[PATCH v3 0/4] PCI: dwc: Add support for configuring lane equalization presets

[Krishna Chaitanya Chundru]

PCIe equalization presets are predefined settings used to optimize
signal integrity by compensating for signal loss and distortion in
high-speed data transmission.

As per PCIe spec 6.0.1 revision section 8.3.3.3 & 4.2.4 for data rates
of 8.0 GT/s, 16.0 GT/s, 32.0 GT/s, and 64.0 GT/s, there is a way to
configure lane equalization presets for each lane to enhance the PCIe
link reliability. Each preset value represents a different combination
of pre-shoot and de-emphasis values. For each data rate, different
registers are defined: for 8.0 GT/s, registers are defined in section
7.7.3.4; for 16.0 GT/s, in section 7.7.5.9, etc. The 8.0 GT/s rate has
an extra receiver preset hint, requiring 16 bits per lane, while the
remaining data rates use 8 bits per lane.

Based on the number of lanes and the supported data rate, read the
device tree property and stores in the presets structure.

Based upon the lane width and supported data rate update lane
equalization registers.

This patch depends on the this dt binding pull request which got recently
merged: https://github.com/devicetree-org/dt-schema/pull/146

Signed-off-by: Krishna Chaitanya Chundru <krishna.chundru@xxxxxxxxxxxxxxxx>
---
Changes in v3:
- In previous series a wrong patch was attached, correct it
- Link to v2: https://lore.kernel.org/r/20241212-preset_v2-v2-0-210430fbcd8a@xxxxxxxxxxxxxxxx

Changes in v2:
- Fix the kernel test robot error
- As suggested by konrad use for loop and read "eq-presets-%ugts", (8 << i)
- Link to v1: https://lore.kernel.org/r/20241116-presets-v1-0-878a837a4fee@xxxxxxxxxxx

---
Krishna Chaitanya Chundru (4):
      arm64: dts: qcom: x1e80100: Add PCIe lane equalization preset properties
      PCI: of: Add API to retrieve equalization presets from device tree
      PCI: dwc: Improve handling of PCIe lane configuration
      PCI: dwc: Add support for configuring lane equalization presets

 arch/arm64/boot/dts/qcom/x1e80100.dtsi            |  8 ++++
 drivers/pci/controller/dwc/pcie-designware-host.c | 42 +++++++++++++++++++++
 drivers/pci/controller/dwc/pcie-designware.c      | 14 ++++++-
 drivers/pci/controller/dwc/pcie-designware.h      |  4 ++
 drivers/pci/of.c                                  | 45 +++++++++++++++++++++++
 drivers/pci/pci.h                                 | 17 ++++++++-
 include/uapi/linux/pci_regs.h                     |  3 ++
 7 files changed, 130 insertions(+), 3 deletions(-)
---
base-commit: 87d6aab2389e5ce0197d8257d5f8ee965a67c4cd
change-id: 20241212-preset_v2-549b7acda9b7

Best regards,
-- 
Krishna Chaitanya Chundru <krishna.chundru@xxxxxxxxxxxxxxxx>