linux-kernel - Re: [PATCH 1/2] i2c: designware: Avoid taking clk

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Message-ID: <aKcYw0Az1fYfNbBr@smile.fi.intel.com>
Date: Thu, 21 Aug 2025 16:01:55 +0300
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Jarkko Nikula <jarkko.nikula@...ux.intel.com>
Cc: Jisheng Zhang <jszhang@...nel.org>,
	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	Jan Dabros <jsd@...ihalf.com>, Andi Shyti <andi.shyti@...nel.org>,
	linux-kernel@...r.kernel.org, linux-i2c@...r.kernel.org
Subject: Re: [PATCH 1/2] i2c: designware: Avoid taking clk_prepare mutex in
 PM callbacks

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.

I believe they try to make an example when clk-si5351 is the provider of
the clock to I²C host controller (DesignWare).

But I'm still not sure about the issues here... Without (even simulated with
specific delay injections) lockdep warnings it would be rather theoretical.

-- 
With Best Regards,
Andy Shevchenko