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 11:08:43 -0600
From:   Mario Limonciello <mario.limonciello@....com>
To:     Mika Westerberg <mika.westerberg@...ux.intel.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

On 11/15/2023 04:40, Mika Westerberg wrote:
> 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.

Right; you mentioned the DVSEC available for hubs in this case.  As I 
don't have one of these to validate it works properly I was thinking 
that should be a follow up.

If you think it should be part of the same series I'll add it, but I'd 
ask if you can please check I did it right on one that reports the DVSEC?

> 
> 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. 

Yeah I'd prefer to avoid the kitchen sink for the first pass and then we 
an add more cases to is_virtual_link later.