Day36 - IIO Triggered Buffer (User-Space Controlled Data Flow)¶

Overview¶

This day introduces the IIO triggered buffer model, transforming the driver from:

Direct mode (read_raw via sysfs)

into:

Event-driven streaming (IRQ → trigger → buffer → /dev/iio:deviceX)

The key concept:

👉 User space controls data flow, not the driver

Architecture¶

Previous (Day34–35)¶

User → read sysfs
      ↓
driver read_raw()
      ↓
I2C read sensor

Day36 (Triggered Buffer)¶

Sensor → DRDY IRQ
           ↓
    IRQ handler
           ↓
    iio_trigger_poll()
           ↓
    pollfunc (driver)
           ↓
    iio_push_to_buffers()
           ↓
    /dev/iio:deviceX
           ↓
        User space

Key Components¶

1. Trigger¶

Represents data-ready event source
Can be:
GPIO interrupt
timer
external trigger

Example:

myadc_iio-dev0

2. Buffer¶

Kernel-side FIFO (kfifo)
Stores sample frames
Exposed via:

/dev/iio:deviceX

3. Scan Elements¶

Define:

which channels are included
ordering in buffer frame

Example:

in_voltage0 → scan_index = 0
in_voltage1 → scan_index = 1
in_voltage2 → scan_index = 2

Important Design Principle¶

⚠️ User-space owns configuration¶

Component	Controlled by
scan elements	user space
trigger binding	user space
buffer enable	user space
data read	user space

👉 Driver must NOT enforce policy

Why driver cannot auto-enable buffer¶

Because:

multiple devices may share trigger
user decides sampling strategy
kernel separates mechanism vs policy

User-Space Control Flow¶

Step 1 - Enable scan elements¶

echo 1 > scan_elements/in_voltage0_en
echo 1 > scan_elements/in_voltage1_en
echo 1 > scan_elements/in_voltage2_en

Step 2 - Bind trigger¶

echo myadc_iio-dev0 > trigger/current_trigger

Step 3 - Configure buffer¶

echo 16 > buffer/length

Step 4 - Enable buffer¶

echo 1 > buffer/enable

Step 5 - Read data¶

cat /dev/iio:device0

Step 6 - Cleanup (IMPORTANT)¶

echo 0 > buffer/enable
echo "" > trigger/current_trigger

Sample Frame Layout¶

For this driver:

[ch0][ch1][ch2]
 2B   2B   2B

Total:

6 bytes per sample

Data Format Example¶

f0 02 54 03 b8 03

Decoded:

Channel	Value
ch0	0x02F0 (752)
ch1	0x0354 (852)
ch2	0x03B8 (952)

Driver Responsibilities¶

Driver MUST:¶

define channels
implement read_raw()
implement trigger handler
push data into buffer

Driver MUST NOT:¶

enable buffer
bind trigger
decide sampling policy

Trigger Handler Flow¶

trigger fires
→ pollfunc executes
→ read sensor channels
→ push sample
→ notify done

Common Pitfalls¶

1. Missing scan_index¶

Duplicate scan index → -EINVAL

2. Buffer still enabled¶

rmmod: module in use

Fix:

echo 0 > buffer/enable

3. Trigger not detached¶

driver cannot unload

Fix:

echo "" > trigger/current_trigger

4. IRQ works but no data¶

Check:

trigger binding
buffer enable
scan elements

Comparison with Previous Days¶

Feature	Day34	Day35	Day36
sysfs read	✔	✔	✔
IRQ	✖	✔	✔
buffer	✖	✖	✔
streaming	✖	✖	✔
user control	low	medium	high

Key Learning¶

1. Separation of concerns¶

driver = data provider
user space = data controller

2. Trigger decoupling¶

sensor event ≠ data read
trigger connects them

3. Buffer = streaming interface¶

replaces repeated sysfs reads
supports high-rate sampling

Next Step¶

Day37:

timestamp channel
variable scan mask support
userspace parsing tool
performance tuning