PCI/VGA: Move non-legacy VGA detection to ADD_DEVICE path

a37c0f4 ("vgaarb: Select a default VGA device even if there's no
legacy VGA") extended the vga_arb_device_init() subsys_initcall so it could
select a non-legacy VGA device as the default.

That failed to consider that PCI devices may be enumerated after
vga_arb_device_init(), e.g., hot-added devices or non-ACPI systems that do
PCI enumeration in pcibios_init().  Devices found then could never be
selected as the default.

One system where this is a problem is the MIPS-based Loongson where an
ASpeed AST2500 VGA device is behind a bridge that doesn't implement the VGA
Enable bit, so legacy resources are not routed to the VGA device. [1]

Fix this by moving the non-legacy VGA device selection from
vga_arb_select_default_device() to vga_arbiter_add_pci_device(), which is
called after every PCI device is enumerated, either by the
vga_arb_device_init() subsys_initcall or as an ADD_DEVICE notifier.

[1] https://lore.kernel.org/r/20210514080025.1828197-6-chenhuacai@loongson.cn

[bhelgaas: commit log, restructure]
Link: https://lore.kernel.org/r/20211015061512.2941859-5-chenhuacai@loongson.cn
Link: https://lore.kernel.org/r/20211015061512.2941859-7-chenhuacai@loongson.cn
Link: https://lore.kernel.org/r/20220224224753.297579-7-helgaas@kernel.org
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Daniel Axtens <dja@axtens.net>
Cc: Zhou Wang <wangzhou1@hisilicon.com>

Author: chenhuacai

drivers/pci/vgaarb.c

@@ -624,6 +624,7 @@ static bool vga_is_boot_device(struct vga_device *vgadev)
 {
 	struct vga_device *boot_vga = vgadev_find(vga_default_device());
 	struct pci_dev *pdev = vgadev->pdev;
+	u16 cmd, boot_cmd;
 
 	/*
 	 * We select the default VGA device in this order:
@@ -661,6 +662,37 @@ static bool vga_is_boot_device(struct vga_device *vgadev)
 	if ((vgadev->owns & VGA_RSRC_LEGACY_MASK) == VGA_RSRC_LEGACY_MASK)
 		return true;
 
+	/*
+	 * If we haven't found a legacy VGA device, accept a non-legacy
+	 * device.  It may have either IO or MEM enabled, and bridges may
+	 * not have PCI_BRIDGE_CTL_VGA enabled, so it may not be able to
+	 * use legacy VGA resources.  Prefer an integrated GPU over others.
+	 */
+	pci_read_config_word(pdev, PCI_COMMAND, &cmd);
+	if (cmd & (PCI_COMMAND_IO | PCI_COMMAND_MEMORY)) {
+
+		/*
+		 * An integrated GPU overrides a previous non-legacy
+		 * device.  We expect only a single integrated GPU, but if
+		 * there are more, we use the *last* because that was the
+		 * previous behavior.
+		 */
+		if (vga_arb_integrated_gpu(&pdev->dev))
+			return true;
+
+		/*
+		 * We prefer the first non-legacy discrete device we find.
+		 * If we already found one, vgadev is no better.
+		 */
+		if (boot_vga) {
+			pci_read_config_word(boot_vga->pdev, PCI_COMMAND,
+					     &boot_cmd);
+			if (boot_cmd & (PCI_COMMAND_IO | PCI_COMMAND_MEMORY))
+				return false;
+		}
+		return true;
+	}
+
 	return false;
 }
 
@@ -1529,30 +1561,8 @@ static struct miscdevice vga_arb_device = {
 
 static void __init vga_arb_select_default_device(void)
 {
-	struct pci_dev *pdev, *found = NULL;
 	struct vga_device *vgadev;
 
-	if (!vga_default_device()) {
-		list_for_each_entry_reverse(vgadev, &vga_list, list) {
-			struct device *dev = &vgadev->pdev->dev;
-			u16 cmd;
-
-			pdev = vgadev->pdev;
-			pci_read_config_word(pdev, PCI_COMMAND, &cmd);
-			if (cmd & (PCI_COMMAND_IO | PCI_COMMAND_MEMORY)) {
-				found = pdev;
-				if (vga_arb_integrated_gpu(dev))
-					break;
-			}
-		}
-	}
-
-	if (found) {
-		vgaarb_info(&found->dev, "setting as boot device (VGA legacy resources not available)\n");
-		vga_set_default_device(found);
-		return;
-	}
-
 	if (!vga_default_device()) {
 		vgadev = list_first_entry_or_null(&vga_list,
 						  struct vga_device, list);