RE: [PATCH] pch_gbe: Adding read memory barriers

On Wed, 2012-05-09 at 11:47 +0100, David Laight wrote:

quoted

Under a strong incoming packet stream and/or high cpu usage,
the pch_gbe driver reports "Receive CRC Error" and discards packet.

It occurred on an Intel ATOM E620T while running a 
300mbit/sec multicast
network stream leading to a ~100% cpu usage.

Adding rmb() calls before considering the network card's status solve
this issue.

Getting it into stable would be perfect as it solves 
reliability issues.

Signed-off-by: Erwan Velu <redacted>
---
  .../net/ethernet/oki-semi/pch_gbe/pch_gbe_main.c   |    3 +++
  1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/drivers/net/ethernet/oki-semi/pch_gbe/pch_gbe_main.c 

b/drivers/net/ethernet/oki-semi/pch_gbe/pch_gbe_main.c
index 8035e5f..ace3654 100644

--- a/drivers/net/ethernet/oki-semi/pch_gbe/pch_gbe_main.c
+++ b/drivers/net/ethernet/oki-semi/pch_gbe/pch_gbe_main.c

@@ -1413,6 +1413,7 @@ static irqreturn_t pch_gbe_intr(int 

irq, void *data)
  			pch_gbe_mac_set_pause_packet(hw);
  		}
  	}
+	smp_rmb(); /* prevent any other reads before*/

Under the assumption that your memory references are uncached,
you only need to stop gcc reordering the object code,
Rather than actually adding one of the 'fence' instructions.

Also, the usual MMIO functions already include compiler barriers.

So you should only need: asm volatile(:::"memory")
NFI which define generates that, the defines in the copy of
sysdep.h I just looked at always include one of the fences.

This is barrier().  But I think this must be intended to control
ordering of fields that are written elsewhere.

Really, this needs a much more specific comment to explain the intent.
Then reviewers can work out whether it actually achieves the intent!

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.