apple-gmux: Add helper for presence detect

Centralize gmux' ACPI HID in a header file and add apple_gmux_present().
This can be used by other drivers to activate quirks specific to dual
GPU MacBook Pros & Mac Pros. The alternative would be to hardcode DMI
or PCI IDs and amend them whenever Apple introduces a new machine.
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>
Reviewed-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/89c23769058a340e5e11d4a7102f3793d3b0c94c.1452525860.git.lukas@wunner.de

Documentation/DocBook/gpu.tmpl

@@ -3451,6 +3451,10 @@ int num_ioctls;</synopsis>
         <title>Backlight control</title>
 !Pdrivers/platform/x86/apple-gmux.c Backlight control
       </sect2>
+      <sect2>
+        <title>Public functions</title>
+!Iinclude/linux/apple-gmux.h
+      </sect2>
     </sect1>
   </chapter>
 

drivers/platform/x86/apple-gmux.c

@@ -19,6 +19,7 @@
 #include <linux/acpi.h>
 #include <linux/pnp.h>
 #include <linux/apple_bl.h>
+#include <linux/apple-gmux.h>
 #include <linux/slab.h>
 #include <linux/delay.h>
 #include <linux/pci.h>
@@ -828,7 +829,7 @@ static void gmux_remove(struct pnp_dev *pnp)
 }
 
 static const struct pnp_device_id gmux_device_ids[] = {
-	{"APP000B", 0},
+	{GMUX_ACPI_HID, 0},
 	{"", 0}
 };
 

include/linux/apple-gmux.h

+/*
+ * apple-gmux.h - microcontroller built into dual GPU MacBook Pro & Mac Pro
+ * Copyright (C) 2015 Lukas Wunner <lukas@wunner.de>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License (version 2) as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef LINUX_APPLE_GMUX_H
+#define LINUX_APPLE_GMUX_H
+
+#include <linux/acpi.h>
+
+#define GMUX_ACPI_HID "APP000B"
+
+/**
+ * apple_gmux_present() - detect if gmux is built into the machine
+ *
+ * Drivers may use this to activate quirks specific to dual GPU MacBook Pros
+ * and Mac Pros, e.g. for deferred probing, runtime pm and backlight.
+ *
+ * Return: %true if gmux is present and the kernel was configured
+ * with CONFIG_APPLE_GMUX, %false otherwise.
+ */
+static inline bool apple_gmux_present(void)
+{
+	return IS_ENABLED(CONFIG_APPLE_GMUX) && acpi_dev_present(GMUX_ACPI_HID);
+}
+
+#endif /* LINUX_APPLE_GMUX_H */