Re: [PATCH v2 8/9] PCI: Exclude PCIe ports used for tunneling in pcie_bandwidth_available()

[Lukas Wunner <lukas@wunner.de>]

On Fri, Nov 03, 2023 at 02:07:57PM -0500, Mario Limonciello wrote:
> The USB4 spec specifies that PCIe ports that are used for tunneling
> PCIe traffic over USB4 fabric will be hardcoded to advertise 2.5GT/s and
> behave as a PCIe Gen1 device. The actual performance of these ports is
> controlled by the fabric implementation.
> 
> Downstream drivers such as amdgpu which utilize pcie_bandwidth_available()
> to program the device will always find the PCIe ports used for
> tunneling as a limiting factor potentially leading to incorrect
> performance decisions.
> 
> To prevent problems in downstream drivers check explicitly for ports
> being used for PCIe tunneling and skip them when looking for bandwidth
> limitations of the hierarchy. If the only device connected is a root port
> used for tunneling then report that device.

I think a better approach would be to define three new bandwidths for
Thunderbolt in enum pci_bus_speed and add appropriate descriptions in
pci_speed_string().  Those three bandwidths would be 10 GBit/s for
Thunderbolt 1, 20 GBit/s for Thunderbolt 2, 40 GBit/s for Thunderbolt 3
and 4.

Code to determine the Thunderbolt generation from the PCI ID already exists
in tb_switch_get_generation().

This will not only address the amdgpu issue you're trying to solve,
but also emit an accurate speed from __pcie_print_link_status().

The speed you're reporting with your approach is not necessarily
accurate because the next non-tunneled device in the hierarchy might
be connected with a far higher PCIe speed than what the Thunderbolt
fabric allows.

Thanks,

Lukas