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

[Paul Cercueil]

Hi Nikolaus,

Le vendredi 10 juillet 2026 à 08:58 +0200, H. Nikolaus Schaller a
écrit :
> 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 (like the Si5351) is registered or modified under the CCF.
> 
> During a clock frequency change, the CCF acquires its global
> 'prepare_lock'
> mutex and calls i2c_transfer() to update the chip registers, stalling
> for the adapter's I2C bus lock.
> 
> Concurrently, a parallel transfer on the same bus (e.g., a GPIO
> expander
> handling LEDs) can hold the I2C adapter lock. Inside the transfer
> path,
> jz4780_i2c_set_speed() calls clk_get_rate() to dynamically calculate
> timings. This call attempts to acquire the blocked CCF
> 'prepare_lock',
> creating a circular dependency that freezes the system or at least
> the involved processes and workers.
> 
> Eliminate the synchronous clk_get_rate() call from the active
> transfer
> path by caching the peripheral clock rate once - inside the private
> jz4780_i2c structure during jz4780_i2c_probe(). Update
> jz4780_i2c_set_speed() to use this cached value, decoupling active
> I2C
> transactions from the CCF internal locks.
> 
> Assisted-by web based Google AI.
> 
> Fixes: ba92222ed63a12 ("i2c: jz4780: Add i2c bus controller driver
> for Ingenic JZ4780")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: H. Nikolaus Schaller <hns@xxxxxxxxxxxxx>
> ---
>  drivers/i2c/busses/i2c-jz4780.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/i2c/busses/i2c-jz4780.c
> b/drivers/i2c/busses/i2c-jz4780.c
> index 664a5471d9335..d729cec9cdf2c 100644
> --- a/drivers/i2c/busses/i2c-jz4780.c
> +++ b/drivers/i2c/busses/i2c-jz4780.c
> @@ -141,6 +141,7 @@ struct jz4780_i2c {
>  	void __iomem		*iomem;
>  	int			 irq;
>  	struct clk		*clk;
> +	unsigned		long clk_rate;

That's a weird spacing. Should probably be "unsigned long" on the left
side.

>  	struct i2c_adapter	 adap;
>  	const struct ingenic_i2c_config *cdata;
>  
> @@ -246,7 +247,7 @@ static int jz4780_i2c_set_target(struct
> jz4780_i2c *i2c, unsigned char address)
>  
>  static int jz4780_i2c_set_speed(struct jz4780_i2c *i2c)
>  {
> -	int dev_clk_khz = clk_get_rate(i2c->clk) / 1000;
> +	int dev_clk_khz = i2c->clk_rate / 1000;

You cache `i2c->clk_rate` but only use it divided by 1000, so maybe
cache `clk_rate_khz` instead?

Cheers,
-Paul

>  	int cnt_high = 0;	/* HIGH period count of the SCL
> clock */
>  	int cnt_low = 0;	/* LOW period count of the SCL clock
> */
>  	int cnt_period = 0;	/* period count of the SCL clock */
> @@ -796,6 +797,8 @@ static int jz4780_i2c_probe(struct
> platform_device *pdev)
>  	if (IS_ERR(i2c->clk))
>  		return PTR_ERR(i2c->clk);
>  
> +	i2c->clk_rate = clk_get_rate(i2c->clk);
> +
>  	ret = of_property_read_u32(pdev->dev.of_node, "clock-
> frequency",
>  				   &clk_freq);
>  	if (ret) {