From: Andrew Lunn <andrew@lunn.ch>
To: Vladimir Oltean <vladimir.oltean@nxp.com>
Cc: David Laight <David.Laight@ACULAB.COM>,
'Florian Fainelli' <f.fainelli@gmail.com>,
"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
"vivien.didelot@gmail.com" <vivien.didelot@gmail.com>,
"kuba@kernel.org" <kuba@kernel.org>,
Christian Eggers <ceggers@arri.de>,
Kurt Kanzenbach <kurt@linutronix.de>
Subject: Re: [RFC PATCH 02/13] net: dsa: implement a central TX reallocation procedure
Date: Mon, 19 Oct 2020 14:19:20 +0200 [thread overview]
Message-ID: <20201019121920.GM456889@lunn.ch> (raw)
In-Reply-To: <20201019103047.oq5ki3jlhnwzz2xv@skbuf>
On Mon, Oct 19, 2020 at 10:30:47AM +0000, Vladimir Oltean wrote:
> 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.
We also have length issues. Most scatter/gather DMA engines require
the fragments are multiple of 4 bytes. Only the last segment does not
have this length restriction. And some of the DSA tag headers are 2
bytes, or 1 byte. So some master devices are going to have to convert
the fragments back to a linear buffer.
Andrew
next prev parent reply other threads:[~2020-10-19 12:19 UTC|newest]
Thread overview: 49+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-17 21:35 [RFC PATCH 00/13] Generic TX reallocation for DSA Vladimir Oltean
2020-10-17 21:35 ` [RFC PATCH 01/13] net: dsa: add plumbing for custom netdev statistics Vladimir Oltean
2020-10-17 22:13 ` Florian Fainelli
2020-10-17 23:15 ` Andrew Lunn
2020-10-18 0:23 ` Vladimir Oltean
2020-10-18 12:02 ` Heiner Kallweit
2020-10-18 12:16 ` Vladimir Oltean
2020-10-18 13:09 ` Heiner Kallweit
2020-10-18 13:48 ` Vladimir Oltean
2020-10-18 14:13 ` Heiner Kallweit
2020-10-18 22:58 ` Vladimir Oltean
2020-10-18 23:11 ` Florian Fainelli
2020-10-19 0:21 ` Vladimir Oltean
2020-10-19 3:49 ` Florian Fainelli
2020-10-19 12:05 ` Andrew Lunn
2020-10-19 16:27 ` Florian Fainelli
2020-10-18 16:49 ` Jakub Kicinski
2020-10-18 17:36 ` Andrew Lunn
2020-10-18 18:26 ` Jakub Kicinski
2020-10-18 18:33 ` Jakub Kicinski
2020-10-17 21:36 ` [RFC PATCH 02/13] net: dsa: implement a central TX reallocation procedure Vladimir Oltean
2020-10-17 22:01 ` Vladimir Oltean
2020-10-17 22:11 ` Florian Fainelli
2020-10-17 22:17 ` Vladimir Oltean
2020-10-18 0:37 ` Florian Fainelli
2020-10-19 8:33 ` David Laight
2020-10-19 10:30 ` Vladimir Oltean
2020-10-19 11:14 ` David Laight
2020-10-19 11:41 ` Vladimir Oltean
2020-10-19 12:19 ` Andrew Lunn [this message]
2020-10-17 22:31 ` Vladimir Oltean
2020-10-18 0:13 ` Vladimir Oltean
2020-10-18 10:36 ` Christian Eggers
2020-10-18 11:42 ` Vladimir Oltean
2020-10-18 11:59 ` Christian Eggers
2020-10-18 12:15 ` Vladimir Oltean
2020-10-17 21:36 ` [RFC PATCH 03/13] net: dsa: tag_ksz: don't allocate additional memory for padding/tagging Vladimir Oltean
2020-10-17 21:36 ` [RFC PATCH 04/13] net: dsa: trailer: " Vladimir Oltean
2020-10-17 21:36 ` [RFC PATCH 05/13] net: dsa: tag_qca: let DSA core deal with TX reallocation Vladimir Oltean
2020-10-17 21:36 ` [RFC PATCH 06/13] net: dsa: tag_ocelot: " Vladimir Oltean
2020-10-17 21:36 ` [RFC PATCH 07/13] net: dsa: tag_mtk: " Vladimir Oltean
2020-10-17 21:36 ` [RFC PATCH 08/13] net: dsa: tag_lan9303: " Vladimir Oltean
2020-10-17 21:36 ` [RFC PATCH 09/13] net: dsa: tag_edsa: " Vladimir Oltean
2020-10-17 21:36 ` [RFC PATCH 10/13] net: dsa: tag_brcm: " Vladimir Oltean
2020-10-17 21:36 ` [RFC PATCH 11/13] net: dsa: tag_dsa: " Vladimir Oltean
2020-10-17 21:36 ` [RFC PATCH 12/13] net: dsa: tag_gswip: " Vladimir Oltean
2020-10-17 22:19 ` Vladimir Oltean
2020-10-17 21:36 ` [RFC PATCH 13/13] net: dsa: tag_ar9331: " Vladimir Oltean
2020-10-17 23:07 ` [RFC PATCH 00/13] Generic TX reallocation for DSA Andrew Lunn
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