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

Hans Zhang
2025-05-13T14:54:23Z

On 2025/5/13 16:04, Niklas Cassel wrote:
> 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@kernel.org>
>> Signed-off-by: Hans Zhang <18255117159@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@kernel.org>

Dear Niklas,

Thank you very much for your review and suggestions. Let's wait for
others' opinions.

Best regards,
Hans