From c13c2b3b3c41b71586209bd8a9615b0c34cb296c Mon Sep 17 00:00:00 2001
From: Devan Lai <devan.lai@gmail.com>
Date: Sat, 17 Oct 2020 17:12:20 +0000
Subject: [PATCH] usb: Allow registration of a single non-contiguous string
 descriptor for WinUSB

Classic WinUSB support is detected by probing for a string descriptor
at index 0xEE with a special string.
usbd_register_extra_string() allows registration of a string at this
index without having to provide 237 other string descriptors

Originally filed as https://github.com/libopencm3/libopencm3/pull/849

WCID reference: https://github.com/pbatard/libwdi/wiki/WCID-Devices
---
 include/libopencm3/usb/usbd.h |  3 +++
 lib/usb/usb.c                 | 16 ++++++++++++++++
 lib/usb/usb_private.h         |  4 ++++
 lib/usb/usb_standard.c        | 16 +++++++++++++++-
 4 files changed, 38 insertions(+), 1 deletion(-)

diff --git a/include/libopencm3/usb/usbd.h b/include/libopencm3/usb/usbd.h
index d2451010..3576d2b2 100644
--- a/include/libopencm3/usb/usbd.h
+++ b/include/libopencm3/usb/usbd.h
@@ -165,6 +165,9 @@ extern int usbd_register_set_config_callback(usbd_device *usbd_dev,
 extern void usbd_register_set_altsetting_callback(usbd_device *usbd_dev,
 					usbd_set_altsetting_callback callback);
 
+/** Registers a non-contiguous string descriptor */
+extern void usbd_register_extra_string(usbd_device *usbd_dev, int index, const char* string);
+
 /* Functions to be provided by the hardware abstraction layer */
 extern void usbd_poll(usbd_device *usbd_dev);
 
diff --git a/lib/usb/usb.c b/lib/usb/usb.c
index e2dd0c27..12bc431c 100644
--- a/lib/usb/usb.c
+++ b/lib/usb/usb.c
@@ -54,6 +54,8 @@ usbd_device *usbd_init(const usbd_driver *driver,
 	usbd_dev->config = conf;
 	usbd_dev->strings = strings;
 	usbd_dev->num_strings = num_strings;
+	usbd_dev->extra_string_idx = 0;
+	usbd_dev->extra_string = NULL;
 	usbd_dev->ctrl_buf = control_buffer;
 	usbd_dev->ctrl_buf_len = control_buffer_size;
 
@@ -94,6 +96,20 @@ void usbd_register_sof_callback(usbd_device *usbd_dev, void (*callback)(void))
 	usbd_dev->user_callback_sof = callback;
 }
 
+void usbd_register_extra_string(usbd_device *usbd_dev, int index, const char* string)
+{
+    /*
+	 * Note: string index 0 is reserved for LANGID requests and cannot
+	 *       be overwritten using this functionality.
+	 */
+	if (string != NULL && index > 0) {
+		usbd_dev->extra_string_idx = index;
+		usbd_dev->extra_string = string;
+	} else {
+		usbd_dev->extra_string_idx = 0;
+	}
+}
+
 void _usbd_reset(usbd_device *usbd_dev)
 {
 	usbd_dev->current_address = 0;
diff --git a/lib/usb/usb_private.h b/lib/usb/usb_private.h
index 3aa3f9ee..9e5e1181 100644
--- a/lib/usb/usb_private.h
+++ b/lib/usb/usb_private.h
@@ -92,6 +92,10 @@ struct _usbd_device {
 
 	const struct _usbd_driver *driver;
 
+	/* Extra, non-contiguous user string descriptor index and value */
+	int extra_string_idx;
+	const char* extra_string;
+
 	/* private driver data */
 
 	uint16_t fifo_mem_top;
diff --git a/lib/usb/usb_standard.c b/lib/usb/usb_standard.c
index 4ce7a8b1..7057f2de 100644
--- a/lib/usb/usb_standard.c
+++ b/lib/usb/usb_standard.c
@@ -179,7 +179,21 @@ usb_standard_get_descriptor(usbd_device *usbd_dev,
 				      sizeof(sd->wData[0]);
 
 			*len = MIN(*len, sd->bLength);
-		} else {
+		} else if (descr_idx == usbd_dev->extra_string_idx) {
+			/* This string is returned as UTF16, hence the
+			 * multiplication
+			 */
+			sd->bLength = strlen(usbd_dev->extra_string) * 2 +
+				      sizeof(sd->bLength) +
+				      sizeof(sd->bDescriptorType);
+
+			*len = MIN(*len, sd->bLength);
+
+			for (i = 0; i < (*len / 2) - 1; i++) {
+				sd->wData[i] =
+					usbd_dev->extra_string[i];
+			}
+        } else {
 			array_idx = descr_idx - 1;
 
 			if (!usbd_dev->strings) {