platform/wmi: Add helper functions for WMI string conversions

WMI strings are encoded using UTF16-LE characters, forcing WMI drivers
to manually convert them to/from standard UTF8 strings. Add a two
helper functions for those tasks.

Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Link: https://patch.msgid.link/20260116204116.4030-4-W_Armin@gmx.de
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>

Documentation/driver-api/wmi.rst

@@ -16,5 +16,8 @@ which will be bound to compatible WMI devices by the driver core.
 .. kernel-doc:: include/linux/wmi.h
    :internal:
 
+.. kernel-doc:: drivers/platform/wmi/string.c
+   :export:
+
 .. kernel-doc:: drivers/platform/wmi/core.c
    :export:

drivers/platform/wmi/Kconfig

@@ -6,6 +6,7 @@
 menuconfig ACPI_WMI
 	tristate "ACPI-WMI support"
 	depends on ACPI && X86
+	select NLS
 	help
 	  This option enables support for the ACPI-WMI driver core.
 

drivers/platform/wmi/Makefile

@@ -4,7 +4,7 @@
 # ACPI WMI core
 #
 
-wmi-y			:= core.o marshalling.o
+wmi-y			:= core.o marshalling.o string.o
 obj-$(CONFIG_ACPI_WMI)	+= wmi.o
 
 # Unit tests

drivers/platform/wmi/string.c

@@ -0,0 +1,92 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * WMI string utility functions.
+ *
+ * Copyright (C) 2025 Armin Wolf <W_Armin@gmx.de>
+ */
+
+#include <linux/build_bug.h>
+#include <linux/compiler_types.h>
+#include <linux/err.h>
+#include <linux/export.h>
+#include <linux/nls.h>
+#include <linux/limits.h>
+#include <linux/types.h>
+#include <linux/wmi.h>
+
+#include <asm/byteorder.h>
+
+static_assert(sizeof(__le16) == sizeof(wchar_t));
+
+/**
+ * wmi_string_to_utf8s - Convert a WMI string into a UTF8 string.
+ * @str: WMI string representation
+ * @dst: Buffer to fill with UTF8 characters
+ * @length: Length of the destination buffer
+ *
+ * Convert as WMI string into a standard UTF8 string. The conversion will stop
+ * once a NUL character is detected or when the buffer is full. Any invalid UTF16
+ * characters will be ignored. The resulting UTF8 string will always be NUL-terminated
+ * when this function returns successfully.
+ *
+ * Return: Length of the resulting UTF8 string or negative errno code on failure.
+ */
+ssize_t wmi_string_to_utf8s(const struct wmi_string *str, u8 *dst, size_t length)
+{
+	/* Contains the maximum number of UTF16 code points to read */
+	int inlen = le16_to_cpu(str->length) / 2;
+	int ret;
+
+	if (length < 1)
+		return -EINVAL;
+
+	/* We must leave room for the NUL character at the end of the destination buffer */
+	ret = utf16s_to_utf8s((__force const wchar_t *)str->chars, inlen, UTF16_LITTLE_ENDIAN, dst,
+			      length - 1);
+	if (ret < 0)
+		return ret;
+
+	dst[ret] = '\0';
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(wmi_string_to_utf8s);
+
+/**
+ * wmi_string_from_utf8s - Convert a UTF8 string into a WMI string.
+ * @str: WMI string representation
+ * @max_chars: Maximum number of UTF16 code points to store inside the WMI string
+ * @src: UTF8 string to convert
+ * @src_length: Length of the source string without any trailing NUL-characters
+ *
+ * Convert a UTF8 string into a WMI string. The conversion will stop when the WMI string is
+ * full. The resulting WMI string will always be NUL-terminated and have its length field set
+ * to and appropriate value when this function returns successfully.
+ *
+ * Return: Number of UTF16 code points inside the WMI string or negative errno code on failure.
+ */
+ssize_t wmi_string_from_utf8s(struct wmi_string *str, size_t max_chars, const u8 *src,
+			      size_t src_length)
+{
+	size_t str_length;
+	int ret;
+
+	if (max_chars < 1)
+		return -EINVAL;
+
+	/* We must leave room for the NUL character at the end of the WMI string */
+	ret = utf8s_to_utf16s(src, src_length, UTF16_LITTLE_ENDIAN, (__force wchar_t *)str->chars,
+			      max_chars - 1);
+	if (ret < 0)
+		return ret;
+
+	str_length = (ret + 1) * sizeof(u16);
+	if (str_length > U16_MAX)
+		return -EOVERFLOW;
+
+	str->length = cpu_to_le16(str_length);
+	str->chars[ret] = '\0';
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(wmi_string_from_utf8s);

include/linux/wmi.h

@@ -62,6 +62,11 @@ struct wmi_string {
 	__le16 chars[];
 } __packed;
 
+ssize_t wmi_string_to_utf8s(const struct wmi_string *str, u8 *dst, size_t length);
+
+ssize_t wmi_string_from_utf8s(struct wmi_string *str, size_t max_chars, const u8 *src,
+			      size_t src_length);
+
 int wmidev_invoke_method(struct wmi_device *wdev, u8 instance, u32 method_id,
 			 const struct wmi_buffer *in, struct wmi_buffer *out);