Nouveau - Nov 2023 - [PATCH v3 7/7] PCI: Exclude PCIe ports used for virtual links in pcie_bandwidth_available()

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 at 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;
+		}
 		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);
-- 
2.34.1

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 at 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);

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 at 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);