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Date:   Thu, 13 Jun 2019 11:02:02 +0200
From:   Stanislaw Gruszka <sgruszka@redhat.com>
To:     Lorenzo Bianconi <lorenzo@kernel.org>
Cc:     Lorenzo Bianconi <lorenzo.bianconi@redhat.com>, 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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Hi

On Thu, Jun 13, 2019 at 10:26:38AM +0200, Lorenzo Bianconi wrote:
> [...]
> 
> > 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.
> 
> Hi Stanislaw,
> 
> reviewing the original patch I think we can't trigger any IOMMU bug since the
> usb buffer length is actually 2048 and not 2048 + skb_shared_info_size:

I'm concerned about alignment and crossing pages boundaries. If you
allocate via page_frag_alloc() buffers, except first one, will have
'not natural' alignment and every second will be spanned across
two pages.

Stanislaw