PCI/ASPM: Don't reconfigure ASPM entering low-power state

Reconfiguring ASPM when a device transitions to low-power state can enable
L1.1/L1.2 substates on the PCIe link at a time when the device is sleeping
and may be unable to exit them. ASPM should be reconfigured on D0 entry
(resume), not on the way down.

pci_set_low_power_state() calls pcie_aspm_pm_state_change() after writing
D3hot to PCI_PM_CTRL. pcie_aspm_pm_state_change() resets link->aspm_capable
to link->aspm_support and then calls pcie_config_aspm_path(), which can
enable ASPM L1.1/L1.2 substates on the PCIe link. If the device cannot
recover the link from L1.2 while in D3hot, subsequent config space reads
return 0xFFFF ("device inaccessible") and pci_power_up() fails with
messages like:

  vfio-pci 0000:5d:00.0: Unable to change power state from D3hot to D0, device inaccessible

This was observed on NVIDIA H100 SXM5 GPUs bound to vfio-pci when Linux
runtime PM suspends them to D3hot: the GPU becomes permanently inaccessible
and disappears from the PCIe bus.

The call to pcie_aspm_pm_state_change() in pci_set_low_power_state() was
restored by f93e71aea6c6 ("Revert "PCI/ASPM: Remove
pcie_aspm_pm_state_change()""), which reverted 08d0cc5f3426 ("PCI/ASPM:
Remove pcie_aspm_pm_state_change()").  The revert was necessary because the
removal broke suspend/resume on certain platforms that required ASPM to be
reconfigured on D0 entry. However, the revert restored the call in both
pci_set_full_power_state() (D0 entry) and pci_set_low_power_state()
(low-power entry).

Only the D0-entry call is needed to fix the suspend/resume regression. The
low-power-entry call is harmful: reconfiguring ASPM immediately after
putting a device into D3hot can enable link substates that the device or
platform cannot exit while the device is sleeping.

Remove the pcie_aspm_pm_state_change() call from pci_set_low_power_state().
ASPM will still be reconfigured correctly when the device returns to D0 via
pci_set_full_power_state().

Fixes: f93e71aea6c6 ("Revert "PCI/ASPM: Remove pcie_aspm_pm_state_change()"")
Signed-off-by: Carlos Bilbao (Lambda) <carlos.bilbao@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20260428040104.78524-1-carlos.bilbao@kernel.org

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