Re: [PATCH v2 2/2] mt76: usb: do not always copy the first part of received frames

[Stanislaw Gruszka <sgruszka@redhat.com>]

On Wed, Jun 12, 2019 at 02:28:48PM +0200, Lorenzo Bianconi wrote:
> [...]
> > 
> > My sense of humor declined quite drastically lastly ;-/
> > 
> > > > > but we can be more cautious since probably copying
> > > > > the first 128B will not make any difference
> > > > 
> > > > Not sure if I understand what you mean.
> > > 
> > > Please correct me if I am wrong but I think max amsdu rx size is 3839B for
> > > mt76. For the sg_en case this frame will span over multiple sg buffers since
> > > sg buffer size is 2048B (2 sg buffers). Moreover if we do not take into account
> > > skb_shared_info when configuring the sg buffer size we will need to always copy
> > > the first 128B of the first buffer since received len will be set to 2048 and
> > > the following if condition will always fail:
> > > 
> > > if (SKB_WITH_OVERHEAD(buf_size) >= MT_DMA_HDR_LEN + len) {
> > > }
> > 
> > Ok, that I understand.
> > 
> > > > > > And skb_shered_info is needed only in first buffer IIUC.
> > > > > > 
> > > > > > Also this patch seems to make first patch unnecessary except for
> > > > > > non sg_en case (in which I think rx AMSDU is broken anyway),
> > > > > > so I would prefer just to apply first patch.
> > > > > 
> > > > > I do not think rx AMSDU is broken for non sg_en case since the max rx value
> > > > > allowed should be 3839 IIRC and we alloc one page in this case
> > > > 
> > > > If that's the case we should be fine, but then I do not understand
> > > > why we allocate 8*2k buffers for sg_en case, isn't that AP can
> > > > sent AMSDU frame 16k big?
> > > 
> > > Sorry I did not get what you mean here, could you please explain?
> > 
> > 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 .

Stanislaw