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

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Date:	Thu, 17 Jun 2010 23:05:59 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	mchan@...adcom.com
Cc:	JBottomley@...ell.com, fujita.tomonori@....ntt.co.jp,
	vapier@...too.org, netdev@...r.kernel.org,
	linux-parisc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: bnx2 fails to compile on parisc because of missing
 get_dma_ops()

On Thu, 17 Jun 2010 06:30:39 -0700
"Michael Chan" <mchan@...adcom.com> wrote:

> James Bottomley wrote:
> 
> > On Thu, 2010-06-17 at 05:54 -0700, Michael Chan wrote:
> > > 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.
> 
> The original patch was an unconditional prefetch.  There was
> some discussion that it might not be correct if the DMA wasn't
> sync'ed yet on some archs.  If the concensus is that it is ok to
> do so, that would be the simplest solution.

As James said, it just adds useless prefetch on incoherent
architectures. sync_single_for_cpu is called later so we can see the
correct data. One useless prefetch is unlikely to lead performance
drop.

You might prefer this v2.

=
From: FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Subject: [PATCH v2] 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.

This patch removes dma_get_ops() and changes bnx2 to do prefetch on
all the architectures. This adds useless prefetch on incoherent
architectures but this is harmless. It is also unlikely to cause the
performance drop.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
---
 drivers/net/bnx2.c |   10 ++++------
 1 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/drivers/net/bnx2.c b/drivers/net/bnx2.c
index 949d7a9..85f1692 100644
--- a/drivers/net/bnx2.c
+++ b/drivers/net/bnx2.c
@@ -3099,12 +3099,10 @@ bnx2_rx_int(struct bnx2 *bp, struct bnx2_napi *bnapi, int budget)
 		skb = rx_buf->skb;
 		prefetchw(skb);
 
-		if (!get_dma_ops(&pdev->dev)->sync_single_for_cpu) {
-			next_rx_buf =
-				&rxr->rx_buf_ring[
-					RX_RING_IDX(NEXT_RX_BD(sw_cons))];
-			prefetch(next_rx_buf->desc);
-		}
+		next_rx_buf =
+			&rxr->rx_buf_ring[RX_RING_IDX(NEXT_RX_BD(sw_cons))];
+		prefetch(next_rx_buf->desc);
+
 		rx_buf->skb = NULL;
 
 		dma_addr = dma_unmap_addr(rx_buf, mapping);
-- 
1.6.5

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