usb: gadget: f_fs: Add the missing get_alt callback

The Apple CarLife iAP gadget has a descriptor in userspace with two
alternate settings. The host sends the set_alt request to configure
alt_setting 0 or 1, and this is verified by the subsequent get_alt
request.

This patch implements and sets the get_alt callback. Without the
get_alt callback, composite.c abruptly concludes the
USB_REQ_GET/SET_INTERFACE request, assuming only one alt setting
for the endpoint.

unlike the uvc and ncm, f_fs gadget is fully implemented in userspace,
and driver just reset the eps and generate the event. so no additional
adaptaion associated with this change is not required in set_alt callback

Signed-off-by: Hardik Gajjar <hgajjar@de.adit-jv.com>
Link: https://lore.kernel.org/r/20240301124708.120394-1-hgajjar@de.adit-jv.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/function/f_fs.c

@@ -45,6 +45,7 @@
 #include "configfs.h"
 
 #define FUNCTIONFS_MAGIC	0xa647361 /* Chosen by a honest dice roll ;) */
+#define MAX_ALT_SETTINGS	2		  /* Allow up to 2 alt settings to be set. */
 
 MODULE_IMPORT_NS(DMA_BUF);
 
@@ -80,6 +81,7 @@ struct ffs_function {
 	short				*interfaces_nums;
 
 	struct usb_function		function;
+	int				cur_alt[MAX_CONFIG_INTERFACES];
 };
 
 
@@ -103,6 +105,7 @@ static int __must_check ffs_func_eps_enable(struct ffs_function *func);
 static int ffs_func_bind(struct usb_configuration *,
 			 struct usb_function *);
 static int ffs_func_set_alt(struct usb_function *, unsigned, unsigned);
+static int ffs_func_get_alt(struct usb_function *f, unsigned int intf);
 static void ffs_func_disable(struct usb_function *);
 static int ffs_func_setup(struct usb_function *,
 			  const struct usb_ctrlrequest *);
@@ -3704,13 +3707,25 @@ static void ffs_reset_work(struct work_struct *work)
 	ffs_data_reset(ffs);
 }
 
+static int ffs_func_get_alt(struct usb_function *f,
+			    unsigned int interface)
+{
+	struct ffs_function *func = ffs_func_from_usb(f);
+	int intf = ffs_func_revmap_intf(func, interface);
+
+	return (intf < 0) ? intf : func->cur_alt[interface];
+}
+
 static int ffs_func_set_alt(struct usb_function *f,
 			    unsigned interface, unsigned alt)
 {
 	struct ffs_function *func = ffs_func_from_usb(f);
 	struct ffs_data *ffs = func->ffs;
 	int ret = 0, intf;
 
+	if (alt > MAX_ALT_SETTINGS)
+		return -EINVAL;
+
 	if (alt != (unsigned)-1) {
 		intf = ffs_func_revmap_intf(func, interface);
 		if (intf < 0)
@@ -3738,8 +3753,10 @@ static int ffs_func_set_alt(struct usb_function *f,
 
 	ffs->func = func;
 	ret = ffs_func_eps_enable(func);
-	if (ret >= 0)
+	if (ret >= 0) {
 		ffs_event_add(ffs, FUNCTIONFS_ENABLE);
+		func->cur_alt[interface] = alt;
+	}
 	return ret;
 }
 
@@ -4066,6 +4083,7 @@ static struct usb_function *ffs_alloc(struct usb_function_instance *fi)
 	func->function.bind    = ffs_func_bind;
 	func->function.unbind  = ffs_func_unbind;
 	func->function.set_alt = ffs_func_set_alt;
+	func->function.get_alt = ffs_func_get_alt;
 	func->function.disable = ffs_func_disable;
 	func->function.setup   = ffs_func_setup;
 	func->function.req_match = ffs_func_req_match;