netdev - RE: [PATCH v1 4/6] net: fec: Increase buffer descriptor entry number

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Date:	Fri, 30 May 2014 08:40:57 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"fugang.duan@...escale.com" <fugang.duan@...escale.com>
Cc:	"ezequiel.garcia@...e-electrons.com" 
	<ezequiel.garcia@...e-electrons.com>,
	David Laight <David.Laight@...LAB.COM>,
	"Frank.Li@...escale.com" <Frank.Li@...escale.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"shawn.guo@...aro.org" <shawn.guo@...aro.org>,
	"bhutchings@...arflare.com" <bhutchings@...arflare.com>,
	"stephen@...workplumber.org" <stephen@...workplumber.org>
Subject: RE: [PATCH v1 4/6] net: fec: Increase buffer descriptor entry number

On Fri, 2014-05-30 at 15:08 +0000, fugang.duan@...escale.com wrote:

> If frag page data is not match the alignment for ethernet DMA controller, there need three descriptor for one MSS:
> One descriptor for headers, one for the first non-align bytes copied from frag page, one for the rest of frag page.
> 

You could avoid the 2nd descriptor, by copying the unaligned part of the
payload into first descriptor (containing headers : about 66 bytes or so
you have room, if not, increase the 128 bytes room to 192 bytes)

> So one frame may cost descriptor number is: 3 x 45 

May... but in general it would be closer of 2 * 45


> 
> So the descriptors slots set to 512 is not big, just is reasonable. Do you think ?

I believe it is fine : that is about 5 64KB TSO packets.



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