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Date: Tue, 23 Apr 2024 08:33:12 +0300
From: Mika Westerberg <mika.westerberg@linux.intel.com>
To: Mario Limonciello <mario.limonciello@amd.com>
Cc: Esther Shimanovich <eshimanovich@chromium.org>,
	Dmitry Torokhov <dmitry.torokhov@gmail.com>,
	Lukas Wunner <lukas@wunner.de>, Bjorn Helgaas <bhelgaas@google.com>,
	linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org,
	Rajat Jain <rajatja@google.com>
Subject: Re: [PATCH v4] PCI: Relabel JHL6540 on Lenovo X1 Carbon 7,8
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On Mon, Apr 22, 2024 at 02:21:18PM -0500, Mario Limonciello wrote:
> On 4/22/2024 14:17, Esther Shimanovich wrote:
> > 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?
> 
> My thought was once you have a device as untrusted, everything else
> connected to it should "also" be untrusted.

I think what you are looking for is that anything behind a PCIe tunnel
should not have this applied. IIRC the AMD GPU or some code there were
going to add identification of "virtual" links to the bandwidth
calculation functionality.

@Mario, do you remember if this was done already and if that could maybe
be re-used here?

The other way I think is something like this:

  - If it does not have "usb4-host-interface" property (or behind a port
    that has that). These are all tunneled (e.g virtual).

  - It is directly connected to a PCIe root port with
    "ExternalFacingPort" and it has sibling device that is "Thunderbolt
    NHI". This is because you can only have "NHI" on a host router
    according to the USB4 spec.

I may be forgetting something though.