x86/amd_nb: Simplify root device search

The "root" device search was introduced to support SMN access for Zen
systems. This device represents a PCIe root complex. It is not the
same as the "CPU/node" devices found at slots 0x18-0x1F.

There may be multiple PCIe root complexes within an AMD node. Such is
the case with server or High-end Desktop (HEDT) systems, etc. Therefore
it is not enough to assume "root <-> AMD node" is a 1-to-1 association.

Currently, this is handled by skipping "extra" root complexes during the
search. However, the hardware provides the PCI bus number of an AMD
node's root device.

Use the hardware info to get the root device's bus and drop the extra
search code and PCI IDs.

Signed-off-by: Yazen Ghannam <yazen.ghannam@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20241206161210.163701-7-yazen.ghannam@amd.com

1 files changed, 56 insertions, 0 deletions

diff --git a/arch/x86/kernel/amd_node.c b/arch/x86/kernel/amd_node.c
index e825cd4426b9..4eea8c7d8090 100644
--- a/arch/x86/kernel/amd_node.c
+++ b/arch/x86/kernel/amd_node.c
@@ -32,3 +32,59 @@ struct pci_dev *amd_node_get_func(u16 node, u8 func)
 
 	return pci_get_domain_bus_and_slot(0, 0, PCI_DEVFN(AMD_NODE0_PCI_SLOT + node, func));
 }
+
+#define DF_BLK_INST_CNT		0x040
+#define	DF_CFG_ADDR_CNTL_LEGACY	0x084
+#define	DF_CFG_ADDR_CNTL_DF4	0xC04
+
+#define DF_MAJOR_REVISION	GENMASK(27, 24)
+
+static u16 get_cfg_addr_cntl_offset(struct pci_dev *df_f0)
+{
+	u32 reg;
+
+	/*
+	 * Revision fields added for DF4 and later.
+	 *
+	 * Major revision of '0' is found pre-DF4. Field is Read-as-Zero.
+	 */
+	if (pci_read_config_dword(df_f0, DF_BLK_INST_CNT, &reg))
+		return 0;
+
+	if (reg & DF_MAJOR_REVISION)
+		return DF_CFG_ADDR_CNTL_DF4;
+
+	return DF_CFG_ADDR_CNTL_LEGACY;
+}
+
+struct pci_dev *amd_node_get_root(u16 node)
+{
+	struct pci_dev *root;
+	u16 cntl_off;
+	u8 bus;
+
+	if (!cpu_feature_enabled(X86_FEATURE_ZEN))
+		return NULL;
+
+	/*
+	 * D18F0xXXX [Config Address Control] (DF::CfgAddressCntl)
+	 * Bits [7:0] (SecBusNum) holds the bus number of the root device for
+	 * this Data Fabric instance. The segment, device, and function will be 0.
+	 */
+	struct pci_dev *df_f0 __free(pci_dev_put) = amd_node_get_func(node, 0);
+	if (!df_f0)
+		return NULL;
+
+	cntl_off = get_cfg_addr_cntl_offset(df_f0);
+	if (!cntl_off)
+		return NULL;
+
+	if (pci_read_config_byte(df_f0, cntl_off, &bus))
+		return NULL;
+
+	/* Grab the pointer for the actual root device instance. */
+	root = pci_get_domain_bus_and_slot(0, bus, 0);
+
+	pci_dbg(root, "is root for AMD node %u\n", node);
+	return root;
+}