Re: [PATCH v2] i2c: jz4780: Cache host clock rate at probe to prevent CCF prepare_lock deadlock

[Andi Shyti]

Hi Nikolaus,

On Sun, Jul 19, 2026 at 10:19:43PM +0200, H. Nikolaus Schaller wrote:
> Fix a severe AB/BA deadlock between the Common Clock Framework (CCF)
> and the I2C adapter lock, which triggers when an I2C-controlled clock
> generator client (like the Si5351) is registered or modified under the CCF.
> 
> During an i2c client clock (generator) frequency change, the CCF acquires its global
> 'prepare_lock' mutex and the driver calls i2c_transfer() to update the client's
> chip registers, stalling for the adapter's I2C bus lock.
> 
> Concurrently, an independent, parallel transfer on the same bus (e.g., a GPIO
> expander handling LEDs) can hold the I2C adapter lock. Inside this parallel
> transfer path, jz4780_i2c_set_speed() calls clk_get_rate() on the host
> controller's input clock to calculate bus timings. This call attempts to acquire
> the blocked CCF 'prepare_lock', creating a circular dependency that freezes
> the system.
> 
> The jz4780 host controller clock itself is static and never changes at runtime.
> 
> However, calling clk_get_rate() inside the active transfer path introduces
> an unnecessary dependency on the CCF internal locks.
> 
> Eliminate this synchronous clk_get_rate() call from the active transfer
> path by caching the static host peripheral clock rate once - inside the private
> jz4780_i2c structure during jz4780_i2c_probe(). Update jz4780_i2c_set_speed()
> to use this cached value, safely decoupling active I2C transactions from the
> CCF internal locks without any risk of stale timings.
> 
> Assisted-by web based Google AI (pinpointing the bug and writing the message).
> 
> Fixes: ba92222ed63a12 ("i2c: jz4780: Add i2c bus controller driver for Ingenic JZ4780")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: H. Nikolaus Schaller <hns@xxxxxxxxxxxxx>

To be clear, I wasn't against the patch itself, I just didn't
like the commit log and I raised questions coming just from
reading the log.

In this v2 you are missing the changelog, please, don't forget
next time to add it.

I merged your patch in i2c/i2c-fixes.

Thanks,
Andi