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

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

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> On Wed, Jun 12, 2019 at 12:21:34PM +0200, Lorenzo Bianconi wrote:
> > 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?
> 
> Yes, sure. Other drivers just use 128 value directly and don't even
> create a macro for that. And if somebody will decide to change
> buf_size it will not be small value.

Ok, I will simplify it in v2

> 
> > > > > 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
> 
> Ok, so L3 and L4 headers should be in linear area of skb and if not 
> network stack will copy them from fragment. But I wonder why other
> drivers just copy ieee80211_hdr and SNAP ? Isn't that if we copy 
> 128B then is possible that part of the payload will be in linear
> area and part in fragment, whereas is expected that payload
> will not be broken into two parts?

I think the payload can be split in multiple skb fragments, the constraint is
just related to header parsing

Regards,
Lorenzo



> 
> Stanislaw

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