[RFC PATCH 0/5] tty: Add KUnit test framework for TTY drivers

[Abhinav Saxena <xandfury@gmail.com>]

This patch series introduces a KUnit testing framework for the TTY
subsystem, enabling deterministic, automated testing of TTY drivers and
core functionality without requiring hardware or userspace interaction.

On an x86_64 build with CONFIG_GCOV enabled, these tests increased
TTY subsystem coverage to approximately 10.6% line coverage and
14.7% function coverage [1].

Problem Statement
-----------------
Testing TTY drivers today requires:
- User-space interaction through device nodes
- Complex setup with ptys or real hardware
- Limited ability to test error paths reliably and deterministically

This series solves these issues by providing in-kernel KUnit tests that
exercise real TTY core paths under controlled, deterministic conditions.

What This Series Provides
-------------------------
1. Reusable test helpers (`tty_test_helpers.h`):
   - Minimal (~150 LOC) infrastructure that any TTY driver should be
   able to use
   - Automatic resource management
   - Integrated into core files under KUnit guard, with
     `EXPORT_SYMBOL_IF_KUNIT()` to keep the production symbol table
     clean

2. Mock TTY driver:
   - Demonstrates how drivers can leverage the helpers
   - Enables deterministic scenarios without hardware

3. Core TTY tests:
   - Validate open/close/read/write/termios paths
   - Exercise hangup, resize, and error handling
   - Ensure real kernel paths are tested, not mocked stubs

4. ttynull driver tests:
   - Validate data sink behavior of the null driver
   - Provide a minimal driver contract baseline

5. Optional coverage support:
   - GCOV integration for test coverage analysis

Future Work
-----------
With this foundation merged, follow-up work can:
- Add more coverage of TTY core functions
- Enable each TTY driver to maintain its own KUnit suite
- Introduce stress tests and race detection
- Extend to include more tests for other tty drivers:
  - UART drivers: test interrupt handling without hardware
  - USB serial: validate disconnect and reconnect sequences
  - PTY drivers: test resize, flow control, and hangups
  - Virtual consoles: test Unicode and input handling
  
Testing
-------
- All patches pass `checkpatch.pl`
- Verified on x86_64 with:
    ./tools/testing/kunit/kunit.py run \
        --kunitconfig=.kunit/ \
        --kunitconfig=drivers/tty/tests/.kunitconfig \
        --arch=x86_64
- All tests pass (working around tty_read wrapper in progress)

Feedback welcome! :)

References
----------
[1] Coverage reports: ttytests.haunted2bwanted.me (alt: linux-9ik.pages.dev)
[2] kunit.dev/third_party/kernel/docs/usage.html#testing-static-functions
[3] KUnit: docs.kernel.org/dev-tools/kunit/
[4] TTY driver API: https://docs.kernel.org/driver-api/serial/
[5] Big thanks to LDD3! (Ch18 especially!)

Signed-off-by: Abhinav Saxena <xandfury@gmail.com>
---
Abhinav Saxena (5):
      tty: Add KUnit test infrastructure configuration
      tty: Add KUnit test helper functions
      tty: Add mock TTY driver for KUnit testing
      tty: Add KUnit tests for core TTY functionality
      tty: Add KUnit tests for ttynull driver

 drivers/tty/Kconfig                  |   9 +
 drivers/tty/Makefile                 |   7 +
 drivers/tty/tests/.kunitconfig       |  44 ++++
 drivers/tty/tests/Kconfig            |  44 ++++
 drivers/tty/tests/Makefile           |   2 +
 drivers/tty/tests/test_tty_io_core.c | 249 ++++++++++++++++++++++
 drivers/tty/tests/test_ttynull.c     | 163 +++++++++++++++
 drivers/tty/tests/tty_mock.c         | 186 +++++++++++++++++
 drivers/tty/tests/tty_mock.h         |  34 +++
 drivers/tty/tests/tty_test_helpers.c | 387 +++++++++++++++++++++++++++++++++++
 drivers/tty/tests/tty_test_helpers.h | 239 +++++++++++++++++++++
 drivers/tty/tty_io.c                 |   4 +
 drivers/tty/ttynull.c                |   5 +
 13 files changed, 1373 insertions(+)
---
base-commit: 8d245acc1e884e89f0808f64d6af3fc91d4903a0
change-id: 20250824-tty-tests-7fcd8b2b093e

Best regards,
-- 
Abhinav Saxena <xandfury@gmail.com>