linux-kernel - RE: [PATCH] rtw88: pci: Use general byte arrays as the elements of RX ring

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Date:   Tue, 30 Jul 2019 09:48:38 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Stanislaw Gruszka' <sgruszka@...hat.com>,
        Jian-Hong Pan <jian-hong@...lessm.com>
CC:     Yan-Hsuan Chuang <yhchuang@...ltek.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        "David S . Miller" <davem@...emloft.net>,
        "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux@...lessm.com" <linux@...lessm.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: RE: [PATCH] rtw88: pci: Use general byte arrays as the elements of RX
 ring

From: Stanislaw Gruszka
> Sent: 30 July 2019 10:36
...
> > +		len = pkt_stat.pkt_len + pkt_offset;
> > +		skb = dev_alloc_skb(len);
> > +		if (WARN_ONCE(!skb, "rx routine starvation\n"))
> >  			goto next_rp;
> >
> >  		/* put the DMA data including rx_desc from phy to new skb */
> > -		skb_put_data(new, skb->data, new_len);
> > +		skb_put_data(skb, rx_desc, len);
> 
> Coping big packets it quite inefficient. What drivers usually do is
> copy only for small packets and for big ones allocate new rx buf
> (drop packet alloc if fail) and pas old buf to network stack via
> skb_add_rx_frag(). See iwlmvm as example.

If you have to do iommu setup/teardown then the breakeven point
for (not) copying may be surprisingly large.
You do need to do the measurements on a range of hardware.
Coping is also likely to affect the L1 cache - unless you can
copy quickly without polluting the cache.

It is all 'swings and roundabouts'.

	David

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