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

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Message-ID: <20231116090042.GF17433@black.fi.intel.com>
Date:   Thu, 16 Nov 2023 11:00:42 +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 Wed, Nov 15, 2023 at 11:08:43AM -0600, Mario Limonciello wrote:
> 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?

I don't think it should be part of this series. I just checked and DVSEC
is only required for hosts so kind of hardware equivalent for the _DSD
property you are using here. For hubs there is no such luxury
unfortunately.

I think I do have hardware here with the DVSEC in place so if you
decide to add it, I should be able to try it.