[PATCH RFC v5 0/4] clk: add support for v1 / v2 clock rate negotiation and kunit tests

[Brian Masney]

The Common Clock Framework is expected to keep a clock’s rate stable
after setting a new rate with:

    clk_set_rate(clk, NEW_RATE);

Clock consumers do not know about the clock hierarchy, sibling clocks,
or the type of clocks involved. However, several longstanding issues
affect how rate changes propagate through the clock tree when
CLK_SET_RATE_PARENT is involved, and the parent's clock rate is changed:

- A clock in some cases can unknowingly change a sibling clock's rate.
  More details about this particular case are documented at:
  https://lore.kernel.org/linux-clk/20250528-clk-wip-v2-v2-2-0d2c2f220442@xxxxxxxxxx/

- No negotiation is done with the sibling clocks, so an inappropriate
  or less than ideal parent rate can be selected.

A selection of some real world examples of where this shows up is at
[1]. DRM needs to run at precise clock rates, and this issue shows up
there, however will also show up in other subsystems that require
precise clock rates, such as sound.

This series introduces kunit tests to illustrate the current behavior in
the clk core. As discussed at Linux Plumbers Conference 2025 in Tokyo
[2], it was suggested to move the negotiation logic into the clk
providers themselves so that only the clks with this issue can have
their rate preserved, and add some common helpers to the clk core.

Note I am marking this series as a RFC because:

- I only have kunit tests, and there's no actual real users of these
  new helpers. (kunit doesn't count.) I'm hoping to get feedback from
  Stephen if this is what he had in mind for an acceptable solution
  based on earlier discussions. In the mean time, I'll see if I can
  get a board that has this issue, or work with someone in another
  company that can test this for me.

- I feel like the names of some of these newly-introduced members
  could be improved. Naming things can be hard.

Changes since v4:
https://lore.kernel.org/linux-clk/20250923-clk-tests-docs-v4-0-9205cb3d3cba@xxxxxxxxxx/
- Reworked based on feedback at Linux Plumbers [2] as described in two
  paragraphs above.
- Dropped gate and mux tests.

Changes since v3:
https://lore.kernel.org/r/20250812-clk-tests-docs-v3-0-054aed58dcd3@xxxxxxxxxx
- Update clk_core struct members (Maxime)
- Add v2 rate negotiation logic and additional kunit tests
- Drop clk_dummy_rate_mhz() in kunit tests; use HZ_PER_MHZ

[1] https://lore.kernel.org/all/20260303115550.9279-1-mitltlatltl@xxxxxxxxx/
    https://lore.kernel.org/lkml/20230825-pll-mipi_keep_rate-v1-0-35bc43570730@xxxxxxxxxxxx/
    https://lore.kernel.org/linux-kernel/20230807-pll-mipi_set_rate_parent-v6-0-f173239a4b59@xxxxxxxxxxxx/
    https://lore.kernel.org/all/20241114065759.3341908-1-victor.liu@xxxxxxx/
    https://lore.kernel.org/linux-clk/20241121-ge-ian-debug-imx8-clk-tree-v1-0-0f1b722588fe@xxxxxxxxxxx/

[2] Fixing Clock Tree Propagation in the Common Clk Framework
    https://www.youtube.com/watch?v=R8TytDzlcFs
    https://lpc.events/event/19/contributions/2152/

Signed-off-by: Brian Masney <bmasney@xxxxxxxxxx>
---
Brian Masney (4):
      clk: test: introduce clk_dummy_div for a mock divider
      clk: test: introduce test suite for sibling rate changes on a divider
      clk: add support for coordinated rate changes from the providers
      clk: test: add kunit test for coordinated rate change

 drivers/clk/clk.c            | 207 +++++++++++++++++++++++++---
 drivers/clk/clk_test.c       | 317 +++++++++++++++++++++++++++++++++++++++++++
 include/linux/clk-provider.h |  39 +++++-
 3 files changed, 546 insertions(+), 17 deletions(-)
---
base-commit: fc7b1a72c6cd5cbbd989c6c32a6486e3e4e3594d
change-id: 20260305-clk-scaling-b3b63cae7624

Best regards,
-- 
Brian Masney <bmasney@xxxxxxxxxx>