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

[Lorenzo Bianconi <lorenzo.bianconi@redhat.com>]

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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.

ack, so I think the second approach will be safer (using roundup instead of
rounddown :))

Regards,
Lorenzo

> 
> Stanislaw
> 

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