[PATCH v4 1/2] PCI: Configure root port MPS during host probing

[Niklas Cassel]

On Sat, May 10, 2025 at 11:56:06PM +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 might not utilize the full
> capabilities supported by the controller hardware. This can result is
> uboptimal 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. By iterating through bridge devices
> under the root bus and identifying PCIe root ports, each port's MPS is
> set to 128 << pcie_mpss to match the device's maximum supported payload
> size. The Max Read Request Size (MRRS) is subsequently adjusted through
> existing companion logic to maintain compatibility with PCIe
> specifications.
> 
> 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.
> 
> Suggested-by: Niklas Cassel <cassel at kernel.org>
> Signed-off-by: Hans Zhang <18255117159 at 163.com>
> ---

Looks good to me, but since this I'm the one who suggested this specific
implementation, it would be good if someone else could review it as well.

Reviewed-by: Niklas Cassel <cassel at kernel.org>