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From:   David Laight <David.Laight@ACULAB.COM>
To:     'Stanislaw Gruszka' <sgruszka@redhat.com>,
        Jian-Hong Pan <jian-hong@endlessm.com>
CC:     Yan-Hsuan Chuang <yhchuang@realtek.com>,
        Kalle Valo <kvalo@codeaurora.org>,
        "David S . Miller" <davem@davemloft.net>,
        "linux-wireless@vger.kernel.org" <linux-wireless@vger.kernel.org>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux@endlessm.com" <linux@endlessm.com>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>
Subject: RE: [PATCH] rtw88: pci: Use general byte arrays as the elements of RX
 ring
Thread-Topic: [PATCH] rtw88: pci: Use general byte arrays as the elements of
 RX ring
Thread-Index: AQHVRro3Kaj7ufhACkSCuQV9Pmu/hqbi59sQ
Date:   Tue, 30 Jul 2019 09:48:38 +0000
Message-ID: <962a8a8e735946d6b3944b7d0e228309@AcuMS.aculab.com>
References: <20190725080925.6575-1-jian-hong@endlessm.com>
 <20190730093533.GC3174@redhat.com>
In-Reply-To: <20190730093533.GC3174@redhat.com>
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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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