Re: [PATCH 1/2] i2c: designware: Avoid taking clk_prepare mutex in PM callbacks

[Jisheng Zhang <jszhang@kernel.org>]

On Thu, Aug 21, 2025 at 04:01:55PM +0300, Andy Shevchenko wrote:
> On Thu, Aug 21, 2025 at 03:45:43PM +0300, Jarkko Nikula wrote:
> > On 8/20/25 7:33 PM, Jisheng Zhang wrote:
> > > On Wed, Aug 20, 2025 at 07:05:42PM +0300, Andy Shevchenko wrote:
> > > > On Wed, Aug 20, 2025 at 11:31:24PM +0800, Jisheng Zhang wrote:
> > > > > This is unsafe, as the runtime PM callbacks are called from the PM
> > > > > workqueue, so this may deadlock when handling an i2c attached clock,
> > > > > which may already hold the clk_prepare mutex from another context.
> > > > 
> > > > Can you be more specific? What is the actual issue in practice?
> > > > Do you have traces and lockdep warnings?
> > > 
> > > Assume we use i2c designware to control any i2c based clks, e.g the
> > > clk-si5351.c driver. In its .clk_prepare, we'll get the prepare_lock
> > > mutex, then we call i2c adapter to operate the regs, to runtime resume
> > > the i2c adapter, we call clk_prepare_enable() which will try to get
> > > the prepare_lock mutex again.
> > > 
> > I'd also like to see the issue here. I'm blind to see what's the relation
> > between the clocks managed by the clk-si5351.c and clocks to the
> > i2c-designware IP.

The key here is: all clks in the system share the same prepare_lock
mutex, so the global prepare_lock mutex is locked by clk-si5351
.prepare(), then in this exact .prepare(), the i2c-designware's runtime
resume will try to lock the same prepare_lock again due to
clk_prepare_enable()
can you plz check clk_prepare_lock() in drivers/clk/clk.c?

And if we take a look at other i2c adapters' drivers, we'll see
some of them have ever met this issue and already fixed it, such
as 

i2c-exynos5, by commit 10ff4c5239a1 ("i2c: exynos5: Fix possible ABBA
deadlock by keeping I2C clock prepared")

i2c-imx, by commit d9a22d713acb ("i2c: imx: avoid taking clk_prepare
mutex in PM callbacks")
> 
> I believe they try to make an example when clk-si5351 is the provider of
> the clock to I²C host controller (DesignWare).

Nope, the example case is using i2c host controller to operate the clk-si5351
> 
> But I'm still not sure about the issues here... Without (even simulated with
> specific delay injections) lockdep warnings it would be rather theoretical.

No, it happened in real world.

> 
> -- 
> With Best Regards,
> Andy Shevchenko
> 
>