[PATCH v4 00/16] PCI: dwc: Cache PCIe capability offset

[Hans Zhang]

Hi,

The DWC PCIe core and its platform drivers repeatedly call
dw_pcie_find_capability(pci, PCI_CAP_ID_EXP) to obtain the offset of the
PCI Express Capability structure, including in paths that run more than
once such as link polling, suspend/resume and interrupt handlers.

Add a cached pcie_cap field to struct dw_pcie and a
dw_pcie_get_pcie_cap() helper that performs the lookup on first use and
caches the result, then convert the core and the 14 platform drivers
that use PCI_CAP_ID_EXP.

After this series, dw_pcie_find_capability(..., PCI_CAP_ID_EXP) no
longer appears in drivers/pci/controller/dwc/ outside of the helper
itself.

Design change in v4
===================

v3 cached the offset at one chosen point (dw_pcie_host_init() after
.init, or a driver's PERST# deassert handler) and then read the raw
field everywhere else. As the review pointed out, that model is
fragile: some paths reach the offset before the chosen caching point
(the qcom .post_init callbacks, qcom-ep icc_update() from the global
IRQ, the i.MX6 endpoint path) and it relies on every endpoint driver
remembering to call the helper after enabling hardware.

v4 turns the cache into a lazy lookup: dw_pcie_get_pcie_cap() is called
at every place that needs the offset. The DBI read therefore happens at
exactly the same point as the open coded dw_pcie_find_capability() call
it replaces, so it is impossible to use the offset before DBI is
accessible, and no driver has to call the helper at a particular time.
The repeated searches are still eliminated.

Sashiko review disposition
==========================

As requested, here is the disposition of every non-pre-existing issue
reported by Sashiko on v3:

- Patch 1, "the search is repeated if the capability is not found":
  not a functional issue. The PCIe Capability is mandatory on DWC
  controllers; if it were missing, the hardware would be unusable and
  the only effect is that the (cheap, error path) search is repeated.
  This is now documented in the helper; no negative-cache flag is
  added.

- Patch 1, data race on pci->pcie_cap: the first lookup can be
  triggered from more than one context, so the field is now accessed
  with READ_ONCE()/WRITE_ONCE(). A concurrent lookup stores the same
  value, hence the race is benign and no lock is needed.

- Patches 2, 3, 6, 8, 16, "pci->pcie_cap may be 0 in Endpoint mode":
  valid for v3, fixed in v4. All users, including the endpoint ones, go
  through dw_pcie_get_pcie_cap(), which searches at the point of use, so
  a stale zero offset can no longer corrupt the standard PCI header.

- Patches 2, 6, 8, "unused cap variable and redundant DBI read in
  dw_pcie_link_get_max_link_width()": valid, fixed in patch 2.

- Patch 2, "the commit message claims a warning is printed": valid. The
  claim was wrong (Mani asked the same question) and has been removed;
  no warning is printed and none is needed, because the helper always
  returns the freshly searched value.

- Patch 10, "icc_update() can run before the PERST# handler caches the
  offset": not possible in v4 and not a new issue. The lookup is done
  inside qcom_pcie_ep_icc_update(), exactly where the open coded search
  was before, so this series does not add a DBI access to the global
  IRQ path. The pre-existing races between the global and PERST# IRQ
  threads are out of scope.

- Patch 11, "the qcom .post_init callbacks run before
  qcom_pcie_clear_aspm_l0s() populated the cache": valid for v3, fixed
  in v4. The .post_init callbacks now obtain the offset on their own.

- Pre-existing issues (rockchip and sophgo INTx IRQ domain leak,
  layerscape dw_pcie_dbi_ro_wr_en()/dis() race, spear13xx phy_init() and
  shared IRQ handling, UltraRISC 32-bit RMW of 16-bit registers): all
  acknowledged, but they are not introduced by this series and are left
  for separate fixes.

The layerscape-ep patch keeps Frank's Reviewed-by tag.

The series is build tested with allmodconfig on x86_64
(drivers/pci/controller/dwc/).

Changes in v4:
- Rework the cache into a lazy lookup done at the point of use; drop the
  explicit caching calls from dw_pcie_host_init() and from the driver
  init/PERST# paths. This removes the whole class of "offset used before
  it is cached" bugs and the need for every endpoint driver to call the
  helper itself.
- Fix the qcom .post_init ordering and the i.MX6, fu740, rockchip,
  UltraRISC and core endpoint paths flagged by Sashiko.
- Fix the redundant lookup and the unused variable in
  dw_pcie_link_get_max_link_width().
- Remove the incorrect "prints a warning" statement from the core patch
  commit message.
- Use READ_ONCE()/WRITE_ONCE() for the cached field.
- Audit the whole dwc subtree: all 14 platform drivers using
  PCI_CAP_ID_EXP are converted, and no direct lookup remains.

Changes in v3:
- Remove automatic caching from dw_pcie_ep_init() to prevent unclocked
  DBI access on EP platforms (qcom-ep, tegra194).
- Add explicit dw_pcie_get_pcie_cap() calls in qcom-ep and tegra194 EP
  paths after hardware enablement.
- Keep Host mode caching unchanged.
- Update commit messages accordingly.

Changes in v2:
- Move dw_pcie_get_pcie_cap() from the beginning of dw_pcie_host_init()
  and dw_pcie_ep_init() to after the hardware initialization callbacks
  (host_init/pre_init). This can prevent the reading of DBI when the
  clock is turned off or when the hardware is not ready.
- Convert all platform drivers to use the cached offset via
  dw_pcie_get_pcie_cap() or directly pci->pcie_cap where safe.
- Fix type mismatches (u16 -> u8).
- Properly split per-driver modifications.

v1 series: https://patchwork.kernel.org/project/linux-pci/cover/20260509135152.2241235-1-18255117159@xxxxxxx/
v2 series: https://patchwork.kernel.org/project/linux-pci/patch/20260530153101.695580-1-18255117159@xxxxxxx/
v3 series: https://patchwork.kernel.org/project/linux-pci/cover/20260720150619.24733-1-18255117159@xxxxxxx/

---

Why splitting into per-driver patches
=====================================
To facilitate review and potential bisection, each platform driver is
modified in its own patch. The core changes are in patches 1-2 and the
follow-up patches convert individual glue drivers. If the maintainers
prefer to squash them, please let me know.

This splitting was carried out solely for the convenience of code
review.

---

Hans Zhang (16):
  PCI: dwc: Add pcie_cap field and caching helper
  PCI: dwc: Use cached PCIe capability offset in core
  PCI: dwc: imx6: Use cached PCIe capability offset
  PCI: dwc: layerscape-ep: Use cached PCIe capability offset
  PCI: dwc: meson: Use cached PCIe capability offset
  PCI: dwc: rockchip: Use cached PCIe capability offset
  PCI: dwc: eswin: Use cached PCIe capability offset
  PCI: dwc: fu740: Use cached PCIe capability offset
  PCI: dwc: intel-gw: Use cached PCIe capability offset
  PCI: dwc: qcom-ep: Use cached PCIe capability offset
  PCI: dwc: qcom: Use cached PCIe capability offset
  PCI: dwc: sophgo: Use cached PCIe capability offset
  PCI: dwc: spacemit-k1: Use cached PCIe capability offset
  PCI: dwc: spear13xx: Use cached PCIe capability offset
  PCI: dwc: tegra194: Use cached PCIe capability offset
  PCI: dwc: ultrarisc: Use cached PCIe capability offset

 drivers/pci/controller/dwc/pci-imx6.c         |  4 +--
 .../pci/controller/dwc/pci-layerscape-ep.c    |  4 +--
 drivers/pci/controller/dwc/pci-meson.c        |  4 +--
 .../pci/controller/dwc/pcie-designware-ep.c   |  2 +-
 drivers/pci/controller/dwc/pcie-designware.c  |  8 ++---
 drivers/pci/controller/dwc/pcie-designware.h  | 34 +++++++++++++++++++
 drivers/pci/controller/dwc/pcie-dw-rockchip.c |  2 +-
 drivers/pci/controller/dwc/pcie-eswin.c       |  2 +-
 drivers/pci/controller/dwc/pcie-fu740.c       |  2 +-
 drivers/pci/controller/dwc/pcie-intel-gw.c    |  2 +-
 drivers/pci/controller/dwc/pcie-qcom-ep.c     | 11 ++++----
 drivers/pci/controller/dwc/pcie-qcom.c        | 17 +++++-----
 drivers/pci/controller/dwc/pcie-sophgo.c      |  5 +--
 drivers/pci/controller/dwc/pcie-spacemit-k1.c |  2 +-
 drivers/pci/controller/dwc/pcie-spear13xx.c   |  2 +-
 drivers/pci/controller/dwc/pcie-tegra194.c    |  6 ++--
 drivers/pci/controller/dwc/pcie-ultrarisc.c   |  2 +-
 17 files changed, 72 insertions(+), 37 deletions(-)


base-commit: 4352b8aee98005853aa63f57d6377282de17a33f
-- 
2.43.0