net: cdc_ncm: respect operator preferred MTU reported by MBIM

According to "Universal Serial Bus Communications Class Subclass
Specification for Mobile Broadband Interface Model, Revision 1.0,
Errata-1" published by USB-IF, the wMTU field of the MBIM extended
functional descriptor indicates the operator preferred MTU for IP data
streams.

This patch modifies cdc_ncm_setup to ensure that the MTU value set on
the usbnet device does not exceed the operator preferred MTU indicated
by wMTU if the MBIM device exposes a MBIM extended functional
descriptor.

Signed-off-by: Ben Chan <benchan@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 18 insertions, 0 deletions

diff --git a/drivers/net/usb/cdc_ncm.c b/drivers/net/usb/cdc_ncm.c
index dbff290ed0e4..e8711a8cfa01 100644
--- a/drivers/net/usb/cdc_ncm.c
+++ b/drivers/net/usb/cdc_ncm.c
@@ -74,6 +74,7 @@ static int cdc_ncm_setup(struct usbnet *dev)
         u8 iface_no;
         int err;
         int eth_hlen;
+        u16 mbim_mtu;
         u16 ntb_fmt_supported;
         __le16 max_datagram_size;
 
@@ -261,6 +262,14 @@ out:
         /* set MTU to max supported by the device if necessary */
         if (dev->net->mtu > ctx->max_datagram_size - eth_hlen)
                 dev->net->mtu = ctx->max_datagram_size - eth_hlen;
+
+        /* do not exceed operater preferred MTU */
+        if (ctx->mbim_extended_desc) {
+                mbim_mtu = le16_to_cpu(ctx->mbim_extended_desc->wMTU);
+                if (mbim_mtu != 0 && mbim_mtu < dev->net->mtu)
+                        dev->net->mtu = mbim_mtu;
+        }
+
         return 0;
 }
 
@@ -399,6 +408,14 @@ int cdc_ncm_bind_common(struct usbnet *dev, struct usb_interface *intf, u8 data_
                         ctx->mbim_desc = (const struct usb_cdc_mbim_desc *)buf;
                         break;
 
+                case USB_CDC_MBIM_EXTENDED_TYPE:
+                        if (buf[0] < sizeof(*(ctx->mbim_extended_desc)))
+                                break;
+
+                        ctx->mbim_extended_desc =
+                                (const struct usb_cdc_mbim_extended_desc *)buf;
+                        break;
+
                 default:
                         break;
                 }
diff --git a/include/linux/usb/cdc_ncm.h b/include/linux/usb/cdc_ncm.h
index c3fa80745996..bdf05fb36729 100644
--- a/include/linux/usb/cdc_ncm.h
+++ b/include/linux/usb/cdc_ncm.h
@@ -93,6 +93,7 @@ struct cdc_ncm_ctx {
 
         const struct usb_cdc_ncm_desc *func_desc;
         const struct usb_cdc_mbim_desc *mbim_desc;
+        const struct usb_cdc_mbim_extended_desc *mbim_extended_desc;
         const struct usb_cdc_ether_desc *ether_desc;
 
         struct usb_interface *control;