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Date:   Thu, 13 Jun 2019 09:51:08 +0200
From:   Stanislaw Gruszka <sgruszka@redhat.com>
To:     Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Cc:     Lorenzo Bianconi <lorenzo@kernel.org>, nbd@nbd.name,
        kvalo@codeaurora.org, linux-wireless@vger.kernel.org
Subject: Re: [PATCH v2 2/2] mt76: usb: do not always copy the first part of
 received frames
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On Wed, Jun 12, 2019 at 04:44:01PM +0200, Lorenzo Bianconi wrote:
> On Jun 12, Stanislaw Gruszka wrote:
> > On Wed, Jun 12, 2019 at 02:59:05PM +0200, Stanislaw Gruszka wrote:
> > > > > If max RX AMSDU size is 3839B I do not see reason why we allocate
> > > > > MT_SG_MAX_SIZE=8 of MT_RX_BUF_SIZE=2k buffers for sg_en case.
> > > > > I thought the reason is that max AMSDU size is 16kB so it fit into
> > > > > 8 sg buffers of 2k.
> > > > > 
> > > > > In other words, for me, looks like either
> > > > > - we can not handle AMSDU for non sg case because we do not
> > > > > allocate big enough buffer
> > > > 
> > > > I think AMSDU is mandatory and we currently support it even for non-sg case
> > > > (since max rx AMSDU is 3839B)
> > > > 
> > > > > or
> > > > > - we can just use one PAGE_SIZE buffer for rx and remove sg
> > > > > buffers for rx completely 
> > > > 
> > > > using sg buffers we can support bigger rx AMSDU size in the future without using
> > > > huge buffers (e.g. we can try to use IEEE80211_MAX_MPDU_LEN_HT_7935 with
> > > > mt76x2u)
> > > 
> > > I think it would be simpler just to allocate 2 pages for 7935B .
> > 
> > And if we could determine that there is no true need to use sg for rx,
> > I think best approach here would be revert f8f527b16db5 in v5.2 to fix
> > regression and remove rx sg in -next. That would make code simpler,
> > allocate 4k instead 16k per packet, allow to use build_skb (4096 - 3839
> > give enough space for shared info) and not use usb hcd bounce buffer.
> 
> I do not think we should drop sg support since:
> - it allow us to rx huge amsdu frames (e.g. IEEE80211_MAX_MPDU_LEN_VHT_11454)
>   using multiple one page buffer. I think there will be new usb devices where we can
>   increase amsdu size (we can increase it even on mt76x2u usb 3.0 devices)

I looked at intel wifi drivers and this is handled by amsdu_size module
parameter, supported values are 4k, 8k and 12k. RX allocation size and
proper values in vht_cap & ht_cap are set accordingly. Assuming (some)
mt76 HW and FW can handle bigger AMSDUs I think we should do similar
thing.

Otherwise looks for me, we just waste memory and have not needed code
for no true reason. 

>   space needed for skb_shared_info is 320B on a x86_64 device

Uhh, I haven't expected that sk_shared_info() is that big, so indeed build_skb
could not used and 128B copy fallback will be necessary.


Stanislaw