linux-kernel - Re: [PATCH v4] PCI: Relabel JHL6540 on Lenovo X1 Carbon 7,8

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Message-ID: <CA+Y6NJEpWpfPqHO6=Z1XFCXZDUq1+g6EFryB+Urq1=h0PhT+fg@mail.gmail.com>
Date: Mon, 22 Apr 2024 15:17:24 -0400
From: Esther Shimanovich <eshimanovich@...omium.org>
To: Mika Westerberg <mika.westerberg@...ux.intel.com>
Cc: Mario Limonciello <mario.limonciello@....com>, Dmitry Torokhov <dmitry.torokhov@...il.com>, 
	Lukas Wunner <lukas@...ner.de>, Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Rajat Jain <rajatja@...gle.com>
Subject: Re: [PATCH v4] PCI: Relabel JHL6540 on Lenovo X1 Carbon 7,8

Thanks for the explanation! I still don't fully understand how that
would work for my use case.

Perhaps it would be better for me to describe the case I am trying to
protect against.

To rehash, this quirk was written for devices with discrete
Thunderbolt controllers.

For example,
CometLake_CPU -> AlpineRidge_Chip -> USB-C Port
This device has the ExternalFacingPort property in ACPI.
My quirk relabels the Alpine Ridge chip as "fixed" and
external-facing, so that devices attached to the USB-C port could be
labeled as "removable"

Let's say we have a TigerLake CPU, which has integrated
Thunderbolt/USB4 capabilities:

TigerLake_ThunderboltCPU -> USB-C Port
This device also has the ExternalFacingPort property in ACPI and lacks
the usb4-host-interface property in the ACPI.

My worry is that someone could take an Alpine Ridge Chip Thunderbolt
Dock and attach it to the TigerLake CPU

TigerLake_ThunderboltCPU -> USB-C Port -> AlpineRidge_Dock

If that were to happen, this quirk would incorrectly label the Alpine
Ridge Dock as "fixed" instead of "removable".

My thinking was that we could prevent this scenario from occurring if
we filtered this quirk not to apply on CPU's like Tiger Lake, with
integrated Thunderbolt/USB4 capabilities.

ExternalFacingPort is found both on the Comet Lake ACPI and also on
the Tiger Lake ACPI. So I can't use that to distinguish between CPUs
which don't have integrated Thunderbolt, like Comet Lake, and CPUs
with integrated Thunderbolt, like Tiger Lake.

I am looking for something that can tell me if the device's Root Port
has the Thunderbolt controller upstream to it or not.
Is there anything like that?
Or perhaps should I add a check which compares the name of the
device's CPU with a list of CPUs that this quirk can be applied to?
Or is there some way I can identify the Thunderbolt controller, then
determine if it's upstream or downstream from the root port?
Or are Alpine Ridge docks not something to worry about at all?