Day33 - Minimal IIO Driver (Single Channel, Direct Mode)¶

Overview¶

This day focuses on implementing a minimal IIO driver to understand:

How IIO devices are registered
How sysfs attributes are generated automatically
How user-space maps to driver callbacks

The driver is intentionally simple:

Single channel (voltage0)
No buffer
No trigger
Only read_raw() supported

Key Concepts¶

1. IIO Device Lifecycle¶

module load
    ↓
platform_device created
    ↓
platform_driver probe()
    ↓
iio_device_register()
    ↓
/sys/bus/iio/devices/iio:deviceX appears

Important¶

An IIO device is NOT created at module load
It is created inside probe()
A matching device must exist for probe to run

2. struct iio_dev¶

struct iio_dev

Represents an IIO device instance.

Similar to:

struct mygpio_dev
struct mysensor_data

But managed by the IIO framework.

3. Channel Definition (`iio_chan_spec`)¶

.type = IIO_VOLTAGE
.indexed = 1
.channel = 0

This directly generates:

in_voltage0_raw
in_voltage0_scale

Key Insight¶

sysfs attribute names are derived from channel metadata, not manually created.

4. info_mask¶

.info_mask_separate =
    BIT(IIO_CHAN_INFO_RAW) |
    BIT(IIO_CHAN_INFO_SCALE);

Controls which attributes appear in sysfs.

Flag	Result
RAW	`in_voltage0_raw`
SCALE	`in_voltage0_scale`

5. read_raw()¶

.read_raw = myiio_read_raw

Handles all data access.

Mapping:

sysfs	mask
in_voltage0_raw	IIO_CHAN_INFO_RAW
in_voltage0_scale	IIO_CHAN_INFO_SCALE

6. Return Value Format¶

Example:

return IIO_VAL_INT;
return IIO_VAL_INT_PLUS_MICRO;

Meaning:

INT → val
INT_PLUS_MICRO → val + val2 * 1e-6

Example:

val = 0
val2 = 1000
→ 0.001

Data Flow¶

sysfs read¶

cat in_voltage0_raw
    ↓
IIO core
    ↓
read_raw(mask = RAW)
    ↓
driver returns value

Comparison with Previous Work¶

vs hwmon¶

hwmon	IIO
Manual sysfs	Auto-generated sysfs
show() functions	read_raw()
Fixed attributes	Channel-driven attributes

vs Character Driver¶

Char driver	IIO
read/write	read_raw
protocol-based	measurement-based
user-defined format	framework-defined format

vs TTY¶

TTY	IIO
byte stream	structured data
flip buffer	IIO buffer (future)
RX-driven	trigger-driven

Limitations of Current Implementation¶

This minimal driver:

Does NOT support buffer
Does NOT support trigger
Does NOT support multi-channel

It behaves like:

single-shot sensor (similar to hwmon)

Key Takeaways¶

IIO sysfs is automatically generated from channel metadata
read_raw() is the central data path
Device must exist before probe runs
IIO abstraction is higher than hwmon

Next Step (Day34)¶

Multi-channel support
Deeper dive into iio_chan_spec
Prepare for buffer / scan elements