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

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

On 11/14/2023 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".

> 
> 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);