Re: [PATCH] PCI/ASPM: Reject sysfs attempts to enable states that are not covered by policy

[Bjorn Helgaas <helgaas@kernel.org>]

On Mon, Jul 20, 2020 at 08:08:59AM +0200, Heiner Kallweit wrote:
> When trying to enable a state that is not covered by the policy,
> then the change request will be silently ignored. That's not too
> nice to the user, therefore reject such attempts explicitly.
> 
> Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
> ---
>  drivers/pci/pcie/aspm.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
> index b17e5ffd3..cd0f30ca9 100644
> --- a/drivers/pci/pcie/aspm.c
> +++ b/drivers/pci/pcie/aspm.c
> @@ -1224,11 +1224,16 @@ static ssize_t aspm_attr_store_common(struct device *dev,
>  {
>  	struct pci_dev *pdev = to_pci_dev(dev);
>  	struct pcie_link_state *link = pcie_aspm_get_link(pdev);
> +	u32 policy_state = policy_to_aspm_state(link);
>  	bool state_enable;
>  
>  	if (strtobool(buf, &state_enable) < 0)
>  		return -EINVAL;
>  
> +	/* reject attempts to enable states not covered by policy */
> +	if (state_enable && state & ~policy_state)
> +		return -EPERM;

I really like the sentiment of this patch, but I don't like the fact
that this test for states being covered by the policy is here by
itself.

There must be some place in the pcie_config_aspm_link() path that does
a similar test and silently ignores things not covered by the policy?
If we could take advantage of *that* test, we won't have to worry
about things getting out of sync if we change that test in the future.

Maybe pcie_config_aspm_link() could return -EPERM if the policy
doesn't allow the requested state, and we could just notice that here?

>  	down_read(&pci_bus_sem);
>  	mutex_lock(&aspm_lock);
>  
> @@ -1241,7 +1246,7 @@ static ssize_t aspm_attr_store_common(struct device *dev,
>  		link->aspm_disable |= state;
>  	}
>  
> -	pcie_config_aspm_link(link, policy_to_aspm_state(link));
> +	pcie_config_aspm_link(link, policy_state);
>  
>  	mutex_unlock(&aspm_lock);
>  	up_read(&pci_bus_sem);
> -- 
> 2.27.0
>