Re: [PATCH v9 2/2] PCI: trace: Add a RAS tracepoint to monitor link speed changes

[Lukas Wunner]

On Fri, Jul 25, 2025 at 04:09:21PM -0500, Bjorn Helgaas wrote:
> > @@ -319,8 +319,7 @@ int pciehp_check_link_status(struct controller *ctrl)
> >  		return -1;
> >  	}
> >  
> > -	pcie_capability_read_word(pdev, PCI_EXP_LNKSTA2, &linksta2);
> > -	__pcie_update_link_speed(ctrl->pcie->port->subordinate, lnk_status, linksta2);
> > +	pcie_update_link_speed(ctrl->pcie->port->subordinate, PCIE_HOTPLUG);
> 
> It kind of bugs me that the hot-add flow reads LNKSTA three times and
> generates both pci_hp_event LINK_UP and link_event tracepoints:
> 
>   pciehp_handle_presence_or_link_change
>     link_active = pciehp_check_link_active()
>       pcie_capability_read_word(PCI_EXP_LNKSTA)
>     if (link_active)
>       ctrl_info(ctrl, "Slot(%s): Link Up\n")

This LNKSTA read decides whether to bring up the slot.
It can't be eliminated.

>       trace_pci_hp_event(PCI_HOTPLUG_LINK_UP)
>       pciehp_enable_slot
>         __pciehp_enable_slot
>           board_added
>             pciehp_check_link_status
>               pcie_capability_read_word(PCI_EXP_LNKSTA)

This is sort of a final check whether the link is (still) active
before commencing device enumeration.  Doesn't look like it can
safely be eliminated either.

>               pcie_update_link_speed
>                 pcie_capability_read_word(PCI_EXP_LNKSTA)
>                 pcie_capability_read_word(PCI_EXP_LNKSTA2)
>                 trace_pcie_link_event(<REASON>)

This third register read is introduced by the present patch and is
indeed somewhat a step back, given that pciehp_check_link_status()
currently deliberately calls __pcie_update_link_speed() to pass
the already read LNKSTA value.

I'm wondering if the tracepoint can be moved down to
__pcie_update_link_speed()?

> And maybe we need both a bare LINK_UP event and a link_event with all
> the details, but again it seems a little weird to me that there are
> two tracepoints when there's really only one event and we know all the
> link_event information from the very first LNKSTA read.

One of the reasons is that a "Link Down" event would have to
contain dummy values for link speed etc, so it seemed cleaner
to separate the hotplug event from the link speed event.

Thanks,

Lukas