What the interviewer wants
- Kernel driver structure and the user interface
- Buffer management and how data reaches userspace without copies
- Overflow, error and timing handling
Constraints
- Data rate: 8 × 1 MS/s × 2 bytes = 16 MB/s, continuous
- FPGA DMA writes into physically contiguous buffers given by the driver; raises an interrupt per completed buffer
- Processing in userspace may stall for up to 200 ms (storage flushes)
- The DMA is not cache-coherent with the CPU
If you ask the interviewer
- Can we use an existing subsystem? Yes. IIO (DMABUF buffers in recent kernels) or a custom char device are both acceptable if justified.
- Is PREEMPT_RT available? It can be enabled.
- Must data never be lost? Loss must be detected and reported with exact sample counts. Avoid it under normal load.
How it's graded
The rubric a senior interviewer would use. Each item is scored 0 to 3 and weighted.
| Buffer design and sizing | weight 3 |
| Zero-copy path to userspace | weight 3 |
| Cache coherency and DMA API | weight 2 |
| Interrupt and wake-up path | weight 2 |
| Overflow detection and timestamps | weight 2 |
| Validation | weight 1 |
How to structure your answer
- Requirements and assumptions: the numbers that drive the design.
- Architecture: blocks, tasks, data flow, memory.
- Key decisions and trade-offs, with the alternatives you rejected.
- Failure modes and recovery.
- Testing and validation.
More design rounds
Write your design in five sections and get it graded against this rubric, with a follow-up question from the AI interviewer.
Answer this round