[PATCH xserver 4/4] xfree86: promote one GPU screen if (NumScreens == 0 && NumGPUScreens > 0)

[Laszlo Ersek]

Aarch64/KVM virtual machines cannot use emulated graphics cards with
linear framebuffers, due to architectural cache coherency issues. (For
this reason, "qemu-system-aarch64" doesn't even include the "virtio-vga"
device model.)

Such guests can use the "virtio-gpu-pci" device well, through the
"modesetting" driver. However, given that "virtio-gpu-pci" is never
recognized as a primary graphics card (which is otherwise correct,
generally speaking), in these guests the X server currently exits with "No
devices detected", unless the user provides a config file with a Device
section that identifies the card by BusID.

Although this behavior matches the documentation precisely, it is
inconvenient for virtual machines, where the X server is expected to start
regardless of the (unpredictable) PCI B/D/F of the "virtio-gpu-pci"
device.

Modify xf86BusConfig() so that if no screen has been found, but there is
at least one GPU screen, one of the GPU screens be promoted to the single
normal screen. (This change should be either unnoticeable on physical
platforms, or beneficial even.) For this, call the
xf86PromoteLastGPUScreenToLastScreen() helper function added in the last
patch.

Update the manual page accordingly. The documentation remains
intentionally vague on what GPU screen gets promoted under the above
circumstances. The statements about multi-head setups, and about
single-head setups using the primary graphics card, are preserved intact.

Cc: Adam Jackson <ajax at redhat.com>
Cc: Dave Airlie <airlied at redhat.com>
Cc: Keith Packard <keithp at keithp.com>
Signed-off-by: Laszlo Ersek <lersek at redhat.com>
---
 hw/xfree86/common/xf86Bus.c  | 19 ++++++++++++++++---
 hw/xfree86/man/xorg.conf.man | 21 ++++++++++++++++-----
 2 files changed, 32 insertions(+), 8 deletions(-)

diff --git a/hw/xfree86/common/xf86Bus.c b/hw/xfree86/common/xf86Bus.c
index dc83e2dde099..572d91fd223b 100644
--- a/hw/xfree86/common/xf86Bus.c
+++ b/hw/xfree86/common/xf86Bus.c
@@ -127,10 +127,23 @@ xf86BusConfig(void)
         xf86CallDriverProbe(xf86DriverList[i], FALSE);
     }
 
-    /* If nothing was detected, return now */
     if (xf86NumScreens == 0) {
-        xf86Msg(X_ERROR, "No devices detected.\n");
-        return FALSE;
+        if (xf86NumGPUScreens == 0) {
+            /* If nothing was detected, return now */
+            xf86Msg(X_ERROR, "No devices detected.\n");
+            return FALSE;
+        }
+
+        /*
+         * If no screen was detected, but there is at least one GPU screen,
+         * promote one of the latter to be the single screen. Note that a
+         * graphics card matched by a Device.BusID (i.e., from the config file)
+         * is added as a screen, not as a GPU screen; so if that happens, we
+         * don't reach this code.
+         */
+        xf86PromoteLastGPUScreenToLastScreen();
+        xf86Msg(X_WARNING, "Use Device.BusID to select a specific secondary "
+                "graphics card.\n");
     }
 
     xf86VGAarbiterInit();
diff --git a/hw/xfree86/man/xorg.conf.man b/hw/xfree86/man/xorg.conf.man
index 94b199e6bac4..36f04febf2f4 100644
--- a/hw/xfree86/man/xorg.conf.man
+++ b/hw/xfree86/man/xorg.conf.man
@@ -1353,11 +1353,22 @@ the
 string has the form
 .BI PCI: bus : device : function
 (e.g., \(lqPCI:1:0:0\(rq might be appropriate for an AGP card).
-This field is usually optional in single-head configurations when using
-the primary graphics card.
-In multi-head configurations, or when using a secondary graphics card in a
-single-head configuration, this entry is mandatory.
-Its main purpose is to make an unambiguous connection between the device
+In multi-head configurations, this entry is mandatory.
+It is usually optional in single-head configurations when using the primary
+graphics card.
+When using a secondary graphics card in a single-head configuration,
+.B BusID
+is recommended; if no graphics card can be identified as primary, nor is any
+card selected by at least one
+.B Device
+section through the
+.B BusID
+field, then an unspecified secondary card (if any) is promoted to primary
+status.
+The main purpose of
+.B BusID
+is to make an unambiguous connection between the
+.B Device
 section and the hardware it is representing.
 This information can usually be found by running the pciaccess tool
 scanpci.
-- 
2.9.2