Subject: Re: [PATCH v9 0/5] I2C Mux per channel bus speed

From: Marcus Folkesson <[email-masked]>
Date: Fri, 8 May 2026 15:14:31 +0200

On Tue, Mar 24, 2026 at 02:54:14PM +0100, 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.
> 
> 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 <[email-masked]>
> ---
> Changes in v9:
> - Fix stray blank line
> - Link to v8: https://lore.kernel.org/r/[email-masked]
> 
> Changes in v8:
> - Fix gramatics and change %d to %u were appropriate
> - Link to v7: https://lore.kernel.org/r/[email-masked]
> 
> Changes in v7:
> - Remove code for finding first mux-locked ancestor
> - Introduce a unlocked (i2c_adapter_set_clk_freq) and unlocked
>   (__i2c_adapter_set_clk_freq) variant
> - Let the locking be handled in __i2c_adapter_set_clk_freq
> - Use I2C_MAX_STANDARD_MODE_FREQ instead of magic numbers where
>   appropriate 
> - Link to v6: https://lore.kernel.org/r/[email-masked]
> 
> Changes in v6:
> - Change logic to find which ancestor to lock with I2C_LOCK_ROOT_ADAPTER
>   It now find the first mux-locked ancestor and then lock its parent.
> 
> - Remove bus_freq_hz in i2c-davinci and only use clock_hz instead
> - Mention in commit message that clock_hz can be used to store frequency in an uniform way
> 
> - Swap order for change freq/deselect to keep symmetry
> - Only allow bus frequency to be lowered in select()
>   This to not allow an intermediate frequency to be set when it is not
>   supposed to
> 
> - check if(ret) instead of ret(<0) where appropriate
> - Fix typos in documentation
> - Change i2c_adapter.clock_hz from int to u32
> - Simplify i2c_adapter_set_clk_freq() by removing 'ret'
> - Link to v5: https://lore.kernel.org/r/[email-masked]
> 
> Changes in v5:
> - Take the lock of the top-most mutex locked mux to make sure that the
>   root is locked
> - Link to v4: https://lore.kernel.org/r/[email-masked]
> 
> Changes in v4:
> - Rebase on master
> - Swap order for printing warning about "channel %u is slower than
>   parent on a non parent-locked mux\n"
> - Fix typo in comment, adaper->adapter
> - Link to v3: https://lore.kernel.org/r/[email-masked]
> 
> Changes in v3:
> - Return -EINVAL if channel is faster than parent (kernel test robot)
> - Link to v2: https://lore.kernel.org/r/[email-masked]
> 
> 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/[email-masked]
> 
> ---

Friendly ping on this series.

Thanks,

Med vänliga hälsningar / Best regards
Marcus Folkesson