Re: Enabling d3 support on hotplug bridges

[Lukas Wunner <lukas@wunner.de>]

On Wed, Sep 23, 2020 at 02:15:09PM +0200, Lukas Wunner wrote:
> I've just taken a look at the ACPI dumps provided by Michal Rostecki
> and Arthur Borsboom in the Gitlab bugs linked below.  The topology
> looks like this:
> 
> 00:01.1        Root Port         [\_SB.PCI0.GPP0]
>   01:00.0      Switch Upstream   [\_SB.PCI0.GPP0.SWUS]
>     02:00.0    Switch Downstream [\_SB.PCI0.GPP0.SWUS.SWDS]
>       03:00.0  dGPU              [\_SB.PCI0.GPP0.SWUS.SWDS.VGA]
>       03:00.1  dGPU Audio        [\_SB.PCI0.GPP0.SWUS.SWDS.HDAU]
> 
> The Root Port is hotplug-capable but is not suspended because we only
> allow that for Thunderbolt hotplug ports or root ports with Microsoft's
> HotPlugSupportInD3 _DSD property.  However, that _DSD is not present
> in the ACPI dumps and the Root Port is obviously not a Thunderbolt
> port either.

I took another, closer look at the ACPI tables and couldn't find anything
specific about the Root Port or the GPU below, save for Power Resources
and corresponding _PS0 / _PS3 methods in the Root Port's namespace.

If a hotplug port is explicitly power manageable by ACPI through these
methods, it should be safe to suspend it to D3.  I wouldn't be surprised
if that's what Windows does.  So I've just submitted a patch to whitelist
such ports for D3.  It has been tested successfully by two users with
affected laptops:

https://lore.kernel.org/linux-pci/cea9071dc46025f0d89cdfcec0642b7bfa45968a.1601614985.git.lukas@wunner.de/

Thanks,

Lukas