[PATCH fixes v4] pci: loongson: Workaround MIPS firmware MRRS settings

[Jiaxun Yang]

This is a partial revert of commit 8b3517f88ff2 ("PCI:
loongson: Prevent LS7A MRRS increases") for MIPS based Loongson.

There are many MIPS based Loongson systems in wild that
shipped with firmware which does not set maximum MRRS properly.

Limiting MRRS to 256 for all as MIPS Loongson comes with higher
MRRS support is considered rare.

It must be done at device enablement stage because hardware will
reset MRRS to inavlid value if a device got disabled.

Cc: stable@xxxxxxxxxxxxxxx
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217680
Fixes: 8b3517f88ff2 ("PCI: loongson: Prevent LS7A MRRS increases")
Signed-off-by: Jiaxun Yang <jiaxun.yang@xxxxxxxxxxx>
---
v4: Improve commit message

This is a partial revert of the origin quirk so there shouldn't
be any drama.
---
 drivers/pci/controller/pci-loongson.c | 38 +++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/drivers/pci/controller/pci-loongson.c b/drivers/pci/controller/pci-loongson.c
index d45e7b8dc530..d184d7b97e54 100644
--- a/drivers/pci/controller/pci-loongson.c
+++ b/drivers/pci/controller/pci-loongson.c
@@ -108,6 +108,44 @@ DECLARE_PCI_FIXUP_EARLY(PCI_VENDOR_ID_LOONGSON,
 DECLARE_PCI_FIXUP_EARLY(PCI_VENDOR_ID_LOONGSON,
 			DEV_LS7A_PCIE_PORT6, loongson_mrrs_quirk);
 
+#ifdef CONFIG_MIPS
+static void loongson_old_mrrs_quirk(struct pci_dev *pdev)
+{
+	struct pci_bus *bus = pdev->bus;
+	struct pci_dev *bridge;
+	static const struct pci_device_id bridge_devids[] = {
+		{ PCI_VDEVICE(LOONGSON, DEV_LS2K_PCIE_PORT0) },
+		{ PCI_VDEVICE(LOONGSON, DEV_LS7A_PCIE_PORT0) },
+		{ PCI_VDEVICE(LOONGSON, DEV_LS7A_PCIE_PORT1) },
+		{ PCI_VDEVICE(LOONGSON, DEV_LS7A_PCIE_PORT2) },
+		{ PCI_VDEVICE(LOONGSON, DEV_LS7A_PCIE_PORT3) },
+		{ PCI_VDEVICE(LOONGSON, DEV_LS7A_PCIE_PORT4) },
+		{ PCI_VDEVICE(LOONGSON, DEV_LS7A_PCIE_PORT5) },
+		{ PCI_VDEVICE(LOONGSON, DEV_LS7A_PCIE_PORT6) },
+		{ 0, },
+	};
+
+	/* look for the matching bridge */
+	while (!pci_is_root_bus(bus)) {
+		bridge = bus->self;
+		bus = bus->parent;
+		/*
+		 * There are still some wild MIPS Loongson firmware won't
+		 * set MRRS properly. Limiting MRRS to 256 as MIPS Loongson
+		 * comes with higher MRRS support is considered rare.
+		 */
+		if (pci_match_id(bridge_devids, bridge)) {
+			if (pcie_get_readrq(pdev) > 256) {
+				pci_info(pdev, "limiting MRRS to 256\n");
+				pcie_set_readrq(pdev, 256);
+			}
+			break;
+		}
+	}
+}
+DECLARE_PCI_FIXUP_ENABLE(PCI_ANY_ID, PCI_ANY_ID, loongson_old_mrrs_quirk);
+#endif
+
 static void loongson_pci_pin_quirk(struct pci_dev *pdev)
 {
 	pdev->pin = 1 + (PCI_FUNC(pdev->devfn) & 3);
-- 
2.34.1