netdev - Re: [PATCH v4] aquantia: Remove the build

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Date:   Fri, 20 Nov 2020 00:23:04 +0000
From:   "Ramsay, Lincoln" <Lincoln.Ramsay@...i.com>
To:     Florian Westphal <fw@...len.de>
CC:     Igor Russkikh <irusskikh@...vell.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Dmitry Bogdanov <dbogdanov@...vell.com>
Subject: Re: [PATCH v4] aquantia: Remove the build_skb path

> > I don't know about benefits/feasibility, but I did wonder if (in the event that the "fast path" is possible), the dma_mapping could use an offset? The page would include the skb header but the dma mapping would not. If that was done though, only 1 RX frame would fit into the page (at least on my system, where the RX frame seems to be 2k and the page is 4k). Also, there's a possibility to set the "order" variable, so that multiple pages are created at once and I'm not sure if this would work in that case.
> 
> Yes, this is what some drivers do, they allocate a page, pass
> pageaddr + headroom_offset everywhere, except build_skb() which gets the
> pageaddr followed by skb_reserve(skb, headroom_offset).

If everyone's happy with the patch as-is, I might just leave it and let people more knowledgeable decide if it's worth adding this optimization back in with this offset logic ;)

Lincoln