platform/x86: asus-wmi: Improve DSTS WMI method ID detection

The DSTS method detection mistakenly selects DCTS instead of DSTS if
nothing is returned when the method ID is not defined in WMNB. As a result,
the control of keyboard backlight is not functional for TUF Gaming series
laptops. Implement detection based on _UID of the WMI device instead.

There is evidence that DCTS is handled by ACPI WMI devices that have _UID
ASUSWMI, whereas none of the devices without ASUSWMI respond to DCTS and
DSTS is used instead [1].

DSDT examples:

FX505GM (_UID ATK):
Method (WMNB, 3, Serialized)
{ ...
    If ((Local0 == 0x53545344))
    {
        ...
        Return (Zero)
    }
    ...
    // No return
}

K54C (_UID ATK):
Method (WMNB, 3, Serialized)
{ ...
    If ((Local0 == 0x53545344))
    {
        ...
        Return (0x02)
    }
    ...
    Return (0xFFFFFFFE)
}

[1] Link: https://lkml.org/lkml/2019/4/11/322Signed-off-by: Yurii Pavlovskyi <yurii.pavlovskyi@gmail.com>
Suggested-by: Daniel Drake <drake@endlessm.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

drivers/hid/hid-asus.c

@@ -396,7 +396,7 @@ static bool asus_kbd_wmi_led_control_present(struct hid_device *hdev)
 	if (!IS_ENABLED(CONFIG_ASUS_WMI))
 		return false;
 
-	ret = asus_wmi_evaluate_method(ASUS_WMI_METHODID_DSTS2,
+	ret = asus_wmi_evaluate_method(ASUS_WMI_METHODID_DSTS,
 				       ASUS_WMI_DEVID_KBD_BACKLIGHT, 0, &value);
 	hid_dbg(hdev, "WMI backlight check: rc %d value %x", ret, value);
 	if (ret)

drivers/platform/x86/asus-wmi.c

@@ -83,6 +83,8 @@ MODULE_LICENSE("GPL");
 #define USB_INTEL_XUSB2PR		0xD0
 #define PCI_DEVICE_ID_INTEL_LYNXPOINT_LP_XHCI	0x9c31
 
+#define ASUS_ACPI_UID_ASUSWMI		"ASUSWMI"
+
 static const char * const ashs_ids[] = { "ATK4001", "ATK4002", NULL };
 
 static bool ashs_present(void)
@@ -1874,6 +1876,8 @@ static int asus_wmi_sysfs_init(struct platform_device *device)
  */
 static int asus_wmi_platform_init(struct asus_wmi *asus)
 {
+	struct device *dev = &asus->platform_device->dev;
+	char *wmi_uid;
 	int rv;
 
 	/* INIT enable hotkeys on some models */
@@ -1903,11 +1907,24 @@ static int asus_wmi_platform_init(struct asus_wmi *asus)
 	 * Note, on most Eeepc, there is no way to check if a method exist
 	 * or note, while on notebooks, they returns 0xFFFFFFFE on failure,
 	 * but once again, SPEC may probably be used for that kind of things.
+	 *
+	 * Additionally at least TUF Gaming series laptops return nothing for
+	 * unknown methods, so the detection in this way is not possible.
+	 *
+	 * There is strong indication that only ACPI WMI devices that have _UID
+	 * equal to "ASUSWMI" use DCTS whereas those with "ATK" use DSTS.
 	 */
-	if (!asus_wmi_evaluate_method(ASUS_WMI_METHODID_DSTS, 0, 0, NULL))
+	wmi_uid = wmi_get_acpi_device_uid(ASUS_WMI_MGMT_GUID);
+	if (!wmi_uid)
+		return -ENODEV;
+
+	if (!strcmp(wmi_uid, ASUS_ACPI_UID_ASUSWMI)) {
+		dev_info(dev, "Detected ASUSWMI, use DCTS\n");
+		asus->dsts_id = ASUS_WMI_METHODID_DCTS;
+	} else {
+		dev_info(dev, "Detected %s, not ASUSWMI, use DSTS\n", wmi_uid);
 		asus->dsts_id = ASUS_WMI_METHODID_DSTS;
-	else
-		asus->dsts_id = ASUS_WMI_METHODID_DSTS2;
+	}
 
 	/* CWAP allow to define the behavior of the Fn+F2 key,
 	 * this method doesn't seems to be present on Eee PCs */

include/linux/platform_data/x86/asus-wmi.h

@@ -18,8 +18,8 @@
 #define ASUS_WMI_METHODID_GDSP		0x50534447 /* Get DiSPlay output */
 #define ASUS_WMI_METHODID_DEVP		0x50564544 /* DEVice Policy */
 #define ASUS_WMI_METHODID_OSVR		0x5256534F /* OS VeRsion */
-#define ASUS_WMI_METHODID_DSTS		0x53544344 /* Device STatuS */
-#define ASUS_WMI_METHODID_DSTS2		0x53545344 /* Device STatuS #2*/
+#define ASUS_WMI_METHODID_DCTS		0x53544344 /* Device status (DCTS) */
+#define ASUS_WMI_METHODID_DSTS		0x53545344 /* Device status (DSTS) */
 #define ASUS_WMI_METHODID_BSTS		0x53545342 /* Bios STatuS ? */
 #define ASUS_WMI_METHODID_DEVS		0x53564544 /* DEVice Set */
 #define ASUS_WMI_METHODID_CFVS		0x53564643 /* CPU Frequency Volt Set */