From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bryan Wu Subject: Phylib polling when doing mdio_read will cause system response and transfer speed drop Date: Tue, 13 Apr 2010 17:27:33 -0700 Message-ID: <4BC50BF5.7080700@canonical.com> Reply-To: bryan.wu@canonical.com Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: netdev@vger.kernel.org, LKML To: afleming@freescale.com, davem@davemloft.net Return-path: Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org 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 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