x 2 root root 0 Jul 21 23:52 events
> -r--r--r-- 1 root root 4.0K Jul 21 23:52 dev
> -rw-r--r-- 1 root root 4.0K Jul 21 23:52 data_fresh_check_enable<-----
> -r--r--r-- 1 root root 4.0K Jul 21 23:52 data_fresh
>
> -> Default value 1 for data_fresh_check_enable
> akshayajpi@...pberrypi:~ $ cat /sys/bus/iio/devices/iio\:device0/data_fresh
> 1
> akshayajpi@...pberrypi:~ $ cat /sys/bus/iio/devices/iio\:device0/data_fresh_check_enable
> 1
>
> -> Disable fresh measurement in the sensor
> akshayajpi@...pberrypi:~ $ i2cset -f -y 1 0x53 0x0 0x0
> akshayajpi@...pberrypi:~ $ cat /sys/bus/iio/devices/iio\:device0/data_fresh_check_enable
> 1
> akshayajpi@...pberrypi:~ $ cat /sys/bus/iio/devices/iio\:device0/data_fresh_check_enable
> 1
> akshayajpi@...pberrypi:~ $ cat /sys/bus/iio/devices/iio\:device0/in_illuminance_raw
> 647
> akshayajpi@...pberrypi:~ $ cat /sys/bus/iio/devices/iio\:device0/data_fresh
> 0
>
> -> Since data_fresh_enable_check is enabled by default, driver skips
> stale data read and emits -EAGAIN.
> akshayajpi@...pberrypi:~ $ cat /sys/bus/iio/devices/iio\:device0/in_illuminance_raw
> cat: '/sys/bus/iio/devices/iio:device0/in_illuminance_raw': Resource temporarily unavailable
> akshayajpi@...pberrypi:~ $ cat /sys/bus/iio/devices/iio\:device0/in_illuminance_raw
> cat: '/sys/bus/iio/devices/iio:device0/in_illuminance_raw': Resource temporarily unavailable
> akshayajpi@...pberrypi:~ $
>
> -> Disable data_fresh_check_en
> akshayajpi@...pberrypi:~ $ echo 0 | sudo tee /sys/bus/iio/devices/iio\:device0/data_fresh_check_enable
> 0
> akshayajpi@...pberrypi:~ $ cat /sys/bus/iio/devices/iio\:device0/data_fresh_check_enable
> 0
> akshayajpi@...pberrypi:~ $ cat /sys/bus/iio/devices/iio\:device0/data_fresh
> 0
>
> -> Since freshness check is disabled, driver reads and prints stale
> data.
> akshayajpi@...pberrypi:~ $ cat /sys/bus/iio/devices/iio\:device0/in_illuminance_raw
> 647
> akshayajpi@...pberrypi:~ $ cat /sys/bus/iio/devices/iio\:device0/in_illuminance_raw
> 647
> akshayajpi@...pberrypi:~ $ cat /sys/bus/iio/devices/iio\:device0/in_illuminance_raw
> 647
>
> -> Re-enabling freshness check, results in driver emitting -EAGAIN.
> akshayajpi@...pberrypi:~ $ echo 1 | sudo tee /sys/bus/iio/devices/iio\:device0/data_fresh_check_enable
> 1
> akshayajpi@...pberrypi:~ $ cat /sys/bus/iio/devices/iio\:device0/data_fresh_check_enable
> 1
> akshayajpi@...pberrypi:~ $ cat /sys/bus/iio/devices/iio\:device0/in_illuminance_raw
> cat: '/sys/bus/iio/devices/iio:device0/in_illuminance_raw': Resource temporarily unavailable
> akshayajpi@...pberrypi:~ $ cat /sys/bus/iio/devices/iio\:device0/in_illuminance_raw
> cat: '/sys/bus/iio/devices/iio:device0/in_illuminance_raw': Resource temporarily unavailable
>
> Documentation/ABI/testing/sysfs-bus-iio | 12 ++++++
> drivers/iio/light/ltr390.c | 55 ++++++++++++++++++++++++-
> 2 files changed, 66 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/ABI/testing/sysfs-bus-iio b/Documentation/ABI/testing/sysfs-bus-iio
> index 5d176d46c15d..da881e16ee3d 100644
> --- a/Documentation/ABI/testing/sysfs-bus-iio
> +++ b/Documentation/ABI/testing/sysfs-bus-iio
> @@ -2397,3 +2397,15 @@ Description:
>
> Provides userspace visibility into data_freshness status which
> can be used for debugging and informational use.
> +
> +What: /sys/.../iio:deviceX/data_fresh_check_enable
> +KernelVersion: 6.16
> +Contact: linux-iio@...r.kernel.org
> +Description:
> + Read-write boolean attribute controlling whether the driver
> + checks the data freshness status bit before reading sensor data.
> +
> + 0 - Freshness check disabled
> + 1 - Enable check; driver will skip reading stale data (default)
> +
> + This flag affects the behavior of data reads.
> diff --git a/drivers/iio/light/ltr390.c b/drivers/iio/light/ltr390.c
> index 5af0ffd3df1d..b13f01835aeb 100644
> --- a/drivers/iio/light/ltr390.c
> +++ b/drivers/iio/light/ltr390.c
> @@ -98,6 +98,7 @@ struct ltr390_data {
> enum ltr390_mode mode;
> int gain;
> int int_time_us;
> + bool data_fresh_check_en;
> };
>
> static const struct regmap_config ltr390_regmap_config = {
> @@ -199,10 +200,40 @@ static ssize_t data_fresh_show(struct device *dev,
> return sysfs_emit(buf, "%d\n", !!(status & LTR390_DATA_STATUS_MASK));
> }
>
> +static ssize_t data_fresh_check_enable_show(struct device *dev,
> + struct device_attribute *attr, char *buf)
> +{
> + struct ltr390_data *data = iio_priv(dev_to_iio_dev(dev));
> +
> + guard(mutex)(&data->lock);
> +
> + return sysfs_emit(buf, "%d\n", data->data_fresh_check_en);
> +}
> +
> +static ssize_t data_fresh_check_enable_store(struct device *dev,
> + struct device_attribute *attr,
> + const char *buf, size_t len)
> +{
> + int ret;
> + bool data_fresh_check_en;
> + struct ltr390_data *data = iio_priv(dev_to_iio_dev(dev));
> +
> + ret = kstrtobool(buf, &data_fresh_check_en);
> + if (ret)
> + return ret;
> +
> + guard(mutex)(&data->lock);
> +
> + data->data_fresh_check_en = data_fresh_check_en;
> + return len;
> +}
> +
> static IIO_DEVICE_ATTR_RO(data_fresh, 0);
> +static IIO_DEVICE_ATTR_RW(data_fresh_check_enable, 0);
>
> static struct attribute *ltr390_attributes[] = {
> &iio_dev_attr_data_fresh.dev_attr.attr,
> + &iio_dev_attr_data_fresh_check_enable.dev_attr.attr,
> NULL
> };
>
> @@ -214,7 +245,7 @@ static int ltr390_read_raw(struct iio_dev *iio_device,
> struct iio_chan_spec const *chan, int *val,
> int *val2, long mask)
> {
> - int ret;
> + int ret, status;
> struct ltr390_data *data = iio_priv(iio_device);
>
> guard(mutex)(&data->lock);
> @@ -226,6 +257,16 @@ static int ltr390_read_raw(struct iio_dev *iio_device,
> if (ret < 0)
> return ret;
>
> + if (data->data_fresh_check_en) {
> + ret = ltr390_register_read(data, LTR390_MAIN_STATUS);
> + if (ret < 0)
> + return ret;
> +
> + status = ret;
> + if (!(status & LTR390_DATA_STATUS_MASK))
> + return -EAGAIN;
> + }
> +
> ret = ltr390_register_read(data, LTR390_UVS_DATA);
> if (ret < 0)
> return ret;
> @@ -236,6 +277,16 @@ static int ltr390_read_raw(struct iio_dev *iio_device,
> if (ret < 0)
> return ret;
>
> + if (data->data_fresh_check_en) {
> + ret = ltr390_register_read(data, LTR390_MAIN_STATUS);
> + if (ret < 0)
> + return ret;
> +
> + status = ret;
> + if (!(status & LTR390_DATA_STATUS_MASK))
> + return -EAGAIN;
> + }
> +
> ret = ltr390_register_read(data, LTR390_ALS_DATA);
> if (ret < 0)
> return ret;
> @@ -687,6 +738,8 @@ static int ltr390_probe(struct i2c_client *client)
> data->gain = 3;
> /* default mode for ltr390 is ALS mode */
> data->mode = LTR390_SET_ALS_MODE;
> + /* default value for data_fresh_check_en is 1 */
> + data->data_fresh_check_en = 1;
I agree that this will break the user behaviour.
Before dumping this off, I wanted to explore an idea.
What if this remains disabled by default, i.e. data->data_fresh_check_en = 0;
So this way the regular sensor read_data calls will not break, and
based on demand,
the application can configure the driver to check for data freshness,
by toggling the sysfs attribute to 1.
i.e. echo 1 | sudo tee /sys/bus/iio/devices/iio\:device0/data_fresh_check_enable
Thanks,
Akshay
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