platform/x86: dell-ddv: Warn if ePPID has a suspicious length

On some systems (like the Dell Inspiron 3505), the acpi operation
region holding the ePPID string is two bytes too short, causing
acpi functions like ToString() to omit the last two bytes.
This does not happen on Windows, supposedly due to their implementation
of ToString() ignoring buffer boundaries.
Inform users if the ePPID length differs from the Dell specification
so they can complain to Dell to fix their BIOS.

Tested on a Dell Inspiron 3505.
Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Link: https://lore.kernel.org/r/20221102212336.380257-2-W_Armin@gmx.deReviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

drivers/platform/x86/dell/dell-wmi-ddv.c

@@ -10,12 +10,14 @@
 #include <linux/acpi.h>
 #include <linux/debugfs.h>
 #include <linux/device.h>
+#include <linux/dev_printk.h>
 #include <linux/kernel.h>
 #include <linux/kstrtox.h>
 #include <linux/math.h>
 #include <linux/module.h>
 #include <linux/limits.h>
 #include <linux/power_supply.h>
+#include <linux/printk.h>
 #include <linux/seq_file.h>
 #include <linux/sysfs.h>
 #include <linux/wmi.h>
@@ -27,6 +29,9 @@
 #define DELL_DDV_SUPPORTED_INTERFACE 2
 #define DELL_DDV_GUID	"8A42EA14-4F2A-FD45-6422-0087F7A7E608"
 
+#define DELL_EPPID_LENGTH	20
+#define DELL_EPPID_EXT_LENGTH	23
+
 enum dell_ddv_method {
 	DELL_DDV_BATTERY_DESIGN_CAPACITY	= 0x01,
 	DELL_DDV_BATTERY_FULL_CHARGE_CAPACITY	= 0x02,
@@ -196,6 +201,10 @@ static ssize_t eppid_show(struct device *dev, struct device_attribute *attr, cha
 	if (ret < 0)
 		return ret;
 
+	if (obj->string.length != DELL_EPPID_LENGTH && obj->string.length != DELL_EPPID_EXT_LENGTH)
+		dev_info_once(&data->wdev->dev, FW_INFO "Suspicious ePPID length (%d)\n",
+			      obj->string.length);
+
 	ret = sysfs_emit(buf, "%s\n", obj->string.pointer);
 
 	kfree(obj);