[PATCH v4 2/2] usb: gadget: u_ether: Fix host MAC address case

[Konrad Gräfe]

The CDC-ECM specification [1] requires to send the host MAC address as
an uppercase hexadecimal string in chapter "5.4 Ethernet Networking
Functional Descriptor":
    The Unicode character is chosen from the set of values 30h through
    39h and 41h through 46h (0-9 and A-F).

However, snprintf(.., "%pm", ..) generates a lowercase MAC address
string. While most host drivers are tolerant to this, UsbNcm.sys on
Windows 10 is not. Instead it uses a different MAC address with all
bytes set to zero including and after the first byte containing a
lowercase letter. On Windows 11 Microsoft fixed it, but apparently they
did not backport the fix.

This change fixes the issue by using "%pmU" to generate an uppercase hex
string to comply with the specification.

[1]: https://www.usb.org/document-library/class-definitions-communication-devices-12, file ECM120.pdf

Fixes: bcd4a1c40bee ("usb: gadget: u_ether: construct with default values and add setters/getters")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Konrad Gräfe <k.graefe@xxxxxxxxxxx>
---

Changes since v3: None

Changes since v2:
* Add uppercase MAC address format string and use that instead of
  manually uppercasing the resulting MAC address string.

Changes since v1:
* Fixed checkpatch.pl warnings

 drivers/usb/gadget/function/u_ether.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/usb/gadget/function/u_ether.c b/drivers/usb/gadget/function/u_ether.c
index 6956ad8ba8dd..70e6b825654c 100644
--- a/drivers/usb/gadget/function/u_ether.c
+++ b/drivers/usb/gadget/function/u_ether.c
@@ -963,7 +963,7 @@ int gether_get_host_addr_cdc(struct net_device *net, char *host_addr, int len)
 		return -EINVAL;
 
 	dev = netdev_priv(net);
-	snprintf(host_addr, len, "%pm", dev->host_mac);
+	snprintf(host_addr, len, "%pmU", dev->host_mac);
 
 	return strlen(host_addr);
 }
-- 
2.34.1