Re: [PATCH v2 1/2] mt76: usb: fix rx A-MSDU support

[Lorenzo Bianconi <lorenzo.bianconi@redhat.com>]

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On Jun 12, Stanislaw Gruszka wrote:
> On Wed, Jun 12, 2019 at 11:45:21AM +0200, Lorenzo Bianconi wrote:
> > > > +mt76u_build_rx_skb(u8 *data, int len, int buf_size,
> > > > +		   int *nsgs)
> > > > +{
> > > > +	int data_len = min(len, MT_SKB_HEAD_LEN);
> 
> Oh, and this looks unneeded as well as for len < MT_SKB_HEAD_LEN=128
> we will go through fast path.

I guess if we remove data_len = min(len, MT_SKB_HEAD_LEN) and even *nsgs = 0 at
the end we are making some assumptions on the value of MT_SKB_HEAD_LEN and
buf_size. In the patch I just avoided them but maybe we can just assume that
MT_SKB_HEAD_LEN and buf_size will not changed in the future. What do you
think?

> 
> > > mt7601u and iwlmvm just copy hdrlen + 8 and put the rest
> > > of the buffer in fragment, which supose to be more efficient,
> > > see comment in iwl_mvm_pass_packet_to_mac80211().
> > 
> > Right here we copy 128B instead of 32 but I think it is good to have L3 and L4
> > header in the linear area of the skb since otherwise the stack will need to
> > align them
> 
> Not sure if understand, I think aliment of L3 & L4 headers will be
> the same, assuming ieee80211 header is aligned the same in fragment
> buffer and in linear area. But if you think this is better to copy those
> to linear area I'm ok with that.

Sorry I have not been so clear. I mean in the stack before accessing a given
header we will run pskb_may_pull() that can end up copying the skb if there is
not enough space in the skb->head

Regards,
Lorenzo

> 
> Stanislaw

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