From be154b598fa54136e2be17d6dd13c8a8bc0078ce Mon Sep 17 00:00:00 2001
From: Foster Snowhill <forst@pen.gy>
Date: Sun, 26 Jan 2025 00:54:09 +0100
Subject: [PATCH] usbnet: ipheth: document scope of NCM implementation
Clarify that the "NCM" implementation in `ipheth` is very limited, as
iOS devices aren't compatible with the CDC NCM specification in regular
tethering mode.
For a standards-compliant implementation, one shall turn to
the `cdc_ncm` module.
Cc: stable@vger.kernel.org # 6.5.x
Signed-off-by: Foster Snowhill <forst@pen.gy>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
drivers/net/usb/ipheth.c | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/drivers/net/usb/ipheth.c b/drivers/net/usb/ipheth.c
index 5347cd7e295b3..a19789b571905 100644
--- a/drivers/net/usb/ipheth.c
+++ b/drivers/net/usb/ipheth.c
@@ -218,6 +218,14 @@ static int ipheth_rcvbulk_callback_legacy(struct urb *urb)
return ipheth_consume_skb(buf, len, dev);
}
+/* In "NCM mode", the iOS device encapsulates RX (phone->computer) traffic
+ * in NCM Transfer Blocks (similarly to CDC NCM). However, unlike reverse
+ * tethering (handled by the `cdc_ncm` driver), regular tethering is not
+ * compliant with the CDC NCM spec, as the device is missing the necessary
+ * descriptors, and TX (computer->phone) traffic is not encapsulated
+ * at all. Thus `ipheth` implements a very limited subset of the spec with
+ * the sole purpose of parsing RX URBs.
+ */
static int ipheth_rcvbulk_callback_ncm(struct urb *urb)
{
struct usb_cdc_ncm_nth16 *ncmh;
--
GitLab