Re: [PATCH] PCI: increase D3 delay for AMD Ryzen5/7 XHCI controllers

[Daniel Drake <drake@endlessm.com>]

On Mon, Oct 14, 2019 at 11:43 PM Bjorn Helgaas <helgaas@kernel.org> wrote:
> Can you tell if this is because the Ryzen7 XHCI controller is out of
> spec, or is the Linux PCI core missing some delay?  If the latter,
> fixing the core might fix other devices as well.
>
> Mika has this patch:
> https://lore.kernel.org/r/20190821124519.71594-1-mika.westerberg@linux.intel.com
> for similar issues, but I think that patch fixes D3cold->D0
> transitions, and your patch appears to be concerned with D3hot->D0
> transitions.

It's actually coming out of D3cold here, however what happens right
before this is that __pci_start_power_transition() calls
pci_platform_power_transition(D0) to leave D3cold state, then
pci_update_current_state() reads PMCSR and updates dev->current_state
to D3hot.

The 20ms delay for these XHCI controllers is needed precisely at this
point - after writing PMCSR to move to D0, and before reading it back
to check the result.
I tried moving the delay immediately before writing PMCSR, but that
doesn't work. Based on that, it seems like it's just a little out of
spec.

With Mika's patch, pcie_wait_downstream_accessible() is called for
these devices after the state transition has already failed. It also
doesn't do any delaying at that point because pci_pcie_type(pdev) ==
0.

Daniel