In our previous blog on Interrupt Management , we discussed how Linux handles interrupts efficiently using a two-part design — the Top Half and the Bottom Half.
If you recall, the Top Half (ISR) responds immediately to hardware events, keeping latency ultra-low.
But the real magic happens after that initial response — in what we call the Bottom Half.
This post is where we dive deeper into how bottom halves work, their different types, and how to decide which one to use in your driver development journey.
⚙️ Why the Bottom Half Exists
When a device interrupts the CPU, Linux must react fast. But many tasks triggered by that interrupt — like copying data, waking user processes, or scheduling I/O — simply can’t or shouldn’t happen inside the interrupt context.
That’s where bottom halves come in.
They handle the heavier, non-time-critical work after the top half finishes acknowledging the interrupt.
In essence:
The Top Half says “Got it!”
The Bottom Half says “Now let’s deal with it properly.”
🧩 The Different Types of Bottom Halves
Linux offers multiple mechanisms to implement bottom halves — each suited to different scenarios. Let’s look at them one by one 👇
1. SoftIRQs
- The most fundamental form of deferred work in Linux.
- Used internally by networking and block I/O subsystems.
- Can run on multiple CPUs in parallel.
- High performance, but rarely used directly in drivers due to complexity.
2. Tasklets
- Built on SoftIRQs but simpler to use.
- Execute in interrupt context but never run two instances of the same tasklet simultaneously.
- Perfect for short, quick deferred tasks that don’t require sleeping.
3. Workqueues
- Execute in process context (via kernel threads), allowing them to sleep, perform I/O, or use locks.
- Ideal for heavier or blocking operations.
- Commonly used in device drivers due to flexibility and safety.
4. Threaded IRQs
- A modern approach that merges top and bottom halves into one threaded interrupt handler.
- The handler itself runs as a kernel thread, meaning it can sleep and behave like process-context code.
- Perfect for drivers needing both responsiveness and complex logic.
💡 Practical Use Cases
Let’s take a few examples from real-world driver development:
📡 Network Drivers
When a network interface (say, Wiznet W5500 SPI-to-Ethernet) raises an interrupt:
- The Top Half identifies which event occurred (packet received, transmit done, etc.) and acknowledges it.
- The Bottom Half — implemented via a Workqueue — processes the packets, updates buffers, and communicates with upper layers.
🔌 USB Drivers
The ISR reacts to the interrupt, while a Threaded IRQ or Workqueue handles data transfers or descriptor parsing.
⚙️ GPIO and Sensor Drivers
A Tasklet or Workqueue can process sensor data asynchronously after an interrupt signals a new reading.
🧠 Choosing the Right Bottom Half
| Scenario | Recommended Mechanism |
|---|---|
| Needs ultra-low latency, no sleep | Tasklet |
| Needs to sleep or perform I/O | Workqueue |
| Networking or block layer tasks | SoftIRQ |
| Complex or flexible interrupt logic | Threaded IRQ |
Choosing the right mechanism isn’t about memorization — it’s about understanding the execution context, timing constraints, and driver requirements.
🔧 Why Practicing Matters More Than Reading
You can’t fully appreciate the elegance of Linux’s interrupt architecture until you implement it yourself.
Seeing how a tasklet works and workqueue fits, or tuning threaded IRQ priorities — that’s where understanding turns into mastery.
And that’s exactly what you’ll do inside our
👉 Project-Oriented Embedded Linux Device Drivers Training
You’ll learn:
✅ Interrupt Management — Top Halves, Bottom Halves, Workqueues
✅ Character Drivers, Synchronization, Process Blocking, and more
✅ 30+ hands-on tasks & an industry-grade driver project
✅ Challenge-based learning format designed to make you job-ready
🎯 Join now: Embedded Linux Device Drivers
🐧 Wrapping Up
The Bottom Half might sound like the “second” part of interrupt handling — but in practice, it’s where the real driver logic lives.
Mastering it means you’re not just responding to hardware — you’re orchestrating it with precision.
If you’ve already understood Interrupt Management, this is your next step to becoming a complete Linux Driver Engineer.
Ready to go hands-on? Let’s build it together 👇
🔗 Embedded Linux Device Drivers