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Subject: Re: [PATCH v3 wireless-drivers 1/3] mt76: usb: fix rx A-MSDU support
From:   Johannes Berg <johannes@sipsolutions.net>
To:     Stanislaw Gruszka <sgruszka@redhat.com>
Cc:     Lorenzo Bianconi <lorenzo.bianconi@redhat.com>,
        Lorenzo Bianconi <lorenzo@kernel.org>, kvalo@codeaurora.org,
        linux-wireless@vger.kernel.org, nbd@nbd.name
Date:   Fri, 14 Jun 2019 13:34:28 +0200
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On Fri, 2019-06-14 at 13:31 +0200, Stanislaw Gruszka wrote:
> On Fri, Jun 14, 2019 at 12:20:59PM +0200, Johannes Berg wrote:
> > On Fri, 2019-06-14 at 12:11 +0200, Lorenzo Bianconi wrote:
> > 
> > > Looking at __ieee80211_amsdu_copy() now I got why other drivers copy hdrlen +
> > > 8, thx :)
> > > In our case reuse_frag is true in __ieee80211_amsdu_copy, so we will end up
> > > copying 32B + ether_len. Anyway I think 32 is a little bit too low and we could get
> > > better performances increasing it a little bit.
> > > A typical use case (e.g IPv6 + TCP):
> > > 
> > > IPv6 = 40B, TCP = 32B --> so 72B..I guess 128B is a good value :)
> > > @Felix, Johannes: what do you think?
> > 
> > I think while we might *allocate* more, I don't think we should *copy*
> > more, since then the TCP payload will no longer be in pages.
> > 
> > It'd probably be better to implement leaving enough tailroom (allocate
> > 128), but copying nothing, unless the *entire* packet fits.
> 
> iwl4965 put entire packet in fragment in il4965_pass_packet_to_mac80211() .
> Initially I thought this is a bug, since mac80211 require header be
> in the linear area, but looks like ieee80211_rx_monitor() copy header
> before rest of mac80211 check it, so 4965 is fine.

Mac80211 should not assume anything about header being present or not,
just like the rest of the network stack.

> Anyway I think the driver should put ieee80211 header in linear area
> and iwlwifi & mt7601u implementation is somewhat optimal.

Yes, it's just an optimisation to do the copy-break (copy small packets
completely) or to copy the header already (since we may have better ways
to do so than skb_copy_bits if we still have a virt pointer to the page,
rather than just the page pointer).

johannes