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From: Bryan Wu on 13 Apr 2010 20:30
Hi Andy and David,

After I posted a patch to add phylib supporting in drivers/net/fec.c, we found
performance drop regressions on Freescale i.MX51 babbage board.

Patch is
http://git.kernel.org/?p=linux/kernel/git/davem/net-next-2.6.git;a=commitdiff;h=e6b043d512fa8d9a3801bf5d72bfa3b8fc3b3cc8.

Bug tracker is here:
https://bugs.launchpad.net/ubuntu/+source/linux-fsl-imx51/+bug/546649

I found the root cause is the polling operation in the mdio_read function. When
we transfer large files, we experienced many times of timeout issue. So I got
several question here:
1. Need I return -ETIMEDOUT when polling timeout. If I don't return -ETIMEOUT,
the performance improved a lot. And after check other drivers, some don't return
anything, some return 0, some return negative value. What's the rule for this
mdio_read polling timeout case.

2. How to do polling busy waiting? Normally, we won't buys wait very long in
polling. But hardware is not perfect every time. Running cpu_relax() 10000 times
in polling will cause our system response very bad when hardware don't set the
flag as we expected. Maybe udelay(25) 10 times or msleep(1) 10 times is better
than that.

I got a patch to recover this issue,
http://kernel.ubuntu.com/git?p=roc/ubuntu-lucid.git;a=commitdiff;h=5d77e3409b319ca84183bf1d2fd158a9c864e03f.

Thanks a lot,
--
Bryan Wu <bryan.wu(a)canonical.com>
Kernel Developer +86.138-1617-6545 Mobile
Ubuntu Kernel Team | Hardware Enablement Team
Canonical Ltd. www.canonical.com
Ubuntu - Linux for human beings | www.ubuntu.com
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