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

["Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>]

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On Sat, 7 Dec 2024, 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.

Hi,

Thanks for finding the reason this far.

Mika mentioned earlier to me this Link Speed stuff is all made up on 
Thunderbolt but I didn't make any changes back then because I thought 
bwctrl is not going to change the speed anyway in that case.

Seem reasonable way to workaround the Thunderbold problem and agreed, 
there's not much point in adding bwctrl for 2.5GT/s only ports.

Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>

-- 
 i.


> 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>
> ---
> 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,
>