linux-kernel - Re: [PATCH V2 1/4] arm: mvebu: increase atomic coherent pool size for armada 370/XP

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Date:	Wed, 07 Nov 2012 00:07:01 +0100
From:	Sören Moch <smoch@....de>
To:	Andrew Lunn <andrew@...n.ch>
CC:	Lior Amsalem <alior@...vell.com>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Ian Molton <ian.molton@...ethink.co.uk>,
	Jason Cooper <jason@...edaemon.net>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux@....linux.org.uk,
	Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>,
	Gregory Clement <gregory.clement@...e-electrons.com>,
	m.szyprowski@...sung.com
Subject: Re: [PATCH V2 1/4] arm: mvebu: increase atomic coherent pool size
 for armada 370/XP


 >>> For Armada 370/XP we have the same problem that for the commit
 >>> cb01b63, so we applied the same solution: "The default 256 KiB
 >>> coherent pool may be too small for some of the Kirkwood devices, so
 >>> increase it to make sure that devices will be able to allocate their
 >>> buffers with GFP_ATOMIC flag"
 >>
 >> I see a regression from linux-3.5 to linux-3.6 and think there might
 >> be a fundamental problem
 >> with this patch. On my Kirkwood system (guruplug server plus) with
 >> linux-3.6.2 I see following
 >> errors and corresponding malfunction even with further increased
 >> (2M, 4M) pool size:
 >>
 >> Oct 19 00:41:22 guru kernel: ERROR: 4096 KiB atomic DMA coherent
 >> pool is too small!
 >> Oct 19 00:41:22 guru kernel: Please increase it with coherent_pool=
 >> kernel parameter!
 >>
 >> So I had to downgrade to linux-3.5 which is running without problems.
 >>
 >> I use SATA and several DVB sticks (em28xx / drxk and dib0700).
 >
 > I'm guess its the DVB sticks which are causing the problems. We have a
 > number of kirkwood devices with two SATA devices which had problems
 > until we extended the coherent_pool. The DVB sticks are probably take
 > more coherent RAM. There was also an issue found recently:
 >
 > http://www.spinics.net/lists/arm-kernel/msg203962.html
 >
 > That conversation has gone quiet, but that could be because the
 > participants are at ELCE.
 >
 >          Andrew

OK, I hope this GFP flag correction will help.

Could there be a fragmentation problem in the coherent_pool with the
different drivers running under heavy load?
With a pool size of 1M I see this error after several minutes, with a 4M
pool I see this error after several 10 minutes. Difficult to test, but not
acceptable on a production system.

Soeren
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