dell-wmi: Check if Dell WMI descriptor structure is valid

After examining existing DSDT ACPI tables of more laptops and looking
into Dell WMI document mentioned in ML dicussion archived at
http://www.spinics.net/lists/platform-driver-x86/msg07220.html we will
parse and check WMI descriptor if contains expected data. It is because
WMI descriptor contains interface version number and it is needed to
know in next commit.

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Tested-by: Gabriele Mazzotta <gabriele.mzt@gmail.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>

drivers/platform/x86/dell-wmi.c

@@ -2,6 +2,7 @@
  * Dell WMI hotkeys
  *
  * Copyright (C) 2008 Red Hat <mjg@redhat.com>
+ * Copyright (C) 2014-2015 Pali Rohár <pali.rohar@gmail.com>
  *
  * Portions based on wistron_btns.c:
  * Copyright (C) 2005 Miloslav Trmac <mitr@volny.cz>
@@ -38,14 +39,18 @@
 #include <acpi/video.h>
 
 MODULE_AUTHOR("Matthew Garrett <mjg@redhat.com>");
+MODULE_AUTHOR("Pali Rohár <pali.rohar@gmail.com>");
 MODULE_DESCRIPTION("Dell laptop WMI hotkeys driver");
 MODULE_LICENSE("GPL");
 
 #define DELL_EVENT_GUID "9DBB5994-A997-11DA-B012-B622A1EF5492"
+#define DELL_DESCRIPTOR_GUID "8D9DDCBC-A997-11DA-B012-B622A1EF5492"
 
 static int acpi_video;
+static u32 dell_wmi_interface_version;
 
 MODULE_ALIAS("wmi:"DELL_EVENT_GUID);
+MODULE_ALIAS("wmi:"DELL_DESCRIPTOR_GUID);
 
 /*
  * Certain keys are flagged as KE_IGNORE. All of these are either
@@ -422,16 +427,87 @@ static void __init find_hk_type(const struct dmi_header *dm, void *dummy)
 	}
 }
 
+/*
+ * Descriptor buffer is 128 byte long and contains:
+ *
+ *       Name             Offset  Length  Value
+ * Vendor Signature          0       4    "DELL"
+ * Object Signature          4       4    " WMI"
+ * WMI Interface Version     8       4    <version>
+ * WMI buffer length        12       4    4096
+ */
+static int __init dell_wmi_check_descriptor_buffer(void)
+{
+	struct acpi_buffer out = { ACPI_ALLOCATE_BUFFER, NULL };
+	union acpi_object *obj;
+	acpi_status status;
+	u32 *buffer;
+
+	status = wmi_query_block(DELL_DESCRIPTOR_GUID, 0, &out);
+	if (ACPI_FAILURE(status)) {
+		pr_err("Cannot read Dell descriptor buffer - %d\n", status);
+		return status;
+	}
+
+	obj = (union acpi_object *)out.pointer;
+	if (!obj) {
+		pr_err("Dell descriptor buffer is empty\n");
+		return -EINVAL;
+	}
+
+	if (obj->type != ACPI_TYPE_BUFFER) {
+		pr_err("Cannot read Dell descriptor buffer\n");
+		kfree(obj);
+		return -EINVAL;
+	}
+
+	if (obj->buffer.length != 128) {
+		pr_err("Dell descriptor buffer has invalid length (%d)\n",
+			obj->buffer.length);
+		if (obj->buffer.length < 16) {
+			kfree(obj);
+			return -EINVAL;
+		}
+	}
+
+	buffer = (u32 *)obj->buffer.pointer;
+
+	if (buffer[0] != 0x4C4C4544 && buffer[1] != 0x494D5720)
+		pr_warn("Dell descriptor buffer has invalid signature (%*ph)\n",
+			8, buffer);
+
+	if (buffer[2] != 0 && buffer[2] != 1)
+		pr_warn("Dell descriptor buffer has unknown version (%d)\n",
+			buffer[2]);
+
+	if (buffer[3] != 4096)
+		pr_warn("Dell descriptor buffer has invalid buffer length (%d)\n",
+			buffer[3]);
+
+	dell_wmi_interface_version = buffer[2];
+
+	pr_info("Detected Dell WMI interface version %u\n",
+		dell_wmi_interface_version);
+
+	kfree(obj);
+	return 0;
+}
+
 static int __init dell_wmi_init(void)
 {
 	int err;
 	acpi_status status;
 
-	if (!wmi_has_guid(DELL_EVENT_GUID)) {
-		pr_warn("No known WMI GUID found\n");
+	if (!wmi_has_guid(DELL_EVENT_GUID) ||
+	    !wmi_has_guid(DELL_DESCRIPTOR_GUID)) {
+		pr_warn("Dell WMI GUID were not found\n");
 		return -ENODEV;
 	}
 
+	err = dell_wmi_check_descriptor_buffer();
+	if (err)
+		return err;
+
 	dmi_walk(find_hk_type, NULL);
 	acpi_video = acpi_video_get_backlight_type() != acpi_backlight_vendor;