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

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Date: Fri, 19 Jan 2024 11:03:18 -0500
From: Esther Shimanovich <eshimanovich@...omium.org>
To: Mika Westerberg <mika.westerberg@...ux.intel.com>
Cc: Dmitry Torokhov <dmitry.torokhov@...il.com>, 
	Mario Limonciello <mario.limonciello@....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

On Thu, Jan 18, 2024 at 1:01 AM Mika Westerberg
<mika.westerberg@...ux.intel.com> wrote:
>
> Well that's pretty much all Intel Titan Ridge and Maple Ridge based
> systems. Some early ones did not use IOMMU but all the rest do.
..
> Before Intel Ice Lake it was all discrete and it is still discrete with
> the Barlow Ridge controller who will have exact same ExternalFacing port
> as the previous models.

Next week I'll try those devices in our inventory to see if I can find
another one with this bug. I'll get back to you on that!

On Fri, Jan 19, 2024 at 2:58 AM Mika Westerberg
<mika.westerberg@...ux.intel.com> wrote:
>
> Now if I understand the reason behind this patch is actually not about
> "removability" that much than about identifying a trusted vs. untrusted
> device and attaching a driver to those. I was under impression that
> there is already a solution to this in ChromeOS kernel. It has an
> allowlist of drivers that are allowed to attach these devices and that
> includes the PCIe port drivers, xhci_hcd and the thunderbolt driver,
> possibly something else too. Is this not working for your case?

This device shouldn’t be treated as a removable thunderbolt device
that is enabled by policy because it is an internal device that should
be trusted in the first place.
Even so, while learning about this problem I tried modifying the
ChromeOS policy but it ended up not fixing the issue because it seems
like there is an expectation for it to see an existing “fixed”
thunderbolt port before it loads anything else. But the fixed
thunderbolt port is prevented from enumerating during bootup, before
the policy has a chance to work.

On Fri, Jan 19, 2024 at 5:23 AM Mika Westerberg
<mika.westerberg@...ux.intel.com> wrote:
>
> The below "might" work:
>
> 1. A device that is directly behind a PCIe root or downstream port that
>    has ->external_facing == 1.
>
> 2. It is a PCIe endpoint.
>
> 3. It is a sibling to or has any of the below PCI IDs (see
>    drivers/thunderbolt/nhi.h for the definitions):

External pci devices seem to have the same kinds of chips in them. So
this wouldn’t distinguish the “fixed but discrete” embedded pci
devices from the “removable” pcie through usb devices. My monitor with
thunderbolt capabilities has the JHL7540 chip in it. From the kernel's
perspective, I have only found that the subsystem id is what
distinguishes these devices.

That is, unless I am missing something in your proposal that would
distinguish a fixed JHL6540 chip from an external JHL6540 chip. Please
correct me on any assumptions I get wrong!