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

[Mario Limonciello <mario.limonciello@amd.com>]

On 11/15/2023 11:04, Mario Limonciello wrote:
> On 11/14/2023 21:23, Lazar, Lijo wrote:
>>
>>
>> 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.
> 
> That's a good point.  How about a matching behavioral change to 
> __pcie_print_link_status() where it looks at the entire hierarchy for 
> any links marked as virtual and prints a message along the lines of:
> 
> "This value may be further limited by virtual links".

I'll wait for some more feedback on the series before posting another 
version, but I did put this together and this is a sample from dmesg of 
the wording I'm planning on using for the next version:

31.504 Gb/s available PCIe bandwidth, this may be further limited by 
conditions of virtual link 0000:00:03.1

> 
>>
>> 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);
>