netdev - RE: [PATCH v3] Renesas Ethernet AVB driver

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Date:	Mon, 27 Apr 2015 09:22:24 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Sergei Shtylyov' <sergei.shtylyov@...entembedded.com>,
	David Miller <davem@...emloft.net>
CC:	"mitsuhiro.kimura.kc@...esas.com" <mitsuhiro.kimura.kc@...esas.com>,
	"f.fainelli@...il.com" <f.fainelli@...il.com>,
	"robh+dt@...nel.org" <robh+dt@...nel.org>,
	"pawel.moll@....com" <pawel.moll@....com>,
	"mark.rutland@....com" <mark.rutland@....com>,
	"ijc+devicetree@...lion.org.uk" <ijc+devicetree@...lion.org.uk>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"galak@...eaurora.org" <galak@...eaurora.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"richardcochran@...il.com" <richardcochran@...il.com>,
	"linux-sh@...r.kernel.org" <linux-sh@...r.kernel.org>,
	"masaru.nagai.vx@...esas.com" <masaru.nagai.vx@...esas.com>
Subject: RE: [PATCH v3] Renesas Ethernet AVB driver

From: Sergei Shtylyov 
> Sent: 24 April 2015 19:27
...
> > If you have ethernet hardware that requires tx or rx buffers to be on
> > 4n boundaries you should send it back as 'not fit for purpose'.
> 
>     The RX buffers can be adjusted with skb_resrerve(), it's only the TX
> buffers that need to be copied...

If the processor can't perform misaligned reads (don't know what is on
your SoC, but I suspect it can't - crossing page boundaries is hard)
then the rx buffer will have to be re-aligned in software.
Even the 'userdata' part will typically end up with an expensive
misaligned buffer copy.

Even on x86 the misaligned transfers are probably measurable.

>     I'm afraid we can't. :-)
>     However, my colleague has suggested a scheme minimizing the copying:
> only up to 3 first bytes need to be copied to the driver's internal buffers,
> the rest can be sent from an skb itself. That would require substantial
> changes to the driver though...

There might be a restriction on the length of buffer fragments.

You might be able to alternate 14 and 1500+ byte receive buffers.
The frame following a slightly overlong one would be 'wrong'.

	David
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