linux-kernel - Re: [PATCHv2] PCI: pciehp: Use appropriate conditions to check the hotplug controller status

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Date: Fri, 14 Jun 2024 21:36:57 +0200
From: Lukas Wunner <lukas@...ner.de>
To: Bjorn Helgaas <helgaas@...nel.org>
Cc: Bitao Hu <yaoma@...ux.alibaba.com>, bhelgaas@...gle.com,
	weirongguang@...inos.cn, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org, kanie@...ux.alibaba.com,
	Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Subject: Re: [PATCHv2] PCI: pciehp: Use appropriate conditions to check the
 hotplug controller status

On Fri, Jun 14, 2024 at 01:41:20PM -0500, Bjorn Helgaas wrote:
> On Tue, May 28, 2024 at 02:42:00PM +0800, Bitao Hu wrote:
> > "present" and "link_active" can be 1 if the status is ready, and 0 if
> > it is not. Both of them can be -ENODEV if reading the config space
> > of the hotplug port failed. That's typically the case if the hotplug
> > port itself was hot-removed. Therefore, this situation can occur:
> > pciehp_card_present() may return 1 and pciehp_check_link_active()
> > may return -ENODEV because the hotplug port was hot-removed in-between
> > the two function calls. In that case we'll emit both "Card present"
> > *and* "Link Up" since both 1 and -ENODEV are considered "true". This
> > is not the expected behavior. Those messages should be emited when
> > "present" and "link_active" are positive.
[...]
> > --- a/drivers/pci/hotplug/pciehp_ctrl.c
> > +++ b/drivers/pci/hotplug/pciehp_ctrl.c
> > @@ -276,10 +276,10 @@ void pciehp_handle_presence_or_link_change(struct controller *ctrl, u32 events)
> >  	case OFF_STATE:
> >  		ctrl->state = POWERON_STATE;
> >  		mutex_unlock(&ctrl->state_lock);
> > -		if (present)
> > +		if (present > 0)
> 
> I completely agree that this is a problem and this patch addresses it.
> But ...
> 
> It seems a little bit weird to me that we even get to this switch
> statement if we got -ENODEV from either pciehp_card_present() or
> pciehp_check_link_active().  If that happens, a config read failed,
> but we're going to go ahead and call pciehp_enable_slot(), which is
> going to do a bunch more config accesses, potentially try to power up
> the slot, etc.
> 
> If a config read failed, it seems like we might want to avoid doing
> some of this stuff.

Hm, good point.  I guess we should change the logical expression instead:

-	if (present <= 0 && link_active <= 0) {
+	if (present < 0 || link_active < 0 || (!present && !link_active)) {


> > -		if (link_active)
> > +		if (link_active > 0)
> >  			ctrl_info(ctrl, "Slot(%s): Link Up\n",
> >  				  slot_name(ctrl));
> 
> These are cases where we misinterpreted -ENODEV as "device is present"
> or "link is active".
> 
> pciehp_ignore_dpc_link_change() and pciehp_slot_reset() also call
> pciehp_check_link_active(), and I think they also interpret -ENODEV as
> "link is active".
> 
> Do we need similar changes there?

Another good observation, both need to check for <= 0 instead of == 0.
Do you want to fix that yourself or would you prefer me (or someone else)
to submit a patch?

Thanks,

Lukas