linux-kernel - Re: [PATCH 2/2] PCI: Ignore PCIe ports used for tunneling in pcie_bandwidth

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Date:   Thu, 2 Nov 2023 18:28:27 +0100
From:   Lukas Wunner <lukas@...ner.de>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Mario Limonciello <mario.limonciello@....com>, bhelgaas@...gle.com,
        mika.westerberg@...ux.intel.com, andreas.noever@...il.com,
        michael.jamet@...el.com, YehezkelShB@...il.com,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-usb@...r.kernel.org, Alexander.Deucher@....com
Subject: Re: [PATCH 2/2] PCI: Ignore PCIe ports used for tunneling in
 pcie_bandwidth_available()

On Thu, Nov 02, 2023 at 10:47:48AM -0500, Bjorn Helgaas wrote:
> On Wed, Nov 01, 2023 at 08:14:31PM -0500, Mario Limonciello wrote:
> > On 11/1/2023 17:52, Bjorn Helgaas wrote:
> > > Is the implication that a tunneling port can *never* be a speed
> > > bottleneck?  That seems to be how this patch would work in practice.
> > 
> > I think that's a stretch we should avoid concluding.
> 
> I'm just reading the description and the patch, which seem to say that
> pcie_bandwidth_available() will never report a tunneling port as the
> limiting port.

If the Thunderbolt host controller is a discrete chip attached with PCIe,
the bandwidth is capped by its Switch Upstream Port.

E.g. the "Light Ridge" Thunderbolt 1 controller's Switch Upstream Port
supports 5 GT/s at x4 width.

In contemporary systems, the Thunderbolt controller is often part of the
CPU SoC, so attached Thunderbolt devices appear below a Root Port.
In that case, there's no such limitation.

Additionally the bandwidth is limited by the Thunderbolt generation:
Thunderbolt 1 had two bidirectional 10 GBit/s channels,
Thunderbolt 2 has 20 GBit/s total, Thunderbolt 3 & 4 has 40 GBit/s total:

https://en.wikipedia.org/wiki/Thunderbolt_(interface)

Hence assuming "unlimited" capacity for Thunderbolt wouldn't be accurate.

Thanks,

Lukas