Day38 - IIO Sysfs Control Interface

🎯 Goal

Implement control plane for IIO driver using sysfs interface.

Replace module parameters with runtime configurable attributes:

• sampling_frequency (standard IIO)
• fifo_enable (driver private)
• watermark (driver private)

Keep existing data path unchanged:

IRQ → trigger → pollfunc → FIFO read → unpack → iio_push_to_buffers

---

🧠 Design Overview

Control Plane vs Data Plane

Type	Interface
Control	sysfs (/sys/bus/iio/devices/iio:deviceX)
Data	buffer device (/dev/iio:deviceX)

---

🧩 Attribute Design

1. sampling_frequency (Standard IIO)

• Implemented via:
• read_raw()
• write_raw()
• Declared in channel:

.info_mask_shared_by_all = BIT(IIO_CHAN_INFO_SAMP_FREQ)

• Exposed as:

/sys/bus/iio/devices/iio:deviceX/sampling_frequency

---

2. fifo_enable (Private Attribute)

• Controls:

• MYADC_CONFIG_FIFO_ENABLE bit

• Implemented via:

• IIO_DEVICE_ATTR

• show/store handler

• Exposed as:

/sys/bus/iio/devices/iio:deviceX/fifo_enable

---

3. watermark (Private Attribute)

• Controls FIFO watermark level

• Only meaningful when FIFO is enabled

• Exposed as:

/sys/bus/iio/devices/iio:deviceX/watermark

---

🔧 Helper Design

All hardware configuration is routed through helper functions:

myadc_set_sample_rate()
myadc_set_fifo_enable()
myadc_set_fifo_watermark()

Design Rules

• Caller must hold data->lock
• Write-through cache:
• Update hardware
• Then update driver state

---

🔒 Concurrency Model

• All control paths use mutex
• Helper functions assume locked context

mutex_lock(&data->lock);
myadc_set_xxx(...);
mutex_unlock(&data->lock);

---

🚫 Runtime Protection

Prevent configuration changes during streaming:

if (iio_buffer_enabled(indio_dev))
    return -EBUSY;

Applied to: - sampling_frequency - fifo_enable - watermark

---

⚙️ Default Configuration

Driver initializes:

MYADC_CONFIG_ENABLE | MYADC_CONFIG_IRQ_ENABLE

• FIFO disabled by default
• IRQ enabled for consistency
• FIFO mode enabled via sysfs only

---

📌 Design Decision

Why not auto-enable DRDY when FIFO is disabled?

• fifo_enable only controls FIFO bit
• Does NOT imply mode switching

Avoid mixing:

• FIFO mode
• DRDY mode

Mode switching should be handled explicitly in future design.

---

🧱 Macro Simplification

Custom macro to reduce boilerplate:

DECLARE_MYADC_IIO_ATTR(name, permission)

Expands to:

IIO_DEVICE_ATTR(name, permission,
                myadc_<name>_show,
                myadc_<name>_store, 0)

---

✅ Summary

• Introduced sysfs-based control plane
• Unified configuration through helper functions
• Preserved existing FIFO data path
• Established clean driver state model