PCI: loongson: Fix supported link speeds for Loongson 3c19/3c29 PCIe

devices proper dynamic speed negotiation.

The "Supported Link Speeds" field reported by Loongson PCIe devices
3c19 and 3c29 is incorrect, which prevents the AMDGPU driver (and
potentially others) from correctly detecting the dynamic link speed
capabilities of the device. As a result, the PCIe card is incorrectly
identified as a GEN1-only device, even when higher speeds are supported.

This patch manually overrides the `supported_speeds` field for these
devices to reflect the actual supported link speeds.

Tested-by: Lain "Fearyncess" Yang <[email protected]>
Signed-off-by: Ayden Meng <[email protected]>

Author: AydenMeng

drivers/pci/quirks.c

@@ -1988,6 +1988,30 @@ DECLARE_PCI_FIXUP_FINAL(PCI_VENDOR_ID_INTEL,	PCI_DEVICE_ID_INTEL_E7525_MCH,	quir
 
 DECLARE_PCI_FIXUP_CLASS_FINAL(PCI_VENDOR_ID_HUAWEI, 0x1610, PCI_CLASS_BRIDGE_PCI, 8, quirk_pcie_mch);
 
+/*
+ * The PCIe devices 3c19/3c29 on Loongson platforms report an incorrect
+ * "Supported Link Speeds" value. By explicitly overriding the
+ * supported_speeds field, the dynamic speed switching functionality
+ * can be enabled.
+ */
+static void loongson_fixup_support_speeds(struct pci_dev *pdev)
+{
+	switch (pdev->bus->max_bus_speed) {
+	case PCIE_SPEED_16_0GT:
+		pdev->supported_speeds |= PCI_EXP_LNKCAP2_SLS_16_0GB;
+	case PCIE_SPEED_8_0GT:
+		pdev->supported_speeds |= PCI_EXP_LNKCAP2_SLS_8_0GB;
+	case PCIE_SPEED_5_0GT:
+		pdev->supported_speeds |= PCI_EXP_LNKCAP2_SLS_5_0GB;
+	case PCIE_SPEED_2_5GT:
+		pdev->supported_speeds |= PCI_EXP_LNKCAP2_SLS_2_5GB;
+	default:
+		break;
+	}
+}
+DECLARE_PCI_FIXUP_FINAL(PCI_VENDOR_ID_LOONGSON, 0x3c19, loongson_fixup_support_speeds);
+DECLARE_PCI_FIXUP_FINAL(PCI_VENDOR_ID_LOONGSON, 0x3c29, loongson_fixup_support_speeds);
+
 /*
  * HiSilicon KunPeng920 and KunPeng930 have devices appear as PCI but are
  * actually on the AMBA bus. These fake PCI devices can support SVA via