Re: [PATCH] i2c: exynos5: Fix possible ABBA deadlock by keeping I2C clock prepared

[Marek Szyprowski <m.szyprowski@samsung.com>]

Hello,

On 2016-04-16 00:04, 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 XU 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
>          ----                    ----
>     lock(sec_core:428:(regmap)->lock);
>                                  lock(prepare_lock);
>                                  lock(sec_core:428:(regmap)->lock);
>     lock(prepare_lock);
>
>    *** DEADLOCK ***
>
> Fix this by only preparing the clock on probe and {en,dis}able in the
> rest of the driver.
>
> This patch is similar to commit 34e81ad5f0b6 ("i2c: s3c2410: fix ABBA
> deadlock by keeping clock prepared") that fixes the same bug in other
> driver for an I2C controller found in Samsung SoCs.

I'm sorry, but this is not the right approach imho. It is just a workaround
applied to specific driver, it also duplicates incorrect clock usage
pattern (there is really no point keeping clock prepared all the time).

IMHO this ABBA deadlock should be really fixed in clocks core (probably
by removing global prepare mutex and replacing it with per clock
controller mutexes). Without a proper patch this issue will hit us again
with other i2c controllers or other drivers as well.

> Reported-by: Anand Moon <linux.amoon@gmail.com>
> Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
>
> ---
>
>   drivers/i2c/busses/i2c-exynos5.c | 20 +++++++++++++++-----
>   1 file changed, 15 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/i2c/busses/i2c-exynos5.c b/drivers/i2c/busses/i2c-exynos5.c
> index b29c7500461a..602633747149 100644
> --- a/drivers/i2c/busses/i2c-exynos5.c
> +++ b/drivers/i2c/busses/i2c-exynos5.c
> @@ -671,7 +671,9 @@ static int exynos5_i2c_xfer(struct i2c_adapter *adap,
>   		return -EIO;
>   	}
>   
> -	clk_prepare_enable(i2c->clk);
> +	ret = clk_enable(i2c->clk);
> +	if (ret)
> +		return ret;
>   
>   	for (i = 0; i < num; i++, msgs++) {
>   		stop = (i == num - 1);
> @@ -695,7 +697,7 @@ static int exynos5_i2c_xfer(struct i2c_adapter *adap,
>   	}
>   
>    out:
> -	clk_disable_unprepare(i2c->clk);
> +	clk_disable(i2c->clk);
>   	return ret;
>   }
>   
> @@ -799,6 +801,10 @@ static int exynos5_i2c_probe(struct platform_device *pdev)
>   
>   	platform_set_drvdata(pdev, i2c);
>   
> +	clk_disable(i2c->clk);
> +
> +	return 0;
> +
>    err_clk:
>   	clk_disable_unprepare(i2c->clk);
>   	return ret;
> @@ -810,6 +816,8 @@ static int exynos5_i2c_remove(struct platform_device *pdev)
>   
>   	i2c_del_adapter(&i2c->adap);
>   
> +	clk_unprepare(i2c->clk);
> +
>   	return 0;
>   }
>   
> @@ -830,16 +838,18 @@ static int exynos5_i2c_resume_noirq(struct device *dev)
>   	struct exynos5_i2c *i2c = platform_get_drvdata(pdev);
>   	int ret = 0;
>   
> -	clk_prepare_enable(i2c->clk);
> +	ret = clk_enable(i2c->clk);
> +	if (ret)
> +		return ret;
>   
>   	ret = exynos5_hsi2c_clock_setup(i2c);
>   	if (ret) {
> -		clk_disable_unprepare(i2c->clk);
> +		clk_disable(i2c->clk);
>   		return ret;
>   	}
>   
>   	exynos5_i2c_init(i2c);
> -	clk_disable_unprepare(i2c->clk);
> +	clk_disable(i2c->clk);
>   	i2c->suspended = 0;
>   
>   	return 0;

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland