netdev.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Ben Hutchings <bhutchings@solarflare.com>
To: David Laight <David.Laight@ACULAB.COM>
Cc: David Miller <davem@davemloft.net>,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	<linux-net-drivers@solarflare.com>,
	Linux Kernel Development <linux-kernel@vger.kernel.org>,
	<linux-s390@vger.kernel.org>, netdev <netdev@vger.kernel.org>
Subject: Re: [PATCHv2 net 1/2] sfc: Delete EFX_PAGE_IP_ALIGN, equivalent to NET_IP_ALIGN
Date: Tue, 14 May 2013 14:48:43 +0100	[thread overview]
Message-ID: <1368539323.4304.81.camel@deadeye.wl.decadent.org.uk> (raw)
In-Reply-To: <AE90C24D6B3A694183C094C60CF0A2F6026B7235@saturn3.aculab.com>

On Tue, 2013-05-14 at 09:32 +0100, David Laight wrote:
> > The two architectures that define CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
> > (powerpc and x86) now both define NET_IP_ALIGN as 0, so there is no
> > need for this optimisation any more.
> 
> Hmmm.... even on x86 there will be a measurable cost
> in misaligned accesses - at least for some workloads.

When they cross cache-line boundaries, yes.

Maybe napi_get_frags() should be adding the 2 byte offset on all
architectures, as the skbs it allocates are never RX DMA buffers.

> If the DMA is able to write to a mis-aligned buffer and
> still perform aligned burst transfers mid-frame then
> 4n+2 aligning the rx buffer should be a win even on x86.

I don't think so.

> Note to hardware engineers: add an option to write two
> bytes of junk before the rx data :-)

There is some hardware with that option.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

  reply	other threads:[~2013-05-14 13:48 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2013-05-13 21:58 Ben Hutchings
2013-05-13 22:01 ` [PATCHv2 net 2/2] sfc: Reduce RX scatter buffer size, and reduce alignment if appropriate Ben Hutchings
2013-05-14  7:09   ` Geert Uytterhoeven
2013-05-14 18:32   ` David Miller
2013-07-23 20:19   ` Ben Hutchings
2013-05-14  8:32 ` [PATCHv2 net 1/2] sfc: Delete EFX_PAGE_IP_ALIGN, equivalent to NET_IP_ALIGN David Laight
2013-05-14 13:48   ` Ben Hutchings [this message]
2013-05-14 18:32 ` David Miller

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=1368539323.4304.81.camel@deadeye.wl.decadent.org.uk \
    --to=bhutchings@solarflare.com \
    --cc=David.Laight@ACULAB.COM \
    --cc=davem@davemloft.net \
    --cc=geert@linux-m68k.org \
    --cc=heiko.carstens@de.ibm.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-net-drivers@solarflare.com \
    --cc=linux-s390@vger.kernel.org \
    --cc=netdev@vger.kernel.org \
    --subject='Re: [PATCHv2 net 1/2] sfc: Delete EFX_PAGE_IP_ALIGN, equivalent to NET_IP_ALIGN' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).