clk: Support spread spectrum and use it in clk-scmi

Sorry for the long delay from v9 -> v10.

Since the assigned-clock-sscs property [1] has been accepted into the device
tree schema, we can now support it in the Linux clock driver. Therefore,
I've picked up the previously submitted work [2] titled "clk: Support
spread spectrum and use it in clk-pll144x and clk-scmi."
As more than six months have passed since [2] was posted, I’m treating this
patchset as a new submission rather than a v3.

- Introduce clk_set_spread_spectrum to set the parameters for enabling
  spread spectrum of a clock.
- Parse 'assigned-clock-sscs' and configure it by default before using the
  clock. This property is parsed before parsing clock rate.
- Enable this feature for clk-scmi on i.MX95.

Because SCMI spec will not include spread spectrum as a standard
extension, we still need to use NXP i.MX OEM extension.

[1] https://github.com/devicetree-org/dt-schema/pull/154
[2] https://lore.kernel.org/all/20250205-clk-ssc-v2-0-fa73083...

Signed-off-by: Peng Fan <peng.fan@nxp.com>
---
Changes in v10:
- For patch 4: per Stephen's comments, write a new testsuite, not modifying rate
  tests. Updated commit log, and dropped the R-b tag.
- Rebased to next-20260609
- Link to v9: https://lore.kernel.org/linux-clk/177743031609.5403.87485...

Changes in v9:
- Rebased to next-20260311
- Fix kunit test by setting return value to 0 when there is no
  assigned-clocks in patch 3
- Link to v8: https://lore.kernel.org/r/20260302-clk-ssc-v7-1-v8-0-2356...

Changes in v8:
- Add R-b from Cristian for patch 5 and patch 6
- Add comment for scmi_clk_oem_info in patch 6
- Rebased to next-20260227
- Link to v7: https://lore.kernel.org/r/20251231-clk-ssc-v7-1-v7-0-380e...

Changes in v7:
- Add R-b from Sebin
- Drop __free usage per comment from Krzysztof in patch 3
- Link to v6: https://lore.kernel.org/linux-clk/20251128-clk-ssc-v6-2-v...

Changes in v6:
- Add kunit build warning
- Update OEM string comparation per Sebin
- Link to v5: https://lore.kernel.org/linux-clk/20251009-clk-ssc-v5-1-v...

Changes in v5:
- Per Stephen, export clk_hw_set_spread_spectrum, use enum for method,
  add const for set_spread_spectrum and rename clk_ss/conf to ss_conf.
- Per Sebin, Cristian, Sudeep, I added clk-scmi-oem.c to support vendor
  extensions.
- Link to v4: https://lore.kernel.org/arm-scmi/aNQPWO6pfA_3mmxf@redhat....

Changes in v4:
- Add R-b for patch 1 from Brian
- Drop unecessary change in patch 4 Per Brian
- Link to v3: https://lore.kernel.org/r/20250912-clk-ssc-version1-v3-0-...

Changes in v3:
- New patch 1 for dt-bindings per comment from Brian
  https://lore.kernel.org/all/aLeEFzXkPog_dt2B@x1/
  This might not be good to add a new dt-binding file in v3. But this is
  quite a simple file that just has four macros to encode modulation
  method. So hope this is fine for DT maintainers.
- Add Brain's R-b for patch 2
- New patch 3 to add Kunit test per Brain. Since Brain helped
  draft part of the code, I added Co-developed-by tag from Brain.
- Link to v2: https://lore.kernel.org/r/20250901-clk-ssc-version1-v2-0-...

Changes in v2:
- Simplify the code in patch 2 per Dan Carpenter and Brian Masney
- Rebased to next-20250829
- Link to v1: https://lore.kernel.org/r/20250812-clk-ssc-version1-v1-0-...

---
Peng Fan (6):
      dt-bindings: clock: Add spread spectrum definition
      clk: Introduce clk_hw_set_spread_spectrum
      clk: conf: Support assigned-clock-sscs
      clk: Add KUnit tests for assigned-clock-sscs
      clk: scmi: Introduce common header for SCMI clock interface
      clk: scmi: Add i.MX95 OEM extension support for SCMI clock driver

 drivers/clk/Makefile                               |  12 +-
 drivers/clk/clk-conf.c                             |  76 ++++++++
 drivers/clk/clk-scmi-oem.c                         | 108 +++++++++++
 drivers/clk/clk-scmi.c                             |  44 ++---
 drivers/clk/clk-scmi.h                             |  51 ++++++
 drivers/clk/clk.c                                  |  27 +++
 drivers/clk/clk_test.c                             | 203 ++++++++++++++++++++-
 drivers/clk/kunit_clk_assigned_rates.h             |  10 +
 .../clk/kunit_clk_assigned_rates_u64_multiple.dtso |   6 +
 ...t_clk_assigned_rates_u64_multiple_consumer.dtso |   6 +
 drivers/clk/kunit_clk_assigned_rates_u64_one.dtso  |   3 +
 .../kunit_clk_assigned_rates_u64_one_consumer.dtso |   3 +
 drivers/clk/kunit_clk_assigned_sscs_multiple.dtso  |  20 ++
 .../kunit_clk_assigned_sscs_multiple_consumer.dtso |  24 +++
 drivers/clk/kunit_clk_assigned_sscs_null.dtso      |  16 ++
 .../clk/kunit_clk_assigned_sscs_null_consumer.dtso |  20 ++
 drivers/clk/kunit_clk_assigned_sscs_one.dtso       |  16 ++
 .../clk/kunit_clk_assigned_sscs_one_consumer.dtso  |  20 ++
 drivers/clk/kunit_clk_assigned_sscs_without.dtso   |  15 ++
 .../kunit_clk_assigned_sscs_without_consumer.dtso  |  19 ++
 drivers/clk/kunit_clk_assigned_sscs_zero.dtso      |  12 ++
 .../clk/kunit_clk_assigned_sscs_zero_consumer.dtso |  16 ++
 include/dt-bindings/clock/clock.h                  |  14 ++
 include/linux/clk-provider.h                       |  31 ++++
 24 files changed, 740 insertions(+), 32 deletions(-)
---
base-commit: 49e02880ec0a8c378e811bc9d85da188d7c6204c
change-id: 20260611-clk-v10-846cfd3e561f

Best regards,
--  
Peng Fan <peng.fan@nxp.com>