usb: host: ch9: Add string descriptor request helper function

subsys/usb/host/usbh_ch9.c

@@ -180,6 +180,15 @@ int usbh_req_desc_cfg(struct usb_device *const udev,
 	return ret;
 }
 
+int usbh_req_desc_str(struct usb_device *const udev,
+		      const uint8_t index, const uint16_t lang_id,
+		      struct net_buf *const desc_buf)
+{
+	uint16_t len = MIN(net_buf_tailroom(desc_buf), UINT8_MAX);
+
+	return usbh_req_desc(udev, USB_DESC_STRING, index, lang_id, len, desc_buf);
+}
+
 int usbh_req_set_address(struct usb_device *const udev,
 			 const uint8_t addr)
 {

subsys/usb/host/usbh_ch9.h

@@ -41,6 +41,10 @@ int usbh_req_desc_cfg(struct usb_device *const udev,
 		      const uint16_t len,
 		      struct usb_cfg_descriptor *const desc);
 
+int usbh_req_desc_str(struct usb_device *const udev,
+		      const uint8_t index, const uint16_t lang_id,
+		      struct net_buf *const desc_buf);
+
 int usbh_req_set_alt(struct usb_device *const udev,
 		     const uint8_t iface,
 		     const uint8_t alt);

subsys/usb/host/usbh_desc.c

@@ -1,6 +1,6 @@
 /*
  * SPDX-FileCopyrightText: Copyright Nordic Semiconductor ASA
- * SPDX-FileCopyrightText: Copyright 2025 NXP
+ * SPDX-FileCopyrightText: Copyright 2025 - 2026 NXP
  * SPDX-License-Identifier: Apache-2.0
  */
 
@@ -10,6 +10,8 @@
 
 #include "usbh_class.h"
 #include "usbh_desc.h"
+#include "usbh_ch9.h"
+#include "usbh_device.h"
 
 LOG_MODULE_REGISTER(usbh_desc, CONFIG_USBH_LOG_LEVEL);
 
@@ -59,6 +61,12 @@ bool usbh_desc_is_valid_endpoint(const void *const desc)
 				  USB_DESC_ENDPOINT);
 }
 
+bool usbh_desc_is_valid_string(const void *const desc)
+{
+	return usbh_desc_is_valid(desc, sizeof(struct usb_string_descriptor),
+				  USB_DESC_STRING);
+}
+
 const void *usbh_desc_get_next(const void *const desc)
 {
 	const struct usb_desc_header *const head = desc;
@@ -218,3 +226,78 @@ const void *usbh_desc_get_next_function(const void *const desc)
 
 	return NULL;
 }
+
+int usbh_desc_get_supported_langs(struct usb_device *const udev, uint16_t *const lang_ids,
+				  const uint8_t lang_ids_len)
+{
+	struct net_buf *buf;
+	uint16_t len;
+	int ret;
+
+	buf = usbh_xfer_buf_alloc(udev, lang_ids_len * sizeof(uint16_t) + 2);
+	if (buf == NULL) {
+		return -ENOMEM;
+	}
+
+	ret = usbh_req_desc_str(udev, 0, 0, buf);
+	if (ret != 0) {
+		goto done;
+	}
+
+	if (!usbh_desc_is_valid_string(buf->data)) {
+		ret = -EBADMSG;
+		goto done;
+	}
+
+	/* Pull bytes of the content */
+	len = net_buf_pull_u8(buf) - 2;
+	/* Drop bDescriptorType */
+	net_buf_pull_u8(buf);
+	len = MIN(len, buf->len) / 2;
+	for (ret = 0; ret < len; ret++) {
+		lang_ids[ret] = net_buf_pull_le16(buf);
+	}
+
+done:
+	if (buf != NULL) {
+		usbh_xfer_buf_free(udev, buf);
+	}
+
+	return ret;
+}
+
+int usbh_desc_str_utf16le_to_ascii(const struct net_buf *const buf, char *const ascii_buf,
+				   const uint16_t ascii_buf_len)
+{
+	struct net_buf_simple tmp_buf;
+	uint16_t utf16le_code;
+	uint16_t len;
+
+	if (!usbh_desc_is_valid_string(buf->data)) {
+		return -EINVAL;
+	}
+
+	net_buf_simple_clone(&buf->b, &tmp_buf);
+
+	/* Pull bytes of the content */
+	len = net_buf_simple_pull_u8(&tmp_buf) - 2;
+	/* Drop bDescriptorType */
+	net_buf_simple_pull_u8(&tmp_buf);
+	len = MIN(MIN(tmp_buf.len, len) / 2, ascii_buf_len - 1);
+	memset(ascii_buf, '\0', ascii_buf_len);
+	for (unsigned int i = 0; i < len; i++) {
+		utf16le_code = net_buf_simple_pull_le16(&tmp_buf);
+
+		if (utf16le_code > 0x7F) {
+			return -EINVAL;
+		}
+
+		ascii_buf[i] = (char)utf16le_code;
+
+		if (utf16le_code == 0) {
+			break;
+		}
+	}
+
+	return 0;
+}

subsys/usb/host/usbh_desc.h

@@ -1,5 +1,6 @@
 /*
  * SPDX-FileCopyrightText: Copyright Nordic Semiconductor ASA
+ * SPDX-FileCopyrightText: Coryright 2025 - 2026 NXP
  * SPDX-License-Identifier: Apache-2.0
  */
 
@@ -121,6 +122,16 @@ bool usbh_desc_is_valid_association(const void *const desc);
  */
 bool usbh_desc_is_valid_endpoint(const void *const desc);
 
+/**
+ * @brief Checks that the pointed descriptor is an string descriptor.
+ *
+ * @param[in] desc The descriptor to validate
+ *
+ * @return true if the descriptor size and type are correct
+ * @return false if the descriptor size or type is wrong
+ */
+bool usbh_desc_is_valid_string(const void *const desc);
+
 /**
  * @brief Get the next function in the descriptor list.
  *
@@ -151,4 +162,36 @@ const void *usbh_desc_get_next_function(const void *const desc);
  */
 const void *usbh_desc_get_next_alt_setting(const void *const desc);
 
+/**
+ * @brief Get supported USB LANGIDs.
+ *
+ * Retrieves the list of language IDs supported by the USB device.
+ *
+ * @param[in] udev Pointer to the USB device.
+ * @param[out] lang_ids Array to store the supported LANGIDs.
+ * @param[in] lang_ids_len Length of the LANGIDs array.
+ *
+ * @retval number of supported IDs.
+ * @retval Negative error code from getting the string descriptor.
+ * @retval -ENOMEM if memory allocation failed.
+ * @retval -EBADMSG if the descriptor is invalid.
+ */
+int usbh_desc_get_supported_langs(struct usb_device *const udev, uint16_t *const lang_ids,
+				  const uint8_t lang_ids_len);
+
+/**
+ * @brief Convert UTF16LE encoded string descriptor to ASCII.
+ *
+ * Converts the UTF16LE encoded string descriptor descriptor to an ASCII string.
+ * The ASCII string is always null-terminated.
+ *
+ * @param[in] buf Buffer containing the string descriptor.
+ * @param[out] ascii_buf Buffer to store the converted ASCII string.
+ * @param[in] ascii_buf_len Maximum length of the ASCII buffer (including null terminator).
+ *
+ * @retval 0 on success.
+ * @retval -EINVAL if descriptor is malformed or conversion is not possilbe.
+ */
+int usbh_desc_str_utf16le_to_ascii(const struct net_buf *const buf, char *const ascii_buf,
+				   const uint16_t ascii_buf_len);
 #endif /* ZEPHYR_INCLUDE_USBH_DESC_H */

tests/subsys/usb/device_next/src/main.c

@@ -9,6 +9,7 @@
 #include <zephyr/usb/usbh.h>
 
 #include "usbh_ch9.h"
+#include "usbh_desc.h"
 #include "usbh_device.h"
 
 #include <zephyr/logging/log.h>
@@ -40,7 +41,9 @@
 static int test_cmp_string_desc(struct net_buf *const buf, const int idx)
 {
 	static struct usbd_desc_node *desc_nd;
-	size_t len;
+	char ascii_str[253];
+	uint16_t len;
+	int err;
 
 	if (idx == test_mfg.str.idx) {
 		desc_nd = &test_mfg;
@@ -52,19 +55,15 @@
 		return -ENOTSUP;
 	}
 
-	if (net_buf_pull_u8(buf) != desc_nd->bLength) {
-		return -EINVAL;
-	}
-
-	if (net_buf_pull_u8(buf) != USB_DESC_STRING) {
-		return -EINVAL;
+	err = usbh_desc_str_utf16le_to_ascii(buf, ascii_str, sizeof(ascii_str));
+	if (err != 0) {
+		return err;
 	}
 
-	LOG_HEXDUMP_DBG(buf->data, buf->len, "");
-	len = MIN(buf->len / 2, desc_nd->bLength / 2);
+	len = MIN(sizeof(ascii_str), desc_nd->bLength / 2);
 	for (size_t i = 0; i < len; i++) {
-		uint16_t a = net_buf_pull_le16(buf);
-		uint16_t b = ((uint8_t *)(desc_nd->ptr))[i];
+		char a = ascii_str[i];
+		char b = ((uint8_t *)(desc_nd->ptr))[i];
 
 		if (a != b) {
 			LOG_INF("%c != %c", a, b);
@@ -77,9 +76,9 @@
 
 ZTEST(device_next, test_get_desc_string)
 {
-	const uint8_t type = USB_DESC_STRING;
-	const uint16_t id = 0x0409;
 	static struct usb_device *udev;
+	const uint16_t our_id = 0x0409;
+	uint16_t lang_ids[2];
 	struct net_buf *buf;
 	int err;
 
@@ -92,19 +91,25 @@
 	err = k_mutex_lock(&udev->mutex, K_MSEC(200));
 	zassert_equal(err, 0, "Failed to lock device");
 
-	err = usbh_req_desc(udev, type, 1, id, UINT8_MAX, buf);
+	err = usbh_desc_get_supported_langs(udev, lang_ids, ARRAY_SIZE(lang_ids));
+	zassert_true(err > 0, 0, "Failed to get LANGIDs");
+
+	zassert_true(err == 1, 0, "Wrong number of LANGIDs");
+	zassert_equal(lang_ids[0], our_id, "Wrong LANGID");
+
+	err = usbh_req_desc_str(udev, 1, our_id, buf);
 	zassert_equal(err, 0, "Transfer status is an error");
 	err = test_cmp_string_desc(buf, 1);
 	zassert_equal(err, 0, "Descriptor comparison failed");
 
 	net_buf_reset(buf);
-	err = usbh_req_desc(udev, type, 2, id, UINT8_MAX, buf);
+	err = usbh_req_desc_str(udev, 2, our_id, buf);
 	zassert_equal(err, 0, "Transfer status is an error");
 	err = test_cmp_string_desc(buf, 2);
 	zassert_equal(err, 0, "Descriptor comparison failed");
 
 	net_buf_reset(buf);
-	err = usbh_req_desc(udev, type, 3, id, UINT8_MAX, buf);
+	err = usbh_req_desc_str(udev, 3, our_id, buf);
 	zassert_equal(err, 0, "Transfer status is an error");
 	err = test_cmp_string_desc(buf, 3);
 	zassert_equal(err, 0, "Descriptor comparison failed");