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usbnet: ipheth: document scope of NCM implementation

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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>
This commit is contained in:
Foster Snowhill 2025-01-26 00:54:09 +01:00 • committed by Paolo Abeni
parent ee591f2b28
commit be154b598f
1 changed files with 8 additions and 0 deletions
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8
drivers/net/usb/ipheth.c
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@ -218,6 +218,14 @@ static int ipheth_rcvbulk_callback_legacy(struct urb *urb)
return ipheth_consume_skb(buf, len, dev); 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) static int ipheth_rcvbulk_callback_ncm(struct urb *urb)
{ {
struct usb_cdc_ncm_nth16 *ncmh; struct usb_cdc_ncm_nth16 *ncmh;