Re: [Regression in 6.14-rc1] System suspend/resume broken by PCI commit 1db806ec06b7c

Ilpo Järvinen
2025-02-04T07:48:24Z

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On Mon, 3 Feb 2025, Rafael J. Wysocki wrote:

> On Mon, Feb 3, 2025 at 9:12 PM Rafael J. Wysocki <rafael@kernel.org> wrote:
> >
> > Hi,

Hi Rafael,

> > The following commit:
> >
> > commit 1db806ec06b7c6e08e8af57088da067963ddf117
> > Author: Jian-Hong Pan <jhp@endlessos.org>
> > Date: Fri Nov 15 15:22:02 2024 +0800
> >
> > PCI/ASPM: Save parent L1SS config in pci_save_aspm_l1ss_state()
> >
> > After 17423360a27a ("PCI/ASPM: Save L1 PM Substates Capability for
> > suspend/resume"), pci_save_aspm_l1ss_state(dev) saves the L1SS state for
> > "dev", and pci_restore_aspm_l1ss_state(dev) restores the state for both
> > "dev" and its parent.
> >
> > The problem is that unless pci_save_state() has been used in some other
> > path and has already saved the parent L1SS state, we will restore junk to
> > the parent, which means the L1 Substates likely won't work correctly.
> >
> > Save the L1SS config for both the device and its parent in
> > pci_save_aspm_l1ss_state(). When restoring, we need both because L1SS must
> > be enabled at the parent (the Downstream Port) before being enabled at the
> > child (the Upstream Port).
> >
> > Link: https://lore.kernel.org/r/20241115072200.37509-3-jhp@endlessos.org
> > Fixes: 17423360a27a ("PCI/ASPM: Save L1 PM Substates Capability for
> > suspend/resume")
> > Closes: https://bugzilla.kernel.org/show_bug.cgi?id=218394
> > Suggested-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
> > Signed-off-by: Jian-Hong Pan <jhp@endlessos.org>
> > [bhelgaas: parallel save/restore structure, simplify commit log, patch at
> > https://lore.kernel.org/r/20241212230340.GA3267194@bhelgaas]
> > Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
> > Tested-by: Jian-Hong Pan <jhp@endlessos.org> # Asus B1400CEAE
> >
> > broke system suspend/resume on my Dell XPS13 9360. It doesn't even
> > pass suspend/resume testing after "echo devices > /sys/power/pm_test".
> >
> > It looks like PCIe links are all down during resume after the above
> > commit, but it is rather hard to collect any data in that state.
> >
> > Reverting the above commit on top of 6.14-rc1 makes things work again,
> > no problem.
> >
> > I'm unsure what exactly the problem is ATM, but I'm going to check a
> > couple of theories.
>
> The attached change makes it work again, FWIW, but moving the
> parent->l1ss check alone below the pdev l1ss saving doesn't help.
>
> So it is either the parent check against NULL or the
> pcie_downstream_port(pdev) one that breaks it. I guess the former,
> but I'll test it tomorrow.

Neither of those is the root cause but it's bit hard to see from the code
itself because the parent->saved_state check your test patch also removed
looks very logical on a glance (but that's the problematic line).

The fix is already here with the explanation:

https://lore.kernel.org/linux-pci/20250131152913.2507-1-ilpo.jarvinen@linux.intel.com/T/#u

--
i.