linux-kernel - Re: [PATCH v8 3/5] amba: Don't unprepare the clocks if device driver wants IRQ safe runtime PM

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Date:	Sat, 01 Nov 2014 01:45:47 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Krzysztof Kozlowski <k.kozlowski@...sung.com>
Cc:	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	Jonathan Corbet <corbet@....net>,
	Russell King <linux@....linux.org.uk>,
	Dan Williams <dan.j.williams@...el.com>,
	Vinod Koul <vinod.koul@...el.com>,
	Ulf Hansson <ulf.hansson@...aro.org>,
	Alan Stern <stern@...land.harvard.edu>,
	linux-pm@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, dmaengine@...r.kernel.org,
	Lars-Peter Clausen <lars@...afoo.de>,
	Michal Simek <michal.simek@...inx.com>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Subject: Re: [PATCH v8 3/5] amba: Don't unprepare the clocks if device driver wants IRQ safe runtime PM

On Monday, October 20, 2014 11:04:46 AM Krzysztof Kozlowski wrote:
> The AMBA bus driver defines runtime Power Management functions which
> disable and unprepare AMBA bus clock. This is problematic for runtime PM
> because unpreparing a clock might sleep so it is not interrupt safe.
> 
> However some drivers may want to implement runtime PM functions in
> interrupt-safe way (see pm_runtime_irq_safe()). In such case the AMBA
> bus driver should only disable/enable the clock in runtime suspend and
> resume callbacks.
> 
> Detect the device driver behavior after calling its probe function and
> store it. During runtime suspend/resume deal with clocks according to
> stored value.
> 
> Signed-off-by: Krzysztof Kozlowski <k.kozlowski@...sung.com>
> Reviewed-by: Ulf Hansson <ulf.hansson@...aro.org>
> ---
>  drivers/amba/bus.c       | 29 +++++++++++++++++++++++++----
>  include/linux/amba/bus.h |  1 +
>  2 files changed, 26 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/amba/bus.c b/drivers/amba/bus.c
> index 47bbdc1b5be3..474434e1b1b9 100644
> --- a/drivers/amba/bus.c
> +++ b/drivers/amba/bus.c
> @@ -95,8 +95,18 @@ static int amba_pm_runtime_suspend(struct device *dev)
>  	struct amba_device *pcdev = to_amba_device(dev);
>  	int ret = pm_generic_runtime_suspend(dev);
>  
> -	if (ret == 0 && dev->driver)
> -		clk_disable_unprepare(pcdev->pclk);
> +	if (ret == 0 && dev->driver) {
> +		/*
> +		 * Drivers should not change pm_runtime_irq_safe()
> +		 * after probe.
> +		 */
> +		WARN_ON(pcdev->irq_safe != pm_runtime_is_irq_safe(dev));
> +
> +		if (pcdev->irq_safe)
> +			clk_disable(pcdev->pclk);
> +		else
> +			clk_disable_unprepare(pcdev->pclk);
> +	}
>  
>  	return ret;
>  }
> @@ -107,7 +117,16 @@ static int amba_pm_runtime_resume(struct device *dev)
>  	int ret;
>  
>  	if (dev->driver) {
> -		ret = clk_prepare_enable(pcdev->pclk);
> +		/*
> +		 * Drivers should not change pm_runtime_irq_safe()
> +		 * after probe.
> +		 */
> +		WARN_ON(pcdev->irq_safe != pm_runtime_is_irq_safe(dev));
> +
> +		if (pcdev->irq_safe)
> +			ret = clk_enable(pcdev->pclk);
> +		else
> +			ret = clk_prepare_enable(pcdev->pclk);
>  		/* Failure is probably fatal to the system, but... */
>  		if (ret)
>  			return ret;
> @@ -198,8 +217,10 @@ static int amba_probe(struct device *dev)
>  		pm_runtime_enable(dev);
>  
>  		ret = pcdrv->probe(pcdev, id);
> -		if (ret == 0)
> +		if (ret == 0) {
> +			pcdev->irq_safe = pm_runtime_is_irq_safe(dev);

This looks racy.

Is it guaranteed that runtime PM callbacks won't be run for the device
after pcdrv->probe() has returned and before setting pcdev->irq_safe?
If not, inconsistent behavior may ensue.

>  			break;
> +		}
>  
>  		pm_runtime_disable(dev);
>  		pm_runtime_set_suspended(dev);
> diff --git a/include/linux/amba/bus.h b/include/linux/amba/bus.h
> index ac02f9bd63dc..c4bae79851fb 100644
> --- a/include/linux/amba/bus.h
> +++ b/include/linux/amba/bus.h
> @@ -32,6 +32,7 @@ struct amba_device {
>  	struct clk		*pclk;
>  	unsigned int		periphid;
>  	unsigned int		irq[AMBA_NR_IRQS];
> +	unsigned int		irq_safe:1;
>  };
>  
>  struct amba_driver {
> 

-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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