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

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

[H. Nikolaus Schaller]

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@vger.kernel.org
Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
---
 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;
 	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;
 	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) {
-- 
2.50.1 (Apple Git-155)

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

[Andi Shyti]

Hi Nikolaus,

On Fri, Jul 10, 2026 at 08:58:35AM +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 (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.

I don't think caching the clock rate once at probe is safe.

If the controller clock rate changes afterwards,
jz4780_i2c_set_speed() will keep using the stale cached value and
calculate incorrect bus timings.

Andi

> 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@vger.kernel.org
> Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>