Re: [PATCH v3] PCI/PM: Only read PCI_PM_CTRL register when available

[Bjorn Helgaas <helgaas@kernel.org>]

On Thu, Aug 24, 2023 at 09:37:38AM +0800, Feiyang Chen wrote:
> When the current state is already PCI_D0, pci_power_up() will return
> 0 even though dev->pm_cap is not set. In that case, we should not
> read the PCI_PM_CTRL register in pci_set_full_power_state().
> 
> There is nothing more needs to be done below in that case.
> Additionally, pci_power_up() has two callers only and the other one
> ignores the return value, so we can safely move the current state
> check from pci_power_up() to pci_set_full_power_state().

Does this fix a bug?  I guess it does, because previously
pci_set_full_power_state() did a config read at 0 + PCI_PM_CTRL, i.e.,
offset 4, which is actually PCI_COMMAND, and set dev->current_state
based on that.  So dev->current_state is now junk, right?

This might account for some "Refused to change power state from %s to D0"
messages.

How did you find this?  It's nice if we can mention a symptom so
people can connect the problem with this fix.

This sounds like something that probably should have a stable tag?

> Fixes: e200904b275c ("PCI/PM: Split pci_power_up()")
> Signed-off-by: Feiyang Chen <chenfeiyang@loongson.cn>
> Reviewed-by: Rafael J. Wysocki <rafael@kernel.org>
> ---
>  drivers/pci/pci.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 60230da957e0..7e90ab7b47a1 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -1242,9 +1242,6 @@ int pci_power_up(struct pci_dev *dev)
>  		else
>  			dev->current_state = state;
>  
> -		if (state == PCI_D0)
> -			return 0;
> -
>  		return -EIO;
>  	}
>  
> @@ -1302,8 +1299,12 @@ static int pci_set_full_power_state(struct pci_dev *dev)
>  	int ret;
>  
>  	ret = pci_power_up(dev);
> -	if (ret < 0)
> +	if (ret < 0) {
> +		if (dev->current_state == PCI_D0)
> +			return 0;
> +
>  		return ret;
> +	}
>  
>  	pci_read_config_word(dev, dev->pm_cap + PCI_PM_CTRL, &pmcsr);
>  	dev->current_state = pmcsr & PCI_PM_CTRL_STATE_MASK;
> -- 
> 2.39.3
>