vga_switcheroo: Support deferred probing of audio clients

Daniel Vetter pointed out that vga_switcheroo_client_probe_defer() could
be needed by audio clients as well. To avoid mistakes when someone adds
conditions for these in the future, constrain the single existing
condition to VGA clients by checking for PCI_BASE_CLASS_DISPLAY. This
encompasses both PCI_CLASS_DISPLAY_VGA as well as PCI_CLASS_DISPLAY_3D,
which is used by some Nvidia Optimus GPUs.

Any future checks for audio clients should then be constrained to
PCI_BASE_CLASS_MULTIMEDIA.

v6: Spun out from commit introducing vga_switcheroo_client_probe_defer()
    to keep it a pure refactoring change. (Emil Velikov, Jani Nikula)

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Emil Velikov <emil.l.velikov@gmail.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/358d58490eb9dda5f270d844b0dce511a2a20828.1464685538.git.lukas@wunner.de

drivers/gpu/vga/vga_switcheroo.c

@@ -331,7 +331,8 @@ EXPORT_SYMBOL(vga_switcheroo_register_client);
  * @id: client identifier
  *
  * Register audio client (audio device on a GPU). The power state of the
- * client is assumed to be ON.
+ * client is assumed to be ON. Beforehand, vga_switcheroo_client_probe_defer()
+ * shall be called to ensure that all prerequisites are met.
  *
  * Return: 0 on success, -ENOMEM on memory allocation error.
  */
@@ -390,13 +391,15 @@ find_active_client(struct list_head *head)
  */
 bool vga_switcheroo_client_probe_defer(struct pci_dev *pdev)
 {
-	/*
-	 * apple-gmux is needed on pre-retina MacBook Pro
-	 * to probe the panel if pdev is the inactive GPU.
-	 */
-	if (apple_gmux_present() && pdev != vga_default_device() &&
-	    !vgasr_priv.handler_flags)
-		return true;
+	if ((pdev->class >> 16) == PCI_BASE_CLASS_DISPLAY) {
+		/*
+		 * apple-gmux is needed on pre-retina MacBook Pro
+		 * to probe the panel if pdev is the inactive GPU.
+		 */
+		if (apple_gmux_present() && pdev != vga_default_device() &&
+		    !vgasr_priv.handler_flags)
+			return true;
+	}
 
 	return false;
 }