[PATCH v2] usb: gadget: ncm: Avoid dropping datagrams of properly parsed NTBs

[Krishna Kurapati]

It is observed sometimes when tethering is used over NCM with Windows 11
as host, at some instances, the gadget_giveback has one byte appended at
the end of a proper NTB. When the NTB is parsed, unwrap call looks for
any leftover bytes in SKB provided by u_ether and if there are any pending
bytes, it treats them as a separate NTB and parses it. But in case the
second NTB (as per unwrap call) is faulty/corrupt, all the datagrams that
were parsed properly in the first NTB and saved in rx_list are dropped.

Adding a few custom traces showed the following:

[002] d..1  7828.532866: dwc3_gadget_giveback: ep1out:
req 000000003868811a length 1025/16384 zsI ==> 0
[002] d..1  7828.532867: ncm_unwrap_ntb: K: ncm_unwrap_ntb toprocess: 1025
[002] d..1  7828.532867: ncm_unwrap_ntb: K: ncm_unwrap_ntb nth: 1751999342
[002] d..1  7828.532868: ncm_unwrap_ntb: K: ncm_unwrap_ntb seq: 0xce67
[002] d..1  7828.532868: ncm_unwrap_ntb: K: ncm_unwrap_ntb blk_len: 0x400
[002] d..1  7828.532868: ncm_unwrap_ntb: K: ncm_unwrap_ntb ndp_len: 0x10
[002] d..1  7828.532869: ncm_unwrap_ntb: K: Parsed NTB with 1 frames

In this case, the giveback is of 1025 bytes and block length is 1024.
The rest 1 byte (which is 0x00) won't be parsed resulting in drop of
all datagrams in rx_list.

Same is case with packets of size 2048:
[002] d..1  7828.557948: dwc3_gadget_giveback: ep1out:
req 0000000011dfd96e length 2049/16384 zsI ==> 0
[002] d..1  7828.557949: ncm_unwrap_ntb: K: ncm_unwrap_ntb nth: 1751999342
[002] d..1  7828.557950: ncm_unwrap_ntb: K: ncm_unwrap_ntb blk_len: 0x800

Lecroy shows one byte coming in extra confirming that the byte is coming
in from PC:

Transfer 2959 - Bytes Transferred(1025)  Timestamp((18.524 843 590)
- Transaction 8391 - Data(1025 bytes) Timestamp(18.524 843 590)
--- Packet 4063861
      Data(1024 bytes)
      Duration(2.117us) Idle(14.700ns) Timestamp(18.524 843 590)
--- Packet 4063863
      Data(1 byte)
      Duration(66.160ns) Time(282.000ns) Timestamp(18.524 845 722)

According to Windows driver, no ZLP is needed if wBlockLength is non-zero,
because the non-zero wBlockLength has already told the function side the
size of transfer to be expected. However, there are in-market NCM devices
that rely on ZLP as long as the wBlockLength is multiple of wMaxPacketSize.
To deal with such devices, it pads an extra 0 at end so the transfer is no
longer multiple of wMaxPacketSize.

Signed-off-by: Krishna Kurapati <quic_kriskura@xxxxxxxxxxx>
---
 drivers/usb/gadget/function/f_ncm.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/gadget/function/f_ncm.c b/drivers/usb/gadget/function/f_ncm.c
index ca5d5f564998..8c314dc98952 100644
--- a/drivers/usb/gadget/function/f_ncm.c
+++ b/drivers/usb/gadget/function/f_ncm.c
@@ -1338,11 +1338,17 @@ static int ncm_unwrap_ntb(struct gether *port,
 	     "Parsed NTB with %d frames\n", dgram_counter);
 
 	to_process -= block_len;
-	if (to_process != 0) {
+
+	if (to_process == 1 &&
+	    (block_len % 512 == 0) &&
+	    (*(unsigned char *)(ntb_ptr + block_len) == 0x00)) {
+		goto done;
+	} else if (to_process > 0) {
 		ntb_ptr = (unsigned char *)(ntb_ptr + block_len);
 		goto parse_ntb;
 	}
 
+done:
 	dev_consume_skb_any(skb);
 
 	return 0;
-- 
2.34.1