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Phylib polling when doing mdio_read will cause system response and transfer speed drop

DISCUSSION Bryan Wu 2 participants 2 messages Apr 14, 2010 – Apr 19, 2010

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Bryan Wu

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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,

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Bryan Wu <[email protected]>
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
#1 lore raw
Quoted 3 lines · Bryan Wu

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:

Same here, I saw the 'MDIO Timeout' Message occasionally.

Quoted 5 lines

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.

Quoted 8 lines

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.

Can't help with the details, but the patch seems to help here, too (and not only because of the removed printk ;)).

Regards,

Wolfram

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Pengutronix e.K.                           | Wolfram Sang                |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |