From dde5e942338322117fa8e30f2531369ce0046f91 Mon Sep 17 00:00:00 2001
From: dylad <dylan.laduranty@mesotic.com>
Date: Sat, 5 Dec 2020 11:51:46 +0100
Subject: [PATCH] usbus: add optional string descriptor for alt interface

---
 sys/include/usb/usbus.h   | 1 +
 sys/usb/usbus/usbus_fmt.c | 7 +++++++
 2 files changed, 8 insertions(+)

diff --git a/sys/include/usb/usbus.h b/sys/include/usb/usbus.h
index 1e5a8c727f..aaa9fbd4cb 100644
--- a/sys/include/usb/usbus.h
+++ b/sys/include/usb/usbus.h
@@ -294,6 +294,7 @@ typedef struct usbus_interface_alt {
                                              descriptor generators */
     usbus_endpoint_t *ep;               /**< List of associated endpoints for
                                              this alternative setting */
+    usbus_string_t *descr;              /**< Descriptor string */
 } usbus_interface_alt_t;
 
 /**
diff --git a/sys/usb/usbus/usbus_fmt.c b/sys/usb/usbus/usbus_fmt.c
index aced79cc11..5584de185f 100644
--- a/sys/usb/usbus/usbus_fmt.c
+++ b/sys/usb/usbus/usbus_fmt.c
@@ -205,6 +205,13 @@ static size_t _fmt_descriptors_iface_alts(usbus_t *usbus,
         len += sizeof(usb_descriptor_interface_t);
         usb_iface.alternate_setting = alts++;
         usb_iface.num_endpoints = _num_endpoints_alt(alt);
+        if (alt->descr) {
+            usb_iface.idx = alt->descr->idx;
+        } else {
+            /* If there is no string descriptor for a given alt interface
+               set the index to 0 to advertise it */
+            usb_iface.idx = 0;
+        }
         usbus_control_slicer_put_bytes(usbus, (uint8_t *)&usb_iface,
                                        sizeof(usb_descriptor_interface_t));
         len += _fmt_descriptors_post(usbus, alt->descr_gen);