netdev - Re: bnx2 fails to compile on parisc because of missing get_dma

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Message-ID: <1276780336.2789.6.camel@mulgrave.site>
Date:	Thu, 17 Jun 2010 08:12:16 -0500
From:	James Bottomley <JBottomley@...ell.com>
To:	Michael Chan <mchan@...adcom.com>
Cc:	'FUJITA Tomonori' <fujita.tomonori@....ntt.co.jp>,
	"vapier@...too.org" <vapier@...too.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-parisc@...r.kernel.org" <linux-parisc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: bnx2 fails to compile on parisc because of missing
 get_dma_ops()

On Thu, 2010-06-17 at 05:54 -0700, Michael Chan wrote:
> FUJITA Tomonori wrote:
> 
> > From: FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
> > Date: Thu, 17 Jun 2010 13:06:15 +0900
> > Subject: [PATCH] bnx2: fix dma_get_ops compilation breakage
> >
> > This removes dma_get_ops() prefetch optimization in bnx2.
> >
> > bnx2 uses dma_get_ops() to see if dma_sync_single_for_cpu() is
> > noop. bnx2 does prefetch if it's noop.
> >
> > But dma_get_ops() isn't available on all the architectures (only the
> > architectures that uses dma_map_ops struct have it). Using
> > dma_get_ops() in drivers leads to compilation breakage on many
> > archtectures.
> >
> > Currently, we don't have a way to see if dma_sync_single_for_cpu() is
> > noop. If it can improve the performance notably, we can add the new
> > DMA API for it.
> 
> This prefetch improves performance noticeably when the driver is
> handling incoming 64-byte packets at a sustained rate.

So why not do it unconditionally?  The worst that can happen is that you
pull in a stale cache line which will get cleaned in the dma_sync, thus
slightly degrading performance on incoherent architectures.

Alternatively, come up with a dma prefetch infrastructure ... all you're
really doing is hinting to the architecture that you'll sync this region
next.

James


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