[PATCH v2] PCI: loongson: Override PCIe bridge supported speeds for older Loongson 3C6000 series steppings

[Ziyao Li via B4 Relay]

From: Ziyao Li <liziyao@xxxxxxxxxxxxx>

Older steppings of the Loongson 3C6000 series incorrectly report the
supported link speeds on their PCIe bridges (device IDs 3c19, 3c29) as
only 2.5 GT/s, despite the upstream bus supporting speeds from 2.5 GT/s
up to 16 GT/s.

As a result, certain PCIe devices would be incorrectly probed as a Gen1-
only, even if higher link speeds are supported, harming performance and
prevents dynamic link speed functionality from being enabled in drivers
such as amdgpu.

Manually override the `supported_speeds` field for affected PCIe bridges
with those found on the upstream bus to correctly reflect the supported
link speeds.

This patch was originally found from AOSC OS[1].

Link: https://github.com/AOSC-Tracking/linux/pull/2 #1
Tested-by: Lain Fearyncess Yang <fsf@xxxxxxxx>
Tested-by: Ayden Meng <aydenmeng@xxxxxxxx>
Signed-off-by: Ayden Meng <aydenmeng@xxxxxxxx>
Signed-off-by: Mingcong Bai <jeffbai@xxxxxxx>
[Xi Ruoyao: Fix falling through logic and add kernel log output.]
Signed-off-by: Xi Ruoyao <xry111@xxxxxxxxxxx>
Link: https://github.com/AOSC-Tracking/linux/commit/4392f441363abdf6fa0a0433d73175a17f493454
[Ziyao Li: move from drivers/pci/quirks.c to drivers/pci/controller/pci-loongson.c]
Signed-off-by: Ziyao Li <liziyao@xxxxxxxxxxxxx>
Tested-by: Mingcong Bai <jeffbai@xxxxxxx>
---
Changes in v2:
- Link to v1: https://lore.kernel.org/r/20250822-loongson-pci1-v1-1-39aabbd11fbd@xxxxxxxxxxxxx
- Move from arch/loongarch/pci/pci.c to drivers/pci/controller/pci-loongson.c
- Fix falling through logic and add kernel log output by Xi Ruoyao
---
 drivers/pci/controller/pci-loongson.c | 39 +++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/drivers/pci/controller/pci-loongson.c b/drivers/pci/controller/pci-loongson.c
index bc630ab8a283..75a1b494b527 100644
--- a/drivers/pci/controller/pci-loongson.c
+++ b/drivers/pci/controller/pci-loongson.c
@@ -176,6 +176,45 @@ static void loongson_pci_msi_quirk(struct pci_dev *dev)
 }
 DECLARE_PCI_FIXUP_FINAL(PCI_VENDOR_ID_LOONGSON, DEV_LS7A_PCIE_PORT5, loongson_pci_msi_quirk);
 
+/*
+ * Older steppings of the Loongson 3C6000 series incorrectly report the
+ * supported link speeds on their PCIe bridges (device IDs 3c19, 3c29) as
+ * only 2.5 GT/s, despite the upstream bus supporting speeds from 2.5 GT/s
+ * up to 16 GT/s.
+ */
+static void quirk_loongson_pci_bridge_supported_speeds(struct pci_dev *pdev)
+{
+	u8 supported_speeds = pdev->supported_speeds;
+
+	switch (pdev->bus->max_bus_speed) {
+	case PCIE_SPEED_16_0GT:
+		supported_speeds |= PCI_EXP_LNKCAP2_SLS_16_0GB;
+		fallthrough;
+	case PCIE_SPEED_8_0GT:
+		supported_speeds |= PCI_EXP_LNKCAP2_SLS_8_0GB;
+		fallthrough;
+	case PCIE_SPEED_5_0GT:
+		supported_speeds |= PCI_EXP_LNKCAP2_SLS_5_0GB;
+		fallthrough;
+	case PCIE_SPEED_2_5GT:
+		supported_speeds |= PCI_EXP_LNKCAP2_SLS_2_5GB;
+		break;
+	default:
+		pci_warn(pdev, "unexpected max bus speed");
+		return;
+	}
+
+	if (supported_speeds != pdev->supported_speeds) {
+		pci_info(pdev, "fixing up supported link speeds: 0x%x => 0x%x",
+			 pdev->supported_speeds, supported_speeds);
+		pdev->supported_speeds = supported_speeds;
+	}
+}
+DECLARE_PCI_FIXUP_HEADER(PCI_VENDOR_ID_LOONGSON, 0x3c19,
+			 quirk_loongson_pci_bridge_supported_speeds);
+DECLARE_PCI_FIXUP_HEADER(PCI_VENDOR_ID_LOONGSON, 0x3c29,
+			 quirk_loongson_pci_bridge_supported_speeds);
+
 static struct loongson_pci *pci_bus_to_loongson_pci(struct pci_bus *bus)
 {
 	struct pci_config_window *cfg;

---
base-commit: ea1013c1539270e372fc99854bc6e4d94eaeff66
change-id: 20250822-loongson-pci1-4ded0d78f1bb

Best regards,
-- 
Ziyao Li <liziyao@xxxxxxxxxxxxx>