Re: [PATCH] PCI / PM: Allow runtime PM without callback functions

[Jean Delvare <jdelvare@suse.de>]

Hi Jarkko,

On Thu, 18 Oct 2018 15:30:38 +0300, Jarkko Nikula wrote:
> Allow PCI core to do runtime PM to devices without needing to use dummy
> runtime PM callback functions if there is no need to do anything device
> specific beyond PCI device power state management.
> 
> Implement this by letting core to change device power state during
> runtime PM transitions even if no callback functions are defined.

Thank you very much for looking into this and providing a fix.

> Fixes: a9c8088c7988 ("i2c: i801: Don't restore config registers on runtime PM")
> Reported-by: Mika Westerberg <mika.westerberg@linux.intel.com>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
> ---
> This is related to my i2c-i801.c fix thread back in June which I completely
> forgot till now: https://lkml.org/lkml/2018/6/27/642
> Discussion back then was that it should be handled in the PCI PM instead
> of having dummy functions in the drivers. I wanted to respin with a
> patch.
> ---
>  drivers/pci/pci-driver.c | 16 ++++++----------
>  1 file changed, 6 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/pci/pci-driver.c b/drivers/pci/pci-driver.c
> index bef17c3fca67..6185b878ede1 100644
> --- a/drivers/pci/pci-driver.c
> +++ b/drivers/pci/pci-driver.c
> @@ -1239,7 +1239,7 @@ static int pci_pm_runtime_suspend(struct device *dev)
>  	struct pci_dev *pci_dev = to_pci_dev(dev);
>  	const struct dev_pm_ops *pm = dev->driver ? dev->driver->pm : NULL;
>  	pci_power_t prev = pci_dev->current_state;
> -	int error;
> +	int error = 0;
>  
>  	/*
>  	 * If pci_dev->driver is not set (unbound), we leave the device in D0,
> @@ -1251,11 +1251,9 @@ static int pci_pm_runtime_suspend(struct device *dev)
>  		return 0;
>  	}
>  
> -	if (!pm || !pm->runtime_suspend)
> -		return -ENOSYS;
> -
>  	pci_dev->state_saved = false;
> -	error = pm->runtime_suspend(dev);
> +	if (pm && pm->runtime_suspend)
> +		error = pm->runtime_suspend(dev);
>  	if (error) {
>  		/*
>  		 * -EBUSY and -EAGAIN is used to request the runtime PM core

Later in this function, pm is dereferenced again. It happens twice in
the "if (error)" condition where it is currently safe (error can't be
non-zero if pm->runtime_suspend() has not been called, and obviously
pm->runtime_suspend() can't have been called if pm was NULL). However
it also happens later without the condition:

	if (!pci_dev->state_saved && pci_dev->current_state != PCI_D0
	    && pci_dev->current_state != PCI_UNKNOWN) {
		WARN_ONCE(pci_dev->current_state != prev,
			"PCI PM: State of device not saved by %pF\n",
			pm->runtime_suspend);
		return 0;
	}

I am no expert of the PM framework but is there no risk to dereference
NULL at this point? Or even if pm is non-NULL, pm->runtime_suspend may
be NULL, leading to a confusing warning message?

More generally, I would feel better if instead of initializing error to
0, we would move under the "if (pm && pm->runtime_suspend)" condition
everything that must not be run if pm->runtime_suspend is not defined.
That would make the possible code flows a lot clearer.

> @@ -1292,7 +1290,7 @@ static int pci_pm_runtime_suspend(struct device *dev)
>  
>  static int pci_pm_runtime_resume(struct device *dev)
>  {
> -	int rc;
> +	int rc = 0;
>  	struct pci_dev *pci_dev = to_pci_dev(dev);
>  	const struct dev_pm_ops *pm = dev->driver ? dev->driver->pm : NULL;
>  
> @@ -1306,14 +1304,12 @@ static int pci_pm_runtime_resume(struct device *dev)
>  	if (!pci_dev->driver)
>  		return 0;
>  
> -	if (!pm || !pm->runtime_resume)
> -		return -ENOSYS;
> -
>  	pci_fixup_device(pci_fixup_resume_early, pci_dev);
>  	pci_enable_wake(pci_dev, PCI_D0, false);
>  	pci_fixup_device(pci_fixup_resume, pci_dev);
>  
> -	rc = pm->runtime_resume(dev);
> +	if (pm && pm->runtime_resume)
> +		rc = pm->runtime_resume(dev);
>  
>  	pci_dev->runtime_d3cold = false;
>  

The rest looks good to me.

Thanks,
-- 
Jean Delvare
SUSE L3 Support