linux-kernel - Re: [PATCH v3 5/7] PCI: ACPI: Detect PCIe root ports that are used for tunneling

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Date:   Wed, 15 Nov 2023 12:40:19 +0200
From:   Mika Westerberg <mika.westerberg@...ux.intel.com>
To:     Mario Limonciello <mario.limonciello@....com>
Cc:     Karol Herbst <kherbst@...hat.com>, Lyude Paul <lyude@...hat.com>,
        Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Lukas Wunner <lukas@...ner.de>,
        Danilo Krummrich <dakr@...hat.com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Xinhui Pan <Xinhui.Pan@....com>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        Pali Rohár <pali@...nel.org>,
        Marek Behún <kabel@...nel.org>,
        "Maciej W . Rozycki" <macro@...am.me.uk>,
        Manivannan Sadhasivam <mani@...nel.org>,
        "open list:DRM DRIVER FOR NVIDIA GEFORCE/QUADRO GPUS" 
        <dri-devel@...ts.freedesktop.org>,
        "open list:DRM DRIVER FOR NVIDIA GEFORCE/QUADRO GPUS" 
        <nouveau@...ts.freedesktop.org>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:RADEON and AMDGPU DRM DRIVERS" 
        <amd-gfx@...ts.freedesktop.org>,
        "open list:PCI SUBSYSTEM" <linux-pci@...r.kernel.org>,
        "open list:ACPI" <linux-acpi@...r.kernel.org>
Subject: Re: [PATCH v3 5/7] PCI: ACPI: Detect PCIe root ports that are used
 for tunneling

Hi Mario,

On Tue, Nov 14, 2023 at 02:07:53PM -0600, Mario Limonciello wrote:
> USB4 routers support a feature called "PCIe tunneling". This
> allows PCIe traffic to be transmitted over USB4 fabric.
> 
> PCIe root ports that are used in this fashion can be discovered
> by device specific data that specifies the USB4 router they are
> connected to. For the PCI core, the specific connection information
> doesn't matter, but it's interesting to know that this root port is
> used for tunneling traffic. This will allow other decisions to be
> made based upon it.
> 
> Detect the `usb4-host-interface` _DSD and if it's found save it
> into a new `is_virtual_link` bit in `struct pci_device`.

While this is fine for the "first" tunneled link, this does not take
into account possible other "virtual" links that lead to the endpoint in
question. Typically for eGPU it only makes sense to plug it directly to
the host but say there is a USB4 hub (with PCIe tunneling capabilities)
in the middle. Now the link from the hub to the eGPU that is also
"virtual" is not marked as such and the bandwidth calculations may not
get what is expected.

It should be possible to map the PCIe ports that go over USB4 links
through router port operation "Get PCIe Downstream Entry Mapping" and
for the Thunderbolt 3 there is the DROM entries (I believe Lukas has
patches for this part already) but I guess it is outside of the scope of
this series. Out of curiosity I tried to look in Windows documentation
if there is such interface for GPUs as we have in Linux but could not
find (which does not mean it does not exist, though).