Re: [REGRESSION] resume with a Thunderbolt dock broke with commit e8b908146d44 "PCI/PM: Increase wait time after resume"

[Lukas Wunner <lukas@wunner.de>]

On Mon, Sep 25, 2023 at 08:48:41AM -0500, Bjorn Helgaas wrote:
> Now pciehp thinks the slot is occupied and the link is up, so we
> re-enumerate the hierarchy.  Is this because thunderbolt did something
> to 06:00.0 that made the link from 05:01.0 come up?

PCIe TLPs are encapsulated into Thunderbolt packets and transmitted
alongside DisplayPort and other data over the same physical link.

For this to work, PCIe tunnels need to be set up between the Thunderbolt
host controller and attached devices.  Once a tunnel is established,
the PCIe link magically goes up and TLPs can be transmitted.

There are two ways to establish those tunnels:

1/ By a firmware in the Thunderbolt host controller.
   (firmware or "internal" connection manager, drivers/thunderbolt/icm.c)

2/ Natively by the kernel.
   (software connection manager)

I'm assuming that the laptop in question exclusively uses the firmware
connection manager, hence the kernel is reliant on that firmware to
establish tunnels and can't really do anything if it fails to do so.

IMO it goes to show that the native approach is more robust.

Thanks,

Lukas