Day22 - I2C hwmon Driver with Polling Mode¶

Objective¶

Enhance the Day21 hwmon I2C sensor driver by adding:

background polling with delayed work
cache-only hwmon read path when polling is enabled
sysfs controls for polling
fallback to on-demand mode when polling is disabled

Environment¶

Target board: Raspberry Pi 5
Kernel: Raspberry Pi Linux 6.12.x
Driver: myi2c_sensor
Sensor side: STM32 fake I2C sensor device
Interface: I2C + hwmon

Features Implemented¶

Polling worker¶

A delayed workqueue periodically:

sends update command to sensor
reads raw temperature registers
converts raw value to milli-degree Celsius
updates internal cache

hwmon read path¶

temp1_input behavior:

polling enabled:
read cached value only
return -EAGAIN if cache is not ready
polling disabled:
fallback to on-demand update logic

Sysfs attributes¶

Driver-specific attributes:

chip_id
temp_raw
config
status
update_period
update_once
cache_interval
polling_enable
polling_interval

Standard hwmon attribute:

temp1_input

Implementation Notes¶

New driver fields¶

Added to struct myi2c_sensor_data:

struct delayed_work poll_work;
bool polling_enable;
unsigned int polling_interval_ms;

Background polling callback¶

Polling worker:

locks driver state
performs sensor update
updates cache
reschedules itself if polling remains enabled

Read path¶

The hwmon read callback uses myi2c_sensor_read_temp_mc().

Behavior:

polling enabled -> cache-only
polling disabled -> update if needed

Build¶

Example:

make

If needed:

make clean
make

Load Driver¶

Example:

sudo insmod myi2c_sensor.ko
dmesg | tail -n 30

Expected result:

probe succeeds
chip ID check passes
hwmon device is registered
first polling work is scheduled

Find hwmon Node¶

Example:

ls /sys/class/hwmon/
for d in /sys/class/hwmon/hwmon*; do
    echo "== $d =="
    cat "$d/name"
done

Find the hwmon node whose name is:

myi2c_sensor

Basic Read Test¶

Example:

cat /sys/class/hwmon/hwmonX/temp1_input

Possible early result:

cat: temp1_input: Resource temporarily unavailable

This means cache is not valid yet and the driver returned -EAGAIN.

Retry after a short delay:

sleep 1
cat /sys/class/hwmon/hwmonX/temp1_input

Check Polling Controls¶

Example:

cat /sys/class/hwmon/hwmonX/polling_enable
cat /sys/class/hwmon/hwmonX/polling_interval

Enable Polling¶

Example:

echo 1 | sudo tee /sys/class/hwmon/hwmonX/polling_enable

Disable Polling¶

Example:

echo 0 | sudo tee /sys/class/hwmon/hwmonX/polling_enable

Expected behavior:

background updates stop
temp1_input falls back to on-demand update logic

Change Polling Interval¶

Example:

echo 1000 | sudo tee /sys/class/hwmon/hwmonX/polling_interval
echo 3000 | sudo tee /sys/class/hwmon/hwmonX/polling_interval

Expected behavior:

shorter interval -> more frequent refresh
longer interval -> slower cache updates

Debug Raw Temperature Path¶

Example:

cat /sys/class/hwmon/hwmonX/temp_raw

This path performs direct access to the sensor and is intended for debugging.

Manual Update¶

Example:

echo 1 | sudo tee /sys/class/hwmon/hwmonX/update_once

This is useful for:

debug
manual refresh
verifying device response

Test Script¶

A helper script was used to verify:

polling enable/disable
polling interval effect
cache-only hwmon behavior

Example:

./test_polling.sh /sys/class/hwmon/hwmonX

The script checks:

enable polling
set polling interval
read temp1_input repeatedly
disable polling
enable polling again

Observed Result¶

Observed behavior matched expectation:

polling mode updated cached values periodically
changing polling interval affected update frequency
disabling polling stopped periodic updates
re-enabling polling resumed updates
hwmon path operated correctly

Conclusion¶

Day22 successfully migrated the driver from a purely read-triggered update model to a polling-based cached model.

This completes the second active update model in the learning path:

Day21: on-demand
Day22: polling
Day23: interrupt-driven (planned)