Re: [PATCH v4] PCI / ACPI: PM: Take _S0W of the target bridge into account in acpi_pci_bridge_d3(()

[Bjorn Helgaas]

On Thu, Jan 12, 2023 at 10:09:21PM +0000, Limonciello, Mario wrote:
> > -----Original Message-----
> > From: Bjorn Helgaas <helgaas@xxxxxxxxxx>
> > Sent: Thursday, January 12, 2023 16:02
> > To: Rafael J. Wysocki <rjw@xxxxxxxxxxxxx>
> > Cc: linux-pci@xxxxxxxxxxxxxxx; Limonciello, Mario
> > <Mario.Limonciello@xxxxxxx>; Rafael J. Wysocki <rafael@xxxxxxxxxx>; Len
> > Brown <lenb@xxxxxxxxxx>; Bjorn Helgaas <bhelgaas@xxxxxxxxxx>; Mika
> > Westerberg <mika.westerberg@xxxxxxxxxxxxxxx>; Mehta, Sanju
> > <Sanju.Mehta@xxxxxxx>; Lukas Wunner <lukas@xxxxxxxxx>; Rafael J .
> > Wysocki <rafael.j.wysocki@xxxxxxxxx>; linux-acpi@xxxxxxxxxxxxxxx; linux-
> > kernel@xxxxxxxxxxxxxxx; Linux PM <linux-pm@xxxxxxxxxxxxxxx>
> > Subject: Re: [PATCH v4] PCI / ACPI: PM: Take _S0W of the target bridge into
> > account in acpi_pci_bridge_d3(()
> > 
> > On Thu, Jan 12, 2023 at 09:51:24PM +0100, Rafael J. Wysocki wrote:
> > > From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> > >
> > > It is generally questionable to allow a PCI bridge to go into D3 if
> > > it has _S0W returning D2 or a shallower power state, so modify
> > > acpi_pci_bridge_d3(() to always take the return value of _S0W for the
> > > target bridge into accout.  That is, make it return 'false' if _S0W
> > > returns D2 or a shallower power state for the target bridge regardless
> > > of its ancestor PCIe Root Port properties.  Of course, this also causes
> > > 'false' to be returned if the PCIe Root Port itself is the target and
> > > its _S0W returns D2 or a shallower power state.
> > >
> > > However, still allow bridges without _S0W that are power-manageable via
> > > ACPI to enter D3 to retain the current code behavior in that case.
> > >
> > > Link: https://lore.kernel.org/linux-pci/20221031223356.32570-1-
> > mario.limonciello@xxxxxxx/
> > > Reported-by: Mario Limonciello <mario.limonciello@xxxxxxx>
> > > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> > 
> > Applied to pci/pm for v6.3, thanks!
> > 
> > It'd be great if we could include a short description of the problems
> > users might see.  I think the original problem was that on some AMD
> > systems we put a USB4 router in D3 when it should remain in D0.  And I
> > assume this means something doesn't wake up when it should?  Or maybe
> > we miss a hotplug event?
> > 
> > If somebody has an example or some text, I'll add it to the commit
> > log.
> 
> Here's a blurb for what happens on AMD side:
> 
> When the platform is configured to not allow the PCIe port used for
> tunneling to wakeup from D3 it will runtime suspend into D0 and the
> USB4 controller which is a consumer will runtime suspend into D3.
> 
> This inconsistency leads to failures to initialize PCIe tunnels for
> USB4 devices.

And what is J. Random User going to see?  DisplayPort not working
ever?  It works to begin with, but not after a suspend?  Devices in a
dock not being able to wake the system?

I don't really know what "PCIe tunnels for USB4 devices not being
initialized" means for me.  I want to know what a problem report from
a non-expert user might look like.

Bjorn