PCI/VGA: Factor out default VGA device selection

Default VGA device selection fails when PCI devices are enumerated after
the vga_arb_device_init() subsys_initcall.

vga_arbiter_add_pci_device() selects the first fully enabled device to
which legacy VGA resources are routed as the default VGA device.  This is
an ADD_DEVICE notifier, so it runs after every PCI device is enumerated.

vga_arb_select_default_device() may select framebuffer devices, partially
enabled GPUs, or non-legacy devices that don't have legacy VGA resources
routed to them as the default VGA device.  But this only happens once, from
the vga_arb_device_init() subsys_initcall, so it doesn't consider devices
enumerated after that:

  acpi_init
    acpi_scan_init
      acpi_pci_root_init         # PCI device enumeration (ACPI systems)

  vga_arb_device_init
    for_each_pci_device
      vga_arbiter_add_pci_device      # ADD_DEVICE notifier
        if (VGA-owner)
          vga_set_default_device      <-- set default VGA
    vga_arb_select_default_device     # only called ONCE
      for_each_vga_device
        if (framebuffer)
          vga_set_default_device      <-- set default VGA to framebuffer
      if (!vga_default_device())
        if (non-legacy, integrated GPU, etc)
          vga_set_default_device      <-- set default VGA
      if (!vga_default_device())
        vga_set_default_device        <-- set default VGA

  pcibios_init
    pcibios_scanbus              # PCI device enumeration (non-ACPI systems)
      ...
        vga_arbiter_add_pci_device    # ADD_DEVICE notification
          if (VGA-owner)
            vga_set_default_device    <-- set default VGA

Note that on non-ACPI systems, vga_arb_select_default_device() runs before
pcibios_init(), so it sees no VGA devices and can never set a framebuffer
device, a non-legacy integrated GPU, etc., as the default device.

Factor out the default VGA device selection to vga_is_boot_device(), called
from vga_arbiter_add_pci_device().

Then we can migrate the default device selection from
vga_arb_select_default_device() to the vga_arbiter_add_pci_device() path.

[bhelgaas: commit log, split to separate patch]
Link: https://lore.kernel.org/r/20211015061512.2941859-4-chenhuacai@loongson.cn
Link: https://lore.kernel.org/r/20220224224753.297579-5-helgaas@kernel.org
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

Author: chenhuacai

drivers/pci/vgaarb.c

@@ -628,6 +628,41 @@ static bool vga_arb_integrated_gpu(struct device *dev)
 #endif
 }
 
+/*
+ * Return true if vgadev is a better default VGA device than the best one
+ * we've seen so far.
+ */
+static bool vga_is_boot_device(struct vga_device *vgadev)
+{
+	struct vga_device *boot_vga = vgadev_find(vga_default_device());
+
+	/*
+	 * We select the default VGA device in this order:
+	 *   Firmware framebuffer (see vga_arb_select_default_device())
+	 *   Legacy VGA device (owns VGA_RSRC_LEGACY_MASK)
+	 *   Non-legacy integrated device (see vga_arb_select_default_device())
+	 *   Non-legacy discrete device (see vga_arb_select_default_device())
+	 *   Other device (see vga_arb_select_default_device())
+	 */
+
+	/*
+	 * A legacy VGA device has MEM and IO enabled and any bridges
+	 * leading to it have PCI_BRIDGE_CTL_VGA enabled so the legacy
+	 * resources ([mem 0xa0000-0xbffff], [io 0x3b0-0x3bb], etc) are
+	 * routed to it.
+	 *
+	 * We use the first one we find, so if we've already found one,
+	 * vgadev is no better.
+	 */
+	if (boot_vga)
+		return false;
+
+	if ((vgadev->owns & VGA_RSRC_LEGACY_MASK) == VGA_RSRC_LEGACY_MASK)
+		return true;
+
+	return false;
+}
+
 /*
  * Rules for using a bridge to control a VGA descendant decoding: if a bridge
  * has only one VGA descendant then it can be used to control the VGA routing
@@ -755,12 +790,10 @@ static bool vga_arbiter_add_pci_device(struct pci_dev *pdev)
 		bus = bus->parent;
 	}
 
-	/* Deal with VGA default device. Use first enabled one
-	 * by default if arch doesn't have it's own hook
-	 */
-	if (vga_default == NULL &&
-	    ((vgadev->owns & VGA_RSRC_LEGACY_MASK) == VGA_RSRC_LEGACY_MASK)) {
-		vgaarb_info(&pdev->dev, "setting as boot VGA device\n");
+	if (vga_is_boot_device(vgadev)) {
+		vgaarb_info(&pdev->dev, "setting as boot VGA device%s\n",
+			    vga_default_device() ?
+			    " (overriding previous)" : "");
 		vga_set_default_device(pdev);
 	}