On Fri, Aug 24, 2018 at 11:42 PM, Peter Wu [off-list ref] wrote:

Are these systems also affected through runtime power management? For
example:

    modprobe nouveau    # should enable runtime PM
    sleep 6             # wait for runtime suspend to kick in
    lspci -s1:          # runtime resume by reading PCI config space

On laptops from about 2015-2016 with a GTX 9xxM this sequence results in
hangs on various laptops
(https://bugzilla.kernel.org/show_bug.cgi?id=156341).

This works fine here. I'm facing a different issue.

quoted

After a lot of experimentation I found a workaround: during resume,
set the value of PCI_PREF_BASE_UPPER32 to 0 on the parent PCI bridge.
Easily done in drivers/pci/quirks.c. Now all nvidia stuff works fine.

I am curious, how did you discover this? While this could work, perhaps
there are alternative workarounds/fixes?

Based on the observation that the following procedure works fine (note
the addition of step 3):

1. Boot
2. Suspend/resume
3. echo rescan > /sys/bus/pci/devices/0000:00:1c.0/rescan
4. Load nouveau driver
5. Start X

I worked through the rescan codepath until I had isolated the specific
code which magically makes things work (in pci_bridge_check_ranges).

Having found that, step 3 in the above test procedure can be replaced
with a simple:
   setpci -s 00:1c.0 0x28.l=0

When you say "parent PCI" bridge, is that actually the device you see in
"lspci -tv"? On a Dell XPS 9560, the GPU is under a different device:

  -[0000:00]-+-00.0  Intel Corporation Xeon E3-1200 v6/7th Gen Core Processor Host Bridge/DRAM Registers
             +-01.0-[01]----00.0  NVIDIA Corporation GP107M [GeForce GTX 1050 Mobile]

 00:01.0 PCI bridge [0604]: Intel Corporation Xeon E3-1200 v5/E3-1500 v5/6th Gen Core Processor PCIe Controller (x16) [8086:1901] (rev 05)

Yes, it's the parent bridge shown by lspci. The address of this varies
from system to system.

quoted

1. Is the Intel PCI bridge misbehaving here? Why does writing the same
value of PCI_PREF_BASE_UPPER32 make any difference at all?

At what point in the suspend code path did you insert this write? It is
possible that the write somehow acted as a fence/memory barrier?

static void quirk_pref_base_upper32(struct pci_dev *dev)
{
       u32 pref_base_upper32;
       pci_read_config_dword(dev, PCI_PREF_BASE_UPPER32, &pref_base_upper32);
       pci_write_config_dword(dev, PCI_PREF_BASE_UPPER32, pref_base_upper32);
}
DECLARE_PCI_FIXUP_RESUME(PCI_VENDOR_ID_INTEL,  0x9d10, quirk_pref_base_upper32);

I don't think it's acting as a barrier. I tried changing this code to
rewrite other registers such as PCI_PREF_MEMORY_BASE and that makes
the bug come back.

quoted

2. Who is responsible for saving and restoring PCI bridge
configuration during suspend and resume? Linux? ACPI? BIOS?

Not sure about PCI bridges, but at least for the PCI Express Capability
registers, it is in control of the OS when control is granted via the
ACPI _OSC method.

I guess you are referring to pci_save_pcie_state(). I can't see
anything equivalent for the bridge registers.

As Windows is probably not affected by this issue, a change must be
possible to make Linux more compatible with Windows. Though I am not
sure what change is needed.

I agree. There's a definite difference with Windows here and it would
be great to find a fix along those lines.

I recently compared PCI configuration space access and ACPI method
invocation using QEMU + VFIO with Linux 4.18, Windows 7 and Windows 10
(1803). There were differences like disabling MSI/interrupts before
suspend, setting the Enable Clock Power Management bit in PCI Express
Link Control and more, but applying these changes were so far not really
successful.

Interesting. Do you know any way that I could spy on Windows' accesses
to the PCI bridge registers?
Looking at at https://wiki.archlinux.org/index.php/PCI_passthrough_via_OVMF
I suspect VFIO would not help me here.
It says:
    Note: If they are grouped with other devices in this manner, pci
root ports and bridges should neither be bound to vfio at boot, nor be
added to the VM.

Thanks
Daniel
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