Re: [PATCH v5 1/2] PCI: Configure root port MPS during host probing

[Bjorn Helgaas <helgaas@kernel.org>]

On Fri, Jun 20, 2025 at 11:55:06PM +0800, Hans Zhang wrote:
> Current PCIe initialization logic may leave root ports operating with
> non-optimal Maximum Payload Size (MPS) settings. While downstream device
> configuration is handled during bus enumeration, root port MPS values
> inherited from firmware or hardware defaults ...

Apparently Root Port MPS configuration is different from that for
downstream devices?

> might not utilize the full
> capabilities supported by the controller hardware. This can result in
> suboptimal data transfer efficiency across the PCIe hierarchy.
> 
> During host controller probing phase, when PCIe bus tuning is enabled,
> the implementation now configures root port MPS settings to their
> hardware-supported maximum values. Specifically, when configuring the MPS
> for a PCIe device, if the device is a root port and the bus tuning is not
> disabled (PCIE_BUS_TUNE_OFF), the MPS is set to 128 << dev->pcie_mpss to
> match the Root Port's maximum supported payload size. The Max Read Request
> Size (MRRS) is subsequently adjusted through existing companion logic to
> maintain compatibility with PCIe specifications.
> 
> Note that this initial setting of the root port MPS to the maximum might
> be reduced later during the enumeration of downstream devices if any of
> those devices do not support the maximum MPS of the root port.
> 
> Explicit initialization at host probing stage ensures consistent PCIe
> topology configuration before downstream devices perform their own MPS
> negotiations. This proactive approach addresses platform-specific
> requirements where controller drivers depend on properly initialized
> root port settings, while maintaining backward compatibility through
> PCIE_BUS_TUNE_OFF conditional checks. Hardware capabilities are fully
> utilized without altering existing device negotiation behaviors.

This last paragraph seems kind of like marketing without any real
content.  Is there something important in there?

Nits:
s/root port/Root Port/

Reword "implementation now configures" to be clear about whether "now"
refers to before this patch or after.

Update the MRRS "to maintain compatibility" part.  I'm dubious about
there being a spec compatibility issue with respect to MRRS.  Cite the
relevant section if there is an issue.

> Suggested-by: Niklas Cassel <cassel@kernel.org>
> Suggested-by: Manivannan Sadhasivam <mani@kernel.org>
> Signed-off-by: Hans Zhang <18255117159@163.com>
> ---
>  drivers/pci/probe.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index 4b8693ec9e4c..9f8803da914c 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -2178,6 +2178,16 @@ static void pci_configure_mps(struct pci_dev *dev)
>  		return;
>  	}
>  
> +	/*
> +	 * Unless MPS strategy is PCIE_BUS_TUNE_OFF (don't touch MPS at all),
> +	 * start off by setting root ports' MPS to MPSS. Depending on the MPS
> +	 * strategy, and the MPSS of the devices below the root port, the MPS
> +	 * of the root port might get overridden later.
> +	 */
> +	if (pci_pcie_type(dev) == PCI_EXP_TYPE_ROOT_PORT &&
> +	    pcie_bus_config != PCIE_BUS_TUNE_OFF)
> +		pcie_set_mps(dev, 128 << dev->pcie_mpss);
> +
>  	if (!bridge || !pci_is_pcie(bridge))
>  		return;
>  
> -- 
> 2.25.1
> 

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