ACPI / init: Make it possible to override _REV

The platform firmware on some systems expects Linux to return "5" as
the supported ACPI revision which makes it expose system configuration
information in a special way.

For example, based on what ACPI exports as the supported revision,
Dell XPS 13 (2015) configures its audio device to either work in HDA
mode or in I2S mode, where the former is supposed to be used on Linux
until the latter is fully supported (in the kernel as well as in user
space).

Since ACPI 6 mandates that _REV should return "2" if ACPI 2 or later
is supported by the OS, a subsequent change will make that happen, so
make it possible to override that on systems where "5" is expected to
be returned for Linux to work correctly one them (such as the Dell
machine mentioned above).

Original-by: Dominik Brodowski <[e-mail]>
Signed-off-by: Rafael J. Wysocki <[e-mail]>

Documentation/kernel-parameters.txt

···
 	acpi_os_name=	[HW,ACPI] Tell ACPI BIOS the name of the OS
 			Format: To spoof as Windows 98: ="Microsoft Windows"
 
+	acpi_rev_override [ACPI] Override the _REV object to return 5 (instead
+			of 2 which is mandated by ACPI 6) as the supported ACPI
+			specification revision (when using this switch, it may
+			be necessary to carry out a cold reboot _twice_ in a
+			row to make it take effect on the platform firmware).
+
 	acpi_osi=	[HW,ACPI] Modify list of supported OS interface strings
 			acpi_osi="string1"	# add string1
 			acpi_osi="!string2"	# remove string2

drivers/acpi/Kconfig

···
 
 	  Say N to delete power /proc/acpi/ directories that have moved to /sys/
 
+config ACPI_REV_OVERRIDE_POSSIBLE
+	bool "Allow supported ACPI revision to be overriden"
+	depends on X86
+	default y
+	help
+	  The platform firmware on some systems expects Linux to return "5" as
+	  the supported ACPI revision which makes it expose system configuration
+	  information in a special way.
+
+	  For example, based on what ACPI exports as the supported revision,
+	  Dell XPS 13 (2015) configures its audio device to either work in HDA
+	  mode or in I2S mode, where the former is supposed to be used on Linux
+	  until the latter is fully supported (in the kernel as well as in user
+	  space).
+
+	  This option enables a DMI-based quirk for the above Dell machine (so
+	  that HDA audio is exposed by the platform firmware to the kernel) and
+	  makes it possible to force the kernel to return "5" as the supported
+	  ACPI revision via the "acpi_rev_override" command line switch.
+
 config ACPI_EC_DEBUGFS
 	tristate "EC read/write access through /sys/kernel/debug/ec"
 	default n

drivers/acpi/blacklist.c

···
 	acpi_osi_setup("!Windows 2012");
 	return 0;
 }
+#ifdef CONFIG_ACPI_REV_OVERRIDE_POSSIBLE
+static int __init dmi_enable_rev_override(const struct dmi_system_id *d)
+{
+	printk(KERN_NOTICE PREFIX "DMI detected: %s (force ACPI _REV to 5)\n",
+	       d->ident);
+	acpi_rev_override_setup(NULL);
+	return 0;
+}
+#endif
 
 static struct dmi_system_id acpi_osi_dmi_table[] __initdata = {
 	{
···
 		     DMI_MATCH(DMI_PRODUCT_NAME, "1015PX"),
 		},
 	},
+
+#ifdef CONFIG_ACPI_REV_OVERRIDE_POSSIBLE
+	/*
+	 * DELL XPS 13 (2015) switches sound between HDA and I2S
+	 * depending on the ACPI _REV callback. If userspace supports
+	 * I2S sufficiently (or if you do not care about sound), you
+	 * can safely disable this quirk.
+	 */
+	{
+	 .callback = dmi_enable_rev_override,
+	 .ident = "DELL XPS 13 (2015)",
+	 .matches = {
+		      DMI_MATCH(DMI_SYS_VENDOR, "Dell Inc."),
+		      DMI_MATCH(DMI_PRODUCT_NAME, "XPS 13 9343"),
+		},
+	},
+#endif
 	{}
 };
 

drivers/acpi/internal.h

···
 #else
 static inline void acpi_cmos_rtc_init(void) {}
 #endif
+int acpi_rev_override_setup(char *str);
 
 extern bool acpi_force_hot_remove;
 

drivers/acpi/osl.c

···
 }
 #endif
 
+#ifdef CONFIG_ACPI_REV_OVERRIDE_POSSIBLE
+static bool acpi_rev_override;
+
+int __init acpi_rev_override_setup(char *str)
+{
+	acpi_rev_override = true;
+	return 1;
+}
+__setup("acpi_rev_override", acpi_rev_override_setup);
+#else
+#define acpi_rev_override	false
+#endif
+
 #define ACPI_MAX_OVERRIDE_LEN 100
 
 static char acpi_os_name[ACPI_MAX_OVERRIDE_LEN];
···
 		printk(KERN_INFO PREFIX "Overriding _OS definition to '%s'\n",
 		       acpi_os_name);
 		*new_val = acpi_os_name;
+	}
+
+	if (!memcmp(init_val->name, "_REV", 4) && acpi_rev_override) {
+		printk(KERN_INFO PREFIX "Overriding _REV return value to 5\n");
+		*new_val = (char *)5;
 	}
 
 	return AE_OK;