drm/nouveau: Defer probe if gmux is present but its driver isn't

gmux is a microcontroller built into dual GPU MacBook Pros.
On pre-retina MBPs, if we're the inactive GPU, we need apple-gmux
to temporarily switch DDC so that we can probe the panel's EDID.

The checks for CONFIG_VGA_ARB and CONFIG_VGA_SWITCHEROO are necessary
because if either of them is disabled but gmux is present, the driver
would never load, even if we're the active GPU. (vga_default_device()
would evaluate to NULL and vga_switcheroo_handler_flags() would
evaluate to 0.)

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=88861
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=61115
Tested-by: Lukas Wunner <lukas@wunner.de>
    [MBP  9,1 2012  intel IVB + nvidia GK107  pre-retina  15"]
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/d9542ca5041178165d3ff286c90cc99634f7d2ce.1452525860.git.lukas@wunner.de

1 files changed, 11 insertions, 0 deletions

diff --git a/drivers/gpu/drm/nouveau/nouveau_drm.c b/drivers/gpu/drm/nouveau/nouveau_drm.c
index 2f2f252e3fb6..bb8498c9b13e 100644
--- a/drivers/gpu/drm/nouveau/nouveau_drm.c
+++ b/drivers/gpu/drm/nouveau/nouveau_drm.c
@@ -22,11 +22,13 @@
  * Authors: Ben Skeggs
  */
 
+#include <linux/apple-gmux.h>
 #include <linux/console.h>
 #include <linux/delay.h>
 #include <linux/module.h>
 #include <linux/pci.h>
 #include <linux/pm_runtime.h>
+#include <linux/vgaarb.h>
 #include <linux/vga_switcheroo.h>
 
 #include "drmP.h"
@@ -312,6 +314,15 @@ static int nouveau_drm_probe(struct pci_dev *pdev,
         bool boot = false;
         int ret;
 
+        /*
+         * apple-gmux is needed on dual GPU MacBook Pro
+         * to probe the panel if we're the inactive GPU.
+         */
+        if (IS_ENABLED(CONFIG_VGA_ARB) && IS_ENABLED(CONFIG_VGA_SWITCHEROO) &&
+            apple_gmux_present() && pdev != vga_default_device() &&
+            !vga_switcheroo_handler_flags())
+                return -EPROBE_DEFER;
+
         /* remove conflicting drivers (vesafb, efifb etc) */
         aper = alloc_apertures(3);
         if (!aper)