Re: [PATCH] PCI/portdrv: Disable bwctrl service if port is fixed at 2.5 GT/s

[Niklas Schnelle <niks@kernel.org>]

On Sat, 2024-12-07 at 19:44 +0100, Niklas Schnelle wrote:
> Trying to enable bwctrl on a Thunderbolt port causes a boot hang on some
> systems though the exact reason is not yet understood. As per the spec
> Thunderbolt PCIe Downstream Ports have a fake Max Link Speed of 2.5 GT/s
> (USB4 v2 sec 11.2.1):
> 
>    "Max Link Speed field in the Link Capabilities Register set to 0001b
>     (data rate of 2.5 GT/s only).
>     Note: These settings do not represent actual throughput.
>     Throughput is implementation specific and based on the USB4 Fabric
>     performance."
> 
> More generally if 2.5 GT/s is the only supported link speed there is no
> point in throtteling as this is already the lowest possible PCIe speed
> so don't advertise the capability stopping bwctrl from being probed on
> these ports.
> 
> Link: https://lore.kernel.org/linux-pci/Z1R4VNwCOlh9Sg9n@wunner.de/
> Fixes: 665745f27487 ("PCI/bwctrl: Re-add BW notification portdrv as PCIe BW controller")
> Tested-by: Niklas Schnelle <niks@kernel.org>
> Signed-off-by: Niklas Schnelle <niks@kernel.org>

Should probably add but forgot:
Suggested-by: Lukas Wunner <lukas@wunner.de>

> ---
> Note: This issue causes a boot hang on my personal workstation see the
> Link for details.
> ---
>  drivers/pci/pcie/portdrv.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/pcie/portdrv.c b/drivers/pci/pcie/portdrv.c
> index 5e10306b63081b1ddd13e0a545418e2a8610c14c..e5f80e4a11aad4ce60b2ce998b40ec9fda8c653d 100644
> --- a/drivers/pci/pcie/portdrv.c
> +++ b/drivers/pci/pcie/portdrv.c
> @@ -270,7 +270,8 @@ static int get_port_device_capability(struct pci_dev *dev)
>  		u32 linkcap;
>  
>  		pcie_capability_read_dword(dev, PCI_EXP_LNKCAP, &linkcap);
> -		if (linkcap & PCI_EXP_LNKCAP_LBNC)
> +		if (linkcap & PCI_EXP_LNKCAP_LBNC &&
> +		    (linkcap & PCI_EXP_LNKCAP_SLS) != PCI_EXP_LNKCAP_SLS_2_5GB)
>  			services |= PCIE_PORT_SERVICE_BWCTRL;
>  	}
>  
> 
> ---
> base-commit: 40384c840ea1944d7c5a392e8975ed088ecf0b37
> change-id: 20241207-fix_bwctrl_thunderbolt-bd1f96b3d98f
> 
> Best regards,