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

Lukas Wunner
2025-07-26T07:52:06Z

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