Re: [PATCH v6 2/4] PCI: host-common: Add link down handling for Root Ports

[Brian Norris <briannorris@chromium.org>]

Hi Lukas,

On Fri, Aug 29, 2025 at 10:35:20AM +0200, Lukas Wunner wrote:
> On Thu, Aug 28, 2025 at 01:25:12PM -0700, Brian Norris wrote:
> > On the flip side: it's not clear
> > PCI_ERS_RESULT_NEED_RESET+pci_channel_io_normal works as documented
> > either. An endpoint might think it's requesting a slot reset, but
> > pcie_do_recovery() will ignore that and skip reset_subordinates()
> > (pci_host_reset_root_port()).
> > 
> > All in all, the docs sound like endpoints _should_ have control over
> > whether we exercise a full port/slot reset for all types of errors. But
> > in practice, we do not actually give it that control. i.e., your commit
> > message is correct, and the docs are not.
> > 
> > I have half a mind to suggest the appended change, so the behavior
> > matches (some of) the docs a little better [1].
> 
> A change similar to the one you're proposing is already queued on the
> pci/aer topic branch for v6.18:
> 
> https://git.kernel.org/pci/pci/c/d0a2dee7d458

Wow, nice coincidence. It's a reminder I should work off the maintainer
/ -next branch, instead of just mainline...

> Here's the corresponding cover letter:
> 
> https://lore.kernel.org/r/cover.1755008151.git.lukas@wunner.de
> 
> There was a discussion why I didn't take the exact same approach you're
> proposing, but only a similar one:
> 
> https://lore.kernel.org/r/aJ2uE6v46Zib30Jh@wunner.de
> https://lore.kernel.org/r/aKHWf3L0NCl_CET5@wunner.de

Wow, that's a ton of great background and explanation. Thanks!

> > Specifically, I'm trying to see what's supposed to happen with
> > PCI_ERS_RESULT_CAN_RECOVER. I see that for pci_channel_io_frozen, almost
> > all endpoint drivers return PCI_ERS_RESULT_NEED_RESET, but if drivers
> > actually return PCI_ERS_RESULT_CAN_RECOVER, it's unclear what should
> > happen.
> > 
> > Today, we don't actually respect it; pcie_do_recovery() just calls
> > reset_subordinates() (pci_host_reset_root_port()) unconditionally. The
> > only thing that return code affects is whether we call
> > report_mmio_enabled() vs report_slot_reset() afterward. This seems odd.
> 
> In the series queued on pci/aer, I've only allowed drivers to opt in
> to a reset on Non-Fatal Errors.  I didn't dare also letting them opt
> out of a reset on Fatal Errors.

Right, I can see where the latter is risky. Frankly, while I have
endpoint drivers suggesting they should be able to do this, I'm not sure
that's a great idea. Or at least, I can see how it would potentially
break other clients, as you explain.

> These changes of behavior are always risky, so it seemed prudent to not
> introduce too many changes at once.  There was no urgent need to also
> change behavior for Fatal Errors for the use case at hand (the xe graphics
> driver).  I went through all drivers with pci_error_handlers to avoid
> breaking any of them.  It's very tedious work, takes weeks.  It would
> be necessary to do that again when changing behavior for Fatal Errors.
> 
> pcieaer-howto.rst justifies the unconditional reset on Fatal Errors by
> saying that the link is unreliable and that a reset is thus required.
> 
> On the other hand, pci-error-recovery.rst (which is a few months older
> than pcieaer-howto.rst) says in section "STEP 3: Link Reset":
> "This is a PCIe specific step and is done whenever a fatal error has been
> detected"
> 
> I'm wondering if the authors of pcieaer-howto.rst took that at face value
> and thought they'd *have* to reset the link on Fatal Errors.
> 
> Looking through the Fatal Errors in PCIe r7.0 sec 6.2.7, I think a reset
> is justified for some of them, but optional for others.  Which leads me
> to believe that the AER driver should actually enforce a reset only for
> certain Fatal Errors, not all of them.  So this seems like something
> worth revisiting in the future.

Hmm, possibly. I haven't looked so closely at the details on all Fatal
Errors, but I may have a look eventually.

> > All in all, the docs sound like endpoints _should_ have control over
> > whether we exercise a full port/slot reset for all types of errors. But
> > in practice, we do not actually give it that control. i.e., your commit
> > message is correct, and the docs are not.
> 
> Indeed the documentation is no longer in sync with the code.  I've just
> submitted a series to rectify that and cc'ed you:
> 
> https://lore.kernel.org/r/cover.1756451884.git.lukas@wunner.de

Thanks! I'll try to take a pass at reviewing, but it may not be prompt.

Thanks again for all the info and work here.

Brian