Re: [linux-pm] [PATCH 1/2] PM / Runtime: Introduce pm_runtime_irq_unsafe()

[Alan Stern]

On Sat, 20 Aug 2011, Rafael J. Wysocki wrote:

> From: Rafael J. Wysocki <rjw@xxxxxxx>
> 
> Add a helper function allowing drivers and subsystems to clear
> the power.irq_safe device flag.

> --- linux.orig/drivers/base/power/runtime.c
> +++ linux/drivers/base/power/runtime.c
> @@ -1109,22 +1109,23 @@ void pm_runtime_no_callbacks(struct devi
>  EXPORT_SYMBOL_GPL(pm_runtime_no_callbacks);
>  
>  /**
> - * pm_runtime_irq_safe - Leave interrupts disabled during callbacks.
> + * __pm_runtime_irq_safe - Manipulate a device's power.irq_safe flag.
>   * @dev: Device to handle
> + * @irq_safe: Whether or not to leave interrupts disabled during callbacks.
>   *
> - * Set the power.irq_safe flag, which tells the PM core that the
> + * Set or unset the power.irq_safe flag, which tells the PM core that the
>   * ->runtime_suspend() and ->runtime_resume() callbacks for this device should
>   * always be invoked with the spinlock held and interrupts disabled.  It also
>   * causes the parent's usage counter to be permanently incremented, preventing
>   * the parent from runtime suspending -- otherwise an irq-safe child might have
>   * to wait for a non-irq-safe parent.
>   */
> -void pm_runtime_irq_safe(struct device *dev)
> +void __pm_runtime_irq_safe(struct device *dev, bool irq_safe)
>  {
>  	if (dev->parent)
>  		pm_runtime_get_sync(dev->parent);
>  	spin_lock_irq(&dev->power.lock);
> -	dev->power.irq_safe = 1;
> +	dev->power.irq_safe = irq_safe;
>  	spin_unlock_irq(&dev->power.lock);

It's not quite this easy.  There are two important aspects that must be
considered.

Firstly, I originally envisioned pm_runtime_irq_safe() being called
just once, before the device is enabled for runtime PM.  If you allow
the flag to be turned on and off like this, you raise the possibility
of races with runtime PM callbacks.  (That is, if a callback occurs at
about the same time as the irq_safe flag is changed, nobody can predict
whether the callback will be invoked with interrupts enabled.)  Maybe
that's something the driver needs to take care of, but it should at
least be mentioned in the documentation.

Secondly, this doesn't manage the parent's usage counter correctly.  
Do the pm_runtime_get_sync(dev->parent) at the beginning only when the
irq_safe flag was off and is being turned on.  And at the end, if the
irq_safe flag was on and is being turned off, do
pm_runtime_put_sync(dev->parent).  See pm_runtime_remove() for why this
matters.  (Also update the documentation; the change to the parent
isn't necessarily permanent any more.)

Alan Stern

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