# Kernel Driver Hardware

> Use when a Linux kernel PR diff touches driver hardware interaction, register access, DMA mapping, memory barriers, IRQ handlers, device power/state machines (Sashiko stage 7). Skip for purely generic software logic (VFS, core networking). Part of the linux-kernel-review suite.

- Skill: `gvago/kernel-driver-hardware` (Agent Skill)
- Install (CLI): `npx skillmds@latest add gvago/kernel-driver-hardware`
- Raw SKILL.md: https://api.skillmd.com/api/skills/gvago/kernel-driver-hardware/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- License: Apache-2.0
- Author: gvago (https://skillmd.com/u/gvago)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/gvago/kernel-driver-hardware

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# kernel-driver-hardware

Lens for hardware-facing driver review (Sashiko stage 7). A hardware
engineer's read of register access, DMA, and device state machines. If the
diff is purely generic software logic (VFS, core networking), this lens
returns no findings, say so and stop.

## How to apply

- Establish the device state machine implied by the driver: probe, open,
  start, stop, reset, suspend/resume, remove. Every hardware touch must be
  legal in the state where it happens.
- Verify ordering guarantees explicitly; never assume the compiler or bus
  preserves your intended order without a barrier.

## Rule index

- H1. Register access legality: clocks and power domains enabled before
  registers are touched; no MMIO after reset/suspend has torn state down.
- H2. DMA correctness: every dma_map has a matching unmap on all paths;
  missing dma_wmb()/dma_rmb() between descriptor writes and doorbell;
  unsafe DMA buffer allocation (stack buffers, unaligned).
- H3. Endianness: missing or double cpu_to_le32/be conversions on
  hardware-shared structures.
- H4. IRQ handling: handlers touching state that is freed or reset
  concurrently; missing synchronize_irq before teardown; enabling IRQs
  before the handler's data is ready.
- H5. Ring/queue state: hardware rings accessed before initialization in
  the current hardware state; producer/consumer index math on wrap.
- H6. Timing: missing required delays after reset/power transitions;
  busy-wait loops without timeout.

## Mapping a finding to the contract

- action_level: action_required for state-machine violations, missing
  barriers on DMA paths, or IRQ/teardown races; remediation_recommended for
  missing timeouts on unlikely-stuck waits.
- category: Correctness.
- evidence: the register/DMA/IRQ operation, the device state at that point,
  and why the ordering or state is wrong. Cite the datasheet constraint
  only as stated in the code/comments, do not invent hardware behavior.

## What NOT to flag

- Generic software logic, return no findings for non-hardware diffs.
- Barrier "hardening" where existing locks already order the accesses.
- Delay-length tuning without evidence the current value fails.

## Sourcing

Stage text adapted from Sashiko (Apache-2.0). For driver-subsystem
specifics (i2c, hid, tty, pci, ata...), kernel-subsystem-guides carries the
per-subsystem invariant files.