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

[Mario Limonciello]

On 4/23/2024 00:33, Mika Westerberg wrote:
> 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?

Yeah there was a series that I worked on a few spins a while back 
specifically in the context of eGPUs to identify virtual links and take 
them out of bandwidth calculations.

It didn't get merged, I recall it got stalled on various feedback and I 
didn't dust it off because the series also did prompt discussions about 
the reasoning that amdgpu was doing this in the first place.  It turned 
out to be a bad assumption in the code and I instead made a change to 
amdgpu to not look at the whole topology but just the link partner 
(466a7d115326ece682c2b60d1c77d1d0b9010b4f if anyone is curious).

> 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.