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

Lazar, Lijo
2023-11-04T06:57:44Z

On 11/4/2023 12:37 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.

The code below ties a generic term 'tunneling' to USB4 spec. I think it
should be something like if (is_USB4 && is_tunneled), exclude from
bandwidth calculations - it should specifically identify usb4 based
tunneling rather than applying to all 'tunneled' cases.

Thanks,
Lijo

>
> 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.
>
> Downstream drivers could make this change on their own 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>
> ---
> drivers/pci/pci.c | 74 +++++++++++++++++++++++++++++++----------------
> 1 file changed, 49 insertions(+), 25 deletions(-)
>
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index d9aa5a39f585..15e37164ce56 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -6223,6 +6223,35 @@ int pcie_set_mps(struct pci_dev *dev, int mps)
> }
> EXPORT_SYMBOL(pcie_set_mps);
>
> +static u32 pcie_calc_bw_limits(struct pci_dev *dev, u32 bw,
> + struct pci_dev **limiting_dev,
> + enum pci_bus_speed *speed,
> + enum pcie_link_width *width)
> +{
> + enum pcie_link_width next_width;
> + enum pci_bus_speed next_speed;
> + u32 next_bw;
> + u16 lnksta;
> +
> + pcie_capability_read_word(dev, PCI_EXP_LNKSTA, &lnksta);
> + next_speed = pcie_link_speed[lnksta & PCI_EXP_LNKSTA_CLS];
> + next_width = (lnksta & PCI_EXP_LNKSTA_NLW) >> PCI_EXP_LNKSTA_NLW_SHIFT;
> + next_bw = next_width * PCIE_SPEED2MBS_ENC(next_speed);
> +
> + /* Check if current device limits the total bandwidth */
> + if (!bw || next_bw <= bw) {
> + bw = next_bw;
> + if (limiting_dev)
> + *limiting_dev = dev;
> + if (speed)
> + *speed = next_speed;
> + if (width)
> + *width = next_width;
> + }
> +
> + return bw;
> +}
> +
> /**
> * pcie_bandwidth_available - determine minimum link settings of a PCIe
> * device and its bandwidth limitation
> @@ -6236,47 +6265,42 @@ EXPORT_SYMBOL(pcie_set_mps);
> * 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 used for transmitting tunneled PCIe traffic over a Thunderbolt
> + * or USB4 link that is part of larger hierarchy. 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)
> {
> - u16 lnksta;
> - enum pci_bus_speed next_speed;
> - enum pcie_link_width next_width;
> - u32 bw, next_bw;
> + struct pci_dev *tdev = NULL;
> + u32 bw = 0;
>
> if (speed)
> *speed = PCI_SPEED_UNKNOWN;
> if (width)
> *width = PCIE_LNK_WIDTH_UNKNOWN;
>
> - bw = 0;
> -
> while (dev) {
> - pcie_capability_read_word(dev, PCI_EXP_LNKSTA, &lnksta);
> -
> - next_speed = pcie_link_speed[lnksta & PCI_EXP_LNKSTA_CLS];
> - next_width = (lnksta & PCI_EXP_LNKSTA_NLW) >>
> - PCI_EXP_LNKSTA_NLW_SHIFT;
> -
> - next_bw = next_width * PCIE_SPEED2MBS_ENC(next_speed);
> -
> - /* Check if current device limits the total bandwidth */
> - if (!bw || next_bw <= bw) {
> - bw = next_bw;
> -
> - if (limiting_dev)
> - *limiting_dev = dev;
> - if (speed)
> - *speed = next_speed;
> - if (width)
> - *width = next_width;
> + if (dev->is_tunneled) {
> + if (!tdev)
> + tdev = dev;
> + goto skip;
> }
> -
> + bw = pcie_calc_bw_limits(dev, bw, limiting_dev, speed, width);
> +skip:
> dev = pci_upstream_bridge(dev);
> }
>
> + /* If nothing "faster" found on link, limit to first tunneled device */
> + if (tdev && !bw)
> + bw = pcie_calc_bw_limits(tdev, bw, limiting_dev, speed, width);
> +
> return bw;
> }
> EXPORT_SYMBOL(pcie_bandwidth_available);