[PATCH] usb: gadget: legacy: ether: fix RNDIS for Windows hosts

[amine-ayaou]

From: Amine Ayaou <amine-ayaou@xxxxxxxxxxx>

Fix RNDIS for Windows hosts by adding Microsoft OS 1.0
descriptor that allows Windows to identify the RNDIS function.

g_ether currently advertises its RNDIS configuration using
the legacy NetChip VID/PID and the RNDIS control interface
class, subclass and protocol values provided by f_rndis
(02/02/FF).

While the Linux rndis_host driver still recognizes the legacy
02/02/FF RNDIS interface descriptors, modern Windows hosts
do not bind the RNDIS driver to g_ether.

Use the existing USB composite framework support for
Microsoft OS 1.0 descriptors to advertise RNDIS when the
RNDIS function is enabled in g_ether.

Use of OS descriptor does not change the existing device IDs
or RNDIS control interface descriptors, keeping compatibility
with older hosts.
(It only adds a string descriptor at index 0xEE and handles
the associated vendor request)

Signed-off-by: Amine Ayaou <amine-ayaou@xxxxxxxxxxx>
---

Notes:
    g_ether's RNDIS is useful when a network interface via USB
    is needed early during boot, before a filesystem is available
    (e.g., nfsroot).
    
    Alternatives for having a network interface via USB during
    boot are configfs and g_ncm.
    Configuring the gadget through configfs requires userspace,
    which is unavailable before mounting the NFS root when
    no initramfs is used.
    NCM (g_ncm) is not an option on hosts that only provide
    RNDIS support.
    
    Tested on BeagleBone Black with a Windows 11 host
    (Version 10.0.26200, Build 26200)
    RNDIS automatically bound with the patch.

 drivers/usb/gadget/legacy/ether.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/gadget/legacy/ether.c b/drivers/usb/gadget/legacy/ether.c
index 99c7fc0d1d597..70de686fa56b7 100644
--- a/drivers/usb/gadget/legacy/ether.c
+++ b/drivers/usb/gadget/legacy/ether.c
@@ -11,6 +11,7 @@
 
 #include <linux/kernel.h>
 #include <linux/netdevice.h>
+#include <linux/nls.h>
 
 #if defined USB_ETH_RNDIS
 #  undef USB_ETH_RNDIS
@@ -92,8 +93,8 @@ static inline bool has_rndis(void)
 
 #include "u_ecm.h"
 #include "u_gether.h"
-#ifdef	USB_ETH_RNDIS
 #include "u_rndis.h"
+#ifdef	USB_ETH_RNDIS
 #include "rndis.h"
 #else
 #define rndis_borrow_net(...) do {} while (0)
@@ -364,6 +365,8 @@ static int eth_bind(struct usb_composite_dev *cdev)
 
 	if (has_rndis()) {
 		/* RNDIS plus ECM-or-Subset */
+		struct f_rndis_opts *rndis_opts = NULL;
+
 		gether_set_gadget(net, cdev->gadget);
 		status = gether_register_netdev(net);
 		if (status)
@@ -387,6 +390,18 @@ static int eth_bind(struct usb_composite_dev *cdev)
 		device_desc.idVendor = cpu_to_le16(RNDIS_VENDOR_NUM);
 		device_desc.idProduct = cpu_to_le16(RNDIS_PRODUCT_NUM);
 		device_desc.bNumConfigurations = 2;
+
+		/* Configure Microsoft OS 1.0 descriptors for RNDIS. */
+		cdev->os_desc_config = &rndis_config_driver;
+		cdev->use_os_string = true;
+		cdev->b_vendor_code = 0xcd; /* arbitrary */
+		utf8s_to_utf16s("MSFT100", 7,
+			UTF16_LITTLE_ENDIAN, (wchar_t *)cdev->qw_sign,
+			OS_STRING_QW_SIGN_LEN / 2);
+		rndis_opts = container_of(fi_rndis, struct f_rndis_opts,
+					  func_inst);
+		memcpy(rndis_opts->rndis_ext_compat_id,
+			"RNDIS\0\0\0" "5162001", 16);
 	}
 
 	/* Allocate string descriptor numbers ... note that string
-- 
2.53.0