[PATCH v5] 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 MRRS setting
may get lost if the parent bridge lost PCI_COMMAND_MASTER, and
we are only sure parent bridge is enabled at this point.

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
v5:
	- Improve commit message and comments.
	- Style fix from Huacai's off-list input.
---
 drivers/pci/controller/pci-loongson.c | 47 ++++++++++++++++++++++++---
 1 file changed, 42 insertions(+), 5 deletions(-)

diff --git a/drivers/pci/controller/pci-loongson.c b/drivers/pci/controller/pci-loongson.c
index d45e7b8dc530..128cc95b236f 100644
--- a/drivers/pci/controller/pci-loongson.c
+++ b/drivers/pci/controller/pci-loongson.c
@@ -80,13 +80,50 @@ DECLARE_PCI_FIXUP_EARLY(PCI_VENDOR_ID_LOONGSON,
 DECLARE_PCI_FIXUP_EARLY(PCI_VENDOR_ID_LOONGSON,
 			DEV_LS7A_LPC, system_bus_quirk);
 
+/*
+ * Some Loongson PCIe ports have h/w limitations of maximum read
+ * request size. They can't handle anything larger than this.
+ * Sane firmware will set proper MRRS at boot, so we only need
+ * no_inc_mrrs for bridges. However, some MIPS Loongson firmware
+ * won't set MRRS properly, and we have to enforce maximum safe
+ * MRRS, which is 256 bytes.
+ */
+#ifdef CONFIG_MIPS
+static void loongson_set_min_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;
+
+		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_mrrs_quirk_old);
+#endif
+
 static void loongson_mrrs_quirk(struct pci_dev *pdev)
 {
-	/*
-	 * Some Loongson PCIe ports have h/w limitations of maximum read
-	 * request size. They can't handle anything larger than this. So
-	 * force this limit on any devices attached under these ports.
-	 */
 	struct pci_host_bridge *bridge = pci_find_host_bridge(pdev->bus);
 
 	bridge->no_inc_mrrs = 1;
-- 
2.34.1