netdev - Re: [RFC PATCH 02/13] net: dsa: implement a central TX reallocation procedure

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Date:   Mon, 19 Oct 2020 10:30:47 +0000
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     David Laight <David.Laight@...LAB.COM>
CC:     'Florian Fainelli' <f.fainelli@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "andrew@...n.ch" <andrew@...n.ch>,
        "vivien.didelot@...il.com" <vivien.didelot@...il.com>,
        "kuba@...nel.org" <kuba@...nel.org>,
        Christian Eggers <ceggers@...i.de>,
        Kurt Kanzenbach <kurt@...utronix.de>
Subject: Re: [RFC PATCH 02/13] net: dsa: implement a central TX reallocation
 procedure

On Mon, Oct 19, 2020 at 08:33:27AM +0000, David Laight wrote:
> Is it possible to send the extra bytes from a separate buffer fragment?
> The entire area could be allocated (coherent) when the rings are
> allocated.
> That would save having to modify the skb at all.
> 
> Even if some bytes of the frame header need 'adjusting' transmitting
> from a copy may be faster - especially on systems with an iommu.
> 
> Many (many) moons ago we found the cutoff point for copying frames
> on a system with an iommu to be around 1k bytes.

Please help me understand better how to implement what you're suggesting.
DSA switches have 3 places where they might insert a tag:
1. Between the source MAC address and the EtherType (this is the most
   common)
2. Before the destination MAC address
3. Before the FCS

I imagine that the most common scenario (1) is also the most difficult
to implement using fragments, since I would need to split the Ethernet
header from the rest of the skb data area, which might defeat the
purpose.

Also, simply integrating these 3 code paths into something generic will
bring challenges of its own.

Lastly, I fully expect the buffers to have proper headroom and tailroom
now (if they don't, then it's worth investigating what's the code path
that doesn't observe our dev->needed_headroom and dev->needed_tailroom).
The reallocation code is there just for clones (and as far as I
understand, fragments won't save us from the need of reallocating the
data areas of clones), and for short frames with DSA switches in case
(3).