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

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

On 11/6/2023 06:52, Ilpo Järvinen wrote:
> On Fri, 3 Nov 2023, 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.
>>
>> 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);
>>
> 
> This patch should be split into two, where one just moves the code to the
> new function.

Good idea, thanks.

> 
> Also note that this will conflict with the FIELD_GET() changes (try to
> not reintroduce non-FIELD_GET() code when you rebase this on top of
> v6.7-rc1 :-)).
> 

Sure, will adjust for that when it's rebased to 6.7-rc1.