[PATCH v3 0/5] I2C Mux per channel bus speed

[Marcus Folkesson <marcus.folkesson@gmail.com>]

This was a RFC on how to implement a feature to have different bus
speeds on different channels with an I2C multiplexer/switch.
As no major complaints on the design came up during the review, I
decided to submit the series without the RFC tag.

The benefit with this feature is that you may group devices after
the fastest bus speed they can handle.
A real-world example is that you could have e.g. a display running @400kHz
and a smart battery running @100kHz using the same I2C controller.

There are many corner cases where this may cause a problem for some
hardware topologies. I've tried to describe those I could think of
in the documentation, see Patch #5.

E.g. one risk is that if the mux driver does not disconnect channels
when Idle, this may cause a higher frequency to "leak" through to
devices that are supposed to run at lower bus speed.
This is not only a "problem" for changing bus speed but could also be
an issue for potential address conflicts.

The implementation is split up into several patches:

Patch #1 Introduce a callback for the i2c controller to set bus speed
Patch #2 Introduce functionality to adjust bus speed depending on mux
         channel.
Patch #3 Cleanup i2c-davinci driver a bit to prepare it for set_clk_freq
Parch #4 Implement set_clk_freq for the i2c-davinci driver
Parch #5 Update documentation with this feature

Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
---
Changes in v3:
- Return -EINVAL if channel is faster than parent (kernel test robot)
- Link to v2: https://lore.kernel.org/r/20251002-i2c-mux-v2-0-b698564cd956@gmail.com

Changes in v2:
- Changed bus_freq field to bus_freq_hz in davinci_i2c_dev (Bartosz Golaszewski)
- Removed idle_state from mux core (Peter Rosin)
- Link to v1: https://lore.kernel.org/r/20250922-i2c-mux-v1-0-28c94a610930@gmail.com

---
Marcus Folkesson (5):
      i2c: core: add callback to change bus frequency
      i2c: mux: add support for per channel bus frequency
      i2c: davinci: calculate bus freq from Hz instead of kHz
      i2c: davinci: add support for setting bus frequency
      docs: i2c: i2c-topology: add section about bus speed

 Documentation/i2c/i2c-topology.rst | 176 +++++++++++++++++++++++++++++++++++++
 drivers/i2c/busses/i2c-davinci.c   |  41 ++++++---
 drivers/i2c/i2c-mux.c              | 116 +++++++++++++++++++++---
 include/linux/i2c.h                |  13 +++
 4 files changed, 324 insertions(+), 22 deletions(-)
---
base-commit: 22f20375f5b71f30c0d6896583b93b6e4bba7279
change-id: 20250913-i2c-mux-b0063de2ae4d

Best regards,
-- 
Marcus Folkesson <marcus.folkesson@gmail.com>