linux-kernel - Re: [PATCH v2 2/2] i2c: exynos5: Fix possible ABBA deadlock by keeping I2C clock prepared

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Date:	Fri, 22 Apr 2016 15:31:46 +0200
From:	Wolfram Sang <wsa@...-dreams.de>
To:	Javier Martinez Canillas <javier@....samsung.com>
Cc:	linux-kernel@...r.kernel.org, Anand Moon <linux.amoon@...il.com>,
	Kukjin Kim <kgene@...nel.org>,
	linux-samsung-soc@...r.kernel.org,
	Krzysztof Kozlowski <k.kozlowski@...sung.com>,
	linux-i2c@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 2/2] i2c: exynos5: Fix possible ABBA deadlock by
 keeping I2C clock prepared

On Sat, Apr 16, 2016 at 09:14:53PM -0400, Javier Martinez Canillas wrote:
> The exynos5 I2C controller driver always prepares and enables a clock
> before using it and then disables unprepares it when the clock is not
> used anymore.
> 
> But this can cause a possible ABBA deadlock in some scenarios since a
> driver that uses regmap to access its I2C registers, will first grab
> the regmap lock and then the I2C xfer function will grab the prepare
> lock when preparing the I2C clock. But since the clock driver also
> uses regmap for I2C accesses, preparing a clock will first grab the
> prepare lock and then the regmap lock when using the regmap API.
> 
> An example of this happens on the Exynos5422 Odroid XU4 board where a
> s2mps11 PMIC is used and both the s2mps11 regulators and clk drivers
> share the same I2C regmap.
> 
> The possible deadlock is reported by the kernel lockdep:
> 
>   Possible unsafe locking scenario:
> 
>         CPU0                    CPU1

Squashed both patches into one for easier backporting, added stable and
applied to for-current, thanks!


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