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From:   Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Date:   Sat, 15 Jun 2019 14:06:29 +0200
Message-ID: <CAJ0CqmX00=d8BsoOfXQdxsh2JXha-d-F6+wAUQCgEYXs=4Z8Eg@mail.gmail.com>
Subject: Re: [PATCH v3 wireless-drivers 1/3] mt76: usb: fix rx A-MSDU support
To:     Stanislaw Gruszka <sgruszka@redhat.com>
Cc:     Johannes Berg <johannes@sipsolutions.net>,
        Lorenzo Bianconi <lorenzo@kernel.org>,
        Kalle Valo <kvalo@codeaurora.org>,
        linux-wireless <linux-wireless@vger.kernel.org>,
        Felix Fietkau <nbd@nbd.name>
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>
> 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.
>
> Anyway I think the driver should put ieee80211 header in linear area
> and iwlwifi & mt7601u implementation is somewhat optimal.

Actually the case is a little bit different for mt76 since we need
even mt76x02_rxwi in the linear area of the received skb.
Taking that into account the requested size to copy will be:
32 + 802.11 hdr + SNAP hdr  = ~ 70B
Moreover to pass rxwi size to usb module we need to add a field in
mt76_driver_ops (e.g rxwi_size).
I will carry out some tests and if there are no differences I will
post a single patch to wireless-drivers using 128B as default size

Regards,
Lorenzo

>
> Stanislaw