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

[Lorenzo Bianconi <lorenzo@kernel.org>]

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> 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:

in mt76u_fill_rx_sg()
data_size = SKB_WITH_OVERHEAD(q->buf_size);

q->buf_size = MT_BUF_WITH_OVERHEAD(MT_RX_BUF_SIZE);

where MT_BUF_WITH_OVERHEAD is
((x) + SKB_DATA_ALIGN(sizeof(struct skb_shared_info)))

anyway we can even set
q->buf_size = PAGE_SIZE
data_size = rounddown(SKB_WITH_OVERHEAD(q->buf_size), usb_endpoint_maxp())

Regards,
Lorenzo

> 
> 
> Stanislaw

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