PCI: keystone: Limit MRSS for all downstream devices

Keystone PCIe controller has a limitation that memory read request size
must not exceed 256 bytes.  This is a hardware limitation.  Add a quirk to
force this limit on all downstream devices by updating MRRS.
Signed-off-by: NMurali Karicheri <m-karicheri2@ti.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>

Documentation/devicetree/bindings/pci/pci-keystone.txt

@@ -63,6 +63,3 @@ Designware DT Properties not applicable for Keystone PCI
 
 1. pcie_bus clock-names not used.  Instead, a phandle to phys is used.
 
-Note for PCI driver usage
-=========================
-Driver requires pci=pcie_bus_perf in the bootargs for proper functioning.

drivers/pci/host/pci-keystone.c

@@ -42,8 +42,53 @@
 /* DEV_STAT_CTRL */
 #define PCIE_CAP_BASE		0x70
 
+/* PCIE controller device IDs */
+#define PCIE_RC_K2HK		0xb008
+#define PCIE_RC_K2E		0xb009
+#define PCIE_RC_K2L		0xb00a
+
 #define to_keystone_pcie(x)	container_of(x, struct keystone_pcie, pp)
 
+static void quirk_limit_mrrs(struct pci_dev *dev)
+{
+	struct pci_bus *bus = dev->bus;
+	struct pci_dev *bridge = bus->self;
+	static const struct pci_device_id rc_pci_devids[] = {
+		{ PCI_DEVICE(PCI_VENDOR_ID_TI, PCIE_RC_K2HK),
+		 .class = PCI_CLASS_BRIDGE_PCI << 8, .class_mask = ~0, },
+		{ PCI_DEVICE(PCI_VENDOR_ID_TI, PCIE_RC_K2E),
+		 .class = PCI_CLASS_BRIDGE_PCI << 8, .class_mask = ~0, },
+		{ PCI_DEVICE(PCI_VENDOR_ID_TI, PCIE_RC_K2L),
+		 .class = PCI_CLASS_BRIDGE_PCI << 8, .class_mask = ~0, },
+		{ 0, },
+	};
+
+	if (pci_is_root_bus(bus))
+		return;
+
+	/* look for the host bridge */
+	while (!pci_is_root_bus(bus)) {
+		bridge = bus->self;
+		bus = bus->parent;
+	}
+
+	if (bridge) {
+		/*
+		 * Keystone PCI controller has a h/w limitation of
+		 * 256 bytes maximum read request size.  It can't handle
+		 * anything higher than this.  So force this limit on
+		 * all downstream devices.
+		 */
+		if (pci_match_id(rc_pci_devids, bridge)) {
+			if (pcie_get_readrq(dev) > 256) {
+				dev_info(&dev->dev, "limiting MRRS to 256\n");
+				pcie_set_readrq(dev, 256);
+			}
+		}
+	}
+}
+DECLARE_PCI_FIXUP_ENABLE(PCI_ANY_ID, PCI_ANY_ID, quirk_limit_mrrs);
+
 static int ks_pcie_establish_link(struct keystone_pcie *ks_pcie)
 {
 	struct pcie_port *pp = &ks_pcie->pp;