Re: [PATCH v10 0/7] I2C Mux per channel bus speed

[Marcus Folkesson]

Hi Andy,

On Wed, Jul 08, 2026 at 10:37:14PM +0300, Andy Shevchenko wrote:
> On Wed, Jul 08, 2026 at 02:16:12PM +0200, Marcus Folkesson wrote:
> > 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.
> > 
> > This patchset has been used and tested heavily the last months
> > on a custom board based on a da850 (DaVinci) platform.
> 
> ...
> 
> > Changes in v10:
> > 
> > - Fix gramatics in documentation
> > - Move {__,}i2c_adapter_set_clk_freq() to i2c-mux.c
> 
> Can you point to the discussion where it was suggested, please?
> It was like ages ago (too many patches to review) and I probably
> missed and/or forgot the rationale for that. Also if we move
> to i2c-mux, why do we leave it in the adapter namespace?

Hrm, it seems like the mailing list wasn't CC:ed for that comment.
It was suggested by Peter and the rationale to remove it was:

"
I'not sure it's a good idea to expose {__,}i2c_adapter_set_clk_freq().

Do we really want to make it so simple for random (non-mux) I2C drivers
to muck about with the bus speed? Any bus speed changes not coordinated
with mux-locked bus speed changes are going to be problematic. This is
shown by the discussion about D2 for the multi-level mux-locked example
(patch 5/5) -> it is problematic even when the bus speed is changed by
the mux code itself.

I think these helpers should be moved from i2c.h to i2c-mux.c.
"

> 
> > - Make set_clk_freq() return actual frequency
> > - Reimplement idle_state (from earlier version)
> > - Implement example on idle_state for ltc4306
> > - Link to v9: https://lore.kernel.org/r/20260324-i2c-mux-v9-0-5292b0608243@xxxxxxxxx
> 
> FWIW, I briefly read the v9 discussion and haven't got the above move,
> seems wasn't discussed there?
> 
> -- 
> With Best Regards,
> Andy Shevchenko
> 
> 

Best regards,
Marcus Folkesson