Hello Alexey,
I have another issue with the Sierra firmware which I hope you can help
me with: The MC7710 device requires at ZLP even if we send
dwNtbOutMaxSize sized NTBs. This is a problem because the current code
explicitly prevents this.
The following code in the v3.8 cdc-ncm was written to keep existing
behaviour from the pre-v3.8 driver:
if (((skb_out->len % le16_to_cpu(ctx->out_ep->desc.wMaxPacketSize)) ==
0) &&
(skb_out->len < le32_to_cpu(ctx->ncm_parm.dwNtbOutMaxSize)) &&
skb_tailroom(skb_out))
*skb_put(skb_out, 1) = 0; /* force short packet */
The previous implementaion looked like this:
6c60408e (Alexey Orishko 2011-05-06 03:01:30 +0000 832) if
(((last_offset < ctx->tx_max) && ((last_offset %
6c60408e (Alexey Orishko 2011-05-06 03:01:30 +0000 833)
le16_to_cpu(ctx->out_ep->desc.wMaxPacketSize)) == 0)) ||
6c60408e (Alexey Orishko 2011-05-06 03:01:30 +0000 834)
(((last_offset == ctx->tx_max) && ((ctx->tx_max %
6c60408e (Alexey Orishko 2011-05-06 03:01:30 +0000 835)
le16_to_cpu(ctx->out_ep->desc.wMaxPacketSize)) == 0)) &&
6c60408e (Alexey Orishko 2011-05-06 03:01:30 +0000 836)
(ctx->tx_max < le32_to_cpu(ctx->ncm_parm.dwNtbOutMaxSize)))) {
900d495a (Alexey Orishko 2010-11-29 23:23:28 +0000 837) /*
force short packet */
900d495a (Alexey Orishko 2010-11-29 23:23:28 +0000 838) *(((u8
*)skb_out->data) + last_offset) = 0;
900d495a (Alexey Orishko 2010-11-29 23:23:28 +0000 839)
last_offset++;
900d495a (Alexey Orishko 2010-11-29 23:23:28 +0000 840) }
The effect is the same: We add a 0 byte if the NTB length is a multiplum
of wMaxPacketSize *except* if the length is equal to dwNtbOutMaxSize.
This exception will happen very often because of the way we pad NTBs.
Now, I have tried to find what the above code was based on, and my guess
is that it is this note in table 3-1 in the CDC NCM spec:
If wBlockLength = 0x0000, the block is terminated by a
short packet. In this case, the USB transfer must still
be shorter than dwNtbInMaxSize or dwNtbOutMax-
Size. If exactly dwNtbInMaxSize or dwNtbOutMaxSize
bytes are sent, and the size is a multiple of wMax-
PacketSize for the given pipe, then no ZLP shall be
sent.
Is that correct? I cannot find any special ZLP handling mentioned
anywhere else in the standard.
If so, then I believe it is a misinterpretation. The above text deals
only with the exceptional case of wBlockLength = 0x0000, which we do not
do. As long as wBlockLength > 0 then I believe the device is in its
full right to expect a ZLP if wBlockLength % wMaxPacketSize is 0.
Would you feel comfortable dropping the additional condition and going
with
if (((skb_out->len % le16_to_cpu(ctx->out_ep->desc.wMaxPacketSize)) ==
0) &&
skb_tailroom(skb_out))
*skb_put(skb_out, 1) = 0; /* force short packet */
? I have verified that this is sufficient to make the Sierra device
work. I will of course test it with the other NCM and MBIM devices I've
got, but that is a very limited set...
The other option I see is making a device specific quirk for this. But
I suspect that Sierra is using the current Qualcomm MBIM implemetation
here, and if so then we are likely to see a large number of similar
devices in the near future. I'd really like to avoid having device
specific quirks for all of them if at all possible.
Bjørn
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