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

[Ilpo Järvinen]

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@xxxxxxx>
> ---
>  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.

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 :-)).

-- 
 i.