[PATCH v9 0/3] PCI: Configure Root Port MPS during host probing

[Hans Zhang <18255117159@163.com>]

Current PCIe enumeration configures each device's Maximum Payload Size
(MPS) top-down to match its upstream bridge.  Root Ports, however, return
early from pci_configure_mps() because they have no upstream bridge, so
their MPS may remain at firmware defaults and fail to use the controller's
full capability.

There is also a hierarchy problem when a device's MPS Supported (MPSS) is
smaller than the MPS already programmed above it.  The existing reduction
only lowers a Root Port.  If a Switch is in between, the Switch ports keep
the larger MPS and the endpoint is left mismatched, which can cause
Malformed TLPs.

This series addresses both issues:

1. Patch 1 fixes pci_configure_mps() so that when a device's MPSS is
   smaller than the MPS already in use above it, the whole Root Port
   hierarchy is reduced as necessary, including Switch ports and
   already-programmed sibling devices.

2. Patch 2 safely initializes Root Port MPS to its maximum supported value
   during host probing.  Patch 1 ensures this larger initial value is
   reduced again when downstream devices require a smaller MPS.
   PCIE_BUS_TUNE_OFF and PCIE_BUS_PEER2PEER are left untouched.

3. Patch 3 removes the now-redundant Meson PCIe driver MPS programming and
   cleans up the resulting dead macros and misleading MRRS helper name.

---
Changes in v9:
- Add patch 1 to reduce the whole Root Port hierarchy when a downstream device
  has a smaller MPSS, addressing the Switch case reported by sashiko.
- Patch 2 keeps the Root Port max-MPS initialization, adds error handling, and
  relies on patch 1 for later reduction.
- Patch 3 drops PCIE_CAP_MAX_PAYLOAD_SIZE and MAX_PAYLOAD_SIZE, renames
  meson_size_to_payload() to meson_size_to_mrrs(), and updates the warning.

Changes in v8:
https://patchwork.kernel.org/project/linux-pci/patch/20260911132145.211380-1-18255117159@163.com/

- Rebase to v7.3-rc1.

Changes in v7:
https://patchwork.kernel.org/project/linux-pci/cover/20251127170908.14850-1-18255117159@163.com/

- Exclude PCIE_BUS_PEER2PEER mode from Root Port MPS configuration
- Remove redundant check for upstream bridge (Root Ports don't have one)
- Improve commit message and code comments as per Bjorn.

Changes for v6:
https://patchwork.kernel.org/project/linux-pci/patch/20251104165125.174168-1-18255117159@163.com/

- Modify the commit message and comments. (Bjorn)
- Patch 1/2 code logic: Add !bridge check to configure MPS only for Root Ports
  without an upstream bridge (root bridges), avoiding incorrect handling of
  non-root-bridge Root Ports (Niklas).

Changes for v5:
https://patchwork.kernel.org/project/linux-pci/patch/20250620155507.1022099-1-18255117159@163.com/

- Use pcie_set_mps directly instead of pcie_write_mps.
- The patch 1 commit message were modified.

Changes for v4:
https://patchwork.kernel.org/project/linux-pci/patch/20250510155607.390687-1-18255117159@163.com/

- The patch [v4 1/2] add a comment to explain why it was done this way.
- The patch [v4 2/2] have not been modified.
- Drop patch [v3 3/3]. The Maintainer of the pci-aardvark.c file suggests
  that this patch cannot be submitted. In addition, Mani also suggests
  dropping this patch until this series of issues is resolved.

Changes for v3:
https://patchwork.kernel.org/project/linux-pci/patch/20250506173439.292460-1-18255117159@163.com/

- The new split is patch 2/3 and 3/3.
- Modify the patch 1/3 according to Niklas' suggestion.

Changes for v2:
https://patchwork.kernel.org/project/linux-pci/patch/20250425095708.32662-1-18255117159@163.com/

- According to the Maintainer's suggestion, limit the setting of MPS
  changes to platforms with controller drivers.
- Delete the MPS code set by the SOC manufacturer.
---

Hans Zhang (3):
  PCI: Match the hierarchy's MPS to a device's MPSS as necessary
  PCI: Configure Root Port MPS during host probing
  PCI: dwc: Remove redundant MPS configuration

 drivers/pci/controller/dwc/pci-meson.c | 25 ++----------
 drivers/pci/probe.c                    | 53 ++++++++++++++++++++++++--
 2 files changed, 52 insertions(+), 26 deletions(-)


base-commit: 08df884136f1c1197bab2a27814404fd329d9aac
-- 
2.34.1


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