Day19 - I2C Sensor Driver (Kernel)¶

Overview¶

In this lab, we implemented a Linux I2C sensor driver using:

STM32 as a fake I2C slave device
Raspberry Pi as I2C master
Linux kernel i2c_driver framework

Key topics:

I2C driver probe/remove lifecycle
Device Tree binding
Register-based device abstraction
sysfs interface design
Synchronization using mutex

System Architecture¶

STM32 (I2C slave)
    ↓
Register Map (fake sensor)
    ↓
Linux I2C Driver
    ↓
sysfs (/sys/bus/i2c/devices/1-0048/)
    ↓
User Space (cat / echo)

Register-Based Device Model¶

The sensor is designed as a register-based device.

Address	Name	Description
0x00	CHIP_ID	Device ID
0x01	CONFIG	Configuration
0x02	TEMP_MSB	Temperature MSB
0x03	TEMP_LSB	Temperature LSB
0x04	STATUS	Status flags
0x05	UPDATE_PERIOD	Update interval
0x06	COMMAND	Command register

I2C Driver Lifecycle¶

Probe¶

static int myi2c_sensor_probe(struct i2c_client *client)

Called when device is matched via Device Tree
Initialize driver context
Verify CHIP_ID
Create sysfs attributes

Remove¶

static void myi2c_sensor_remove(struct i2c_client *client)

Remove sysfs group
Release allocated resources

Device Tree Binding¶

/dts-v1/;
/plugin/;

/ {
    compatible = "brcm,bcm2712";

    fragment@0 {
        target = <&i2c1>;
        __overlay__ {
            status = "okay";

            myi2c_sensor@48 {
                compatible = "myvendor,myi2c-sensor";
                reg = <0x48>;
            };
        };
    };
};

Sysfs Interface Design¶

Data Plane vs Control Plane¶

This driver separates:

Data plane → sensor readings
Control plane → configuration / commands

Sysfs Attributes¶

Attribute	Type	Description
chip_id	RO	Device ID
temp_raw	RO	Raw sensor value
temp_input	RO	Temperature (milli-degree Celsius)
update_once	WO	Trigger measurement
config	RW	Configuration register
status	RO	Status register
update_period	RW	Update interval

Temperature Representation¶

The driver exports temperature in milli-degree Celsius (m°C).

Example:

raw value = 2529 → 25.29°C
temp_input = 25290

Reason:

sysfs is a stable kernel ABI
values must be machine-readable
formatting should be done in user space

Synchronization¶

All I2C operations are protected using a mutex:

mutex_lock(&data->lock);
/* I2C access */
mutex_unlock(&data->lock);

This prevents race conditions between concurrent sysfs accesses.

Key Design Decisions¶

Use register abstraction (real hardware model)
Separate control/data plane
Use sysfs instead of ioctl
Use milli-degree for standardization
Use mutex for safe access

Known Limitations¶

No interrupt-based data ready
No caching mechanism
Not integrated with hwmon subsystem
Polling-based update

Next Step¶

Convert driver to hwmon subsystem
Support standard interface:

/sys/class/hwmon/hwmonX/temp1_input