Re: [PATCH v3 7/7] PCI: Exclude PCIe ports used for virtual links in pcie_bandwidth_available()

["Lazar, Lijo" <lijo.lazar@amd.com>]

On 11/15/2023 1:37 AM, 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.
> 
> Callers for pcie_bandwidth_available() will always find the PCIe ports
> used for tunneling as a limiting factor potentially leading to incorrect
> performance decisions.
> 
> To prevent such problems check explicitly for ports that are marked as
> virtual links or as thunderbolt controllers and skip them when looking
> for bandwidth limitations of the hierarchy. If the only device connected
> is a port used for tunneling then report that device.
> 
> Callers to pcie_bandwidth_available() could make this change on their
> own as well but then they wouldn't be able to detect other potential
> speed bottlenecks from the hierarchy without duplicating
> pcie_bandwidth_available() logic.
> 
> Link: https://gitlab.freedesktop.org/drm/amd/-/issues/2925#note_2145860
> Link: https://www.usb.org/document-library/usb4r-specification-v20
>        USB4 V2 with Errata and ECN through June 2023
>        Section 11.2.1
> Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
> ---
> v2->v3:
>   * Split from previous patch version
>   * Look for thunderbolt or virtual link
> ---
>   drivers/pci/pci.c | 19 +++++++++++++++++++
>   1 file changed, 19 insertions(+)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 0ff7883cc774..b1fb2258b211 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -6269,11 +6269,20 @@ static u32 pcie_calc_bw_limits(struct pci_dev *dev, u32 bw,
>    * limiting_dev, speed, and width pointers are supplied) information about
>    * that point.  The bandwidth returned is in Mb/s, i.e., megabits/second of
>    * raw bandwidth.
> + *
> + * This excludes the bandwidth calculation that has been returned from a
> + * PCIe device that is used for transmitting tunneled PCIe traffic over a virtual
> + * link part of larger hierarchy. Examples include Thunderbolt3 and USB4 links.
> + * The calculation is excluded because the USB4 specification specifies that the
> + * max speed returned from PCIe configuration registers for the tunneling link is
> + * always PCI 1x 2.5 GT/s.  When only tunneled devices are present, the bandwidth
> + * returned is the bandwidth available from the first tunneled device.
>    */
>   u32 pcie_bandwidth_available(struct pci_dev *dev, struct pci_dev **limiting_dev,
>   			     enum pci_bus_speed *speed,
>   			     enum pcie_link_width *width)
>   {
> +	struct pci_dev *vdev = NULL;
>   	u32 bw = 0;
>   
>   	if (speed)
> @@ -6282,10 +6291,20 @@ u32 pcie_bandwidth_available(struct pci_dev *dev, struct pci_dev **limiting_dev,
>   		*width = PCIE_LNK_WIDTH_UNKNOWN;
>   
>   	while (dev) {
> +		if (dev->is_virtual_link || dev->is_thunderbolt) {
> +			if (!vdev)
> +				vdev = dev;
> +			goto skip;
> +		}

One problem with this is it *silently* ignores the bandwidth limiting 
device - the bandwidth may not be really available if there are virtual 
links in between. That is a change in behavior from the messages shown 
in __pcie_print_link_status.

Thanks,
Lijo

>   		bw = pcie_calc_bw_limits(dev, bw, limiting_dev, speed, width);
> +skip:
>   		dev = pci_upstream_bridge(dev);
>   	}
>   
> +	/* If nothing "faster" found on hierarchy, limit to first virtual link */
> +	if (vdev && !bw)
> +		bw = pcie_calc_bw_limits(vdev, bw, limiting_dev, speed, width);
> +
>   	return bw;
>   }
>   EXPORT_SYMBOL(pcie_bandwidth_available);