[PATCH] usb: gadget: f_midi: allow customizing the USB MIDI interface string through configfs

Victor Krawiec
2025-09-24T15:48:38Z

When using f_midi from configfs the USB MIDI interface string is hardcoded
to 'MIDI function'.

This USB string descriptor is used by some third-party OS or software to
display the name of the MIDI device

Since we add an additional string option a new macro block was created to
factorize declarations

Signed-off-by: Victor Krawiec <victor.krawiec@arturia.com>
---
drivers/usb/gadget/function/f_midi.c | 108 +++++++++++++++------------
drivers/usb/gadget/function/u_midi.h | 8 +-
2 files changed, 66 insertions(+), 50 deletions(-)

diff --git a/drivers/usb/gadget/function/f_midi.c b/drivers/usb/gadget/function/f_midi.c
index da82598fcef8..0a8af7d507d9 100644
--- a/drivers/usb/gadget/function/f_midi.c
+++ b/drivers/usb/gadget/function/f_midi.c
@@ -875,6 +875,7 @@ static int f_midi_bind(struct usb_configuration *c, struct usb_function *f)
struct usb_composite_dev *cdev = c->cdev;
struct f_midi *midi = func_to_midi(f);
struct usb_string *us;
+ struct f_midi_opts *opts;
int status, n, jack = 1, i = 0, endpoint_descriptor_index = 0;

midi->gadget = cdev->gadget;
@@ -883,6 +884,10 @@ static int f_midi_bind(struct usb_configuration *c, struct usb_function *f)
if (status < 0)
goto fail_register;

+ opts = container_of(f->fi, struct f_midi_opts, func_inst);
+ if (opts->interface_string_allocated && opts->interface_string)
+ midi_string_defs[STRING_FUNC_IDX].s = opts->interface_string;
+
/* maybe allocate device-global string ID */
us = usb_gstrings_attach(c->cdev, midi_strings,
ARRAY_SIZE(midi_string_defs));
@@ -1178,59 +1183,62 @@ end: \
\
CONFIGFS_ATTR(f_midi_opts_, name);

+#define F_MIDI_OPT_STRING(name) \
+static ssize_t f_midi_opts_##name##_show(struct config_item *item, char *page) \
+{ \
+ struct f_midi_opts *opts = to_f_midi_opts(item); \
+ ssize_t result; \
+ \
+ mutex_lock(&opts->lock); \
+ if (opts->name) { \
+ result = strscpy(page, opts->name, PAGE_SIZE); \
+ } else { \
+ page[0] = 0; \
+ result = 0; \
+ } \
+ \
+ mutex_unlock(&opts->lock); \
+ \
+ return result; \
+} \
+ \
+static ssize_t f_midi_opts_##name##_store(struct config_item *item, \
+ const char *page, size_t len) \
+{ \
+ struct f_midi_opts *opts = to_f_midi_opts(item); \
+ int ret; \
+ char *c; \
+ \
+ mutex_lock(&opts->lock); \
+ if (opts->refcnt > 1) { \
+ ret = -EBUSY; \
+ goto end; \
+ } \
+ \
+ c = kstrndup(page, len, GFP_KERNEL); \
+ if (!c) { \
+ ret = -ENOMEM; \
+ goto end; \
+ } \
+ if (opts->name##_allocated) \
+ kfree(opts->name); \
+ opts->name = c; \
+ opts->name##_allocated = true; \
+ ret = len; \
+end: \
+ mutex_unlock(&opts->lock); \
+ return ret; \
+} \
+ \
+CONFIGFS_ATTR(f_midi_opts_, name);
+
F_MIDI_OPT_SIGNED(index, true, SNDRV_CARDS);
F_MIDI_OPT(buflen, false, 0);
F_MIDI_OPT(qlen, false, 0);
F_MIDI_OPT(in_ports, true, MAX_PORTS);
F_MIDI_OPT(out_ports, true, MAX_PORTS);
-
-static ssize_t f_midi_opts_id_show(struct config_item *item, char *page)
-{
- struct f_midi_opts *opts = to_f_midi_opts(item);
- ssize_t result;
-
- mutex_lock(&opts->lock);
- if (opts->id) {
- result = strscpy(page, opts->id, PAGE_SIZE);
- } else {
- page[0] = 0;
- result = 0;
- }
-
- mutex_unlock(&opts->lock);
-
- return result;
-}
-
-static ssize_t f_midi_opts_id_store(struct config_item *item,
- const char *page, size_t len)
-{
- struct f_midi_opts *opts = to_f_midi_opts(item);
- int ret;
- char *c;
-
- mutex_lock(&opts->lock);
- if (opts->refcnt > 1) {
- ret = -EBUSY;
- goto end;
- }
-
- c = kstrndup(page, len, GFP_KERNEL);
- if (!c) {
- ret = -ENOMEM;
- goto end;
- }
- if (opts->id_allocated)
- kfree(opts->id);
- opts->id = c;
- opts->id_allocated = true;
- ret = len;
-end:
- mutex_unlock(&opts->lock);
- return ret;
-}
-
-CONFIGFS_ATTR(f_midi_opts_, id);
+F_MIDI_OPT_STRING(id);
+F_MIDI_OPT_STRING(interface_string);

static struct configfs_attribute *midi_attrs[] = {
&f_midi_opts_attr_index,
@@ -1239,6 +1247,7 @@ static struct configfs_attribute *midi_attrs[] = {
&f_midi_opts_attr_in_ports,
&f_midi_opts_attr_out_ports,
&f_midi_opts_attr_id,
+ &f_midi_opts_attr_interface_string,
NULL,
};

@@ -1264,6 +1273,8 @@ static void f_midi_free_inst(struct usb_function_instance *f)
if (free) {
if (opts->id_allocated)
kfree(opts->id);
+ if (opts->interface_string_allocated)
+ kfree(opts->interface_string);
kfree(opts);
}
}
@@ -1280,6 +1291,7 @@ static struct usb_function_instance *f_midi_alloc_inst(void)
opts->func_inst.free_func_inst = f_midi_free_inst;
opts->index = SNDRV_DEFAULT_IDX1;
opts->id = SNDRV_DEFAULT_STR1;
+ opts->interface_string = SNDRV_DEFAULT_STR1;
opts->buflen = 512;
opts->qlen = 32;
opts->in_ports = 1;
diff --git a/drivers/usb/gadget/function/u_midi.h b/drivers/usb/gadget/function/u_midi.h
index 2e400b495cb8..c9c396301a43 100644
--- a/drivers/usb/gadget/function/u_midi.h
+++ b/drivers/usb/gadget/function/u_midi.h
@@ -15,11 +15,15 @@

#include <linux/usb/composite.h>

+#define F_MIDI_OPT_STRING_DECLARE(name) \
+ char *name; \
+ bool name##_allocated; \
+
struct f_midi_opts {
struct usb_function_instance func_inst;
int index;
- char *id;
- bool id_allocated;
+ F_MIDI_OPT_STRING_DECLARE(id);
+ F_MIDI_OPT_STRING_DECLARE(interface_string);
unsigned int in_ports;
unsigned int out_ports;
unsigned int buflen;

base-commit: f83ec76bf285bea5727f478a68b894f5543ca76e
--
2.43.0