linux-kernel - Re: [2.6.35-rc3] select useful number of entries for DMA debugging...

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Message-ID: <AANLkTin24TP3qcCb4c0-dLfqysyBDYSrPiO3UXtp-Y1n@mail.gmail.com>
Date:	Sun, 11 Jul 2010 12:54:31 +0100
From:	Daniel J Blueman <daniel.blueman@...il.com>
To:	Joerg Roedel <joro@...tes.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [2.6.35-rc3] select useful number of entries for DMA debugging...

On 10 July 2010 16:43, Joerg Roedel <joro@...tes.org> wrote:
> On Sat, Jul 10, 2010 at 03:52:04PM +0100, Daniel J Blueman wrote:
>> On 9 July 2010 22:33, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>> > On Mon, Jul 5, 2010 at 1:35 AM, Daniel J Blueman
>> > <daniel.blueman@...il.com> wrote:
>> >> When booting 2.6.35-rc3 on some different x86 boxes with DMA debugging
>> >> enabled, I've consistently seen it exhaust the allocated entries during
>> >> boot, giving 'DMA-API: debugging out of memory - disabling'.
>> >>
>> >> Increase number of entries to allow DMA debugging again.
>> >
>> > Rather than increase the default that gets allocated whenever anybody
>> > enables the DMA debugging, I'd really prefer to see people use the
>> > kernel command line option if they run out. After all, it's a (pretty
>> > esoteric) debug option, and the number of required entries depends on
>> > machine configuration. I'd rather not make the default cover a huge
>> > number, when you could just add
>> >
>> >   dma_debug_entries=65536
>> >
>> > on the kernel boot command line instead for machines that want/need it..
>>
>> That said, I am seeing the DMA pool exhaust on a single socket Core i5
>> system with Intel graphics and no other adapters - seems like a fairly
>> common case. If eg 25% of developers will be using similar to this,
>> maybe it's good to make DMA debugging less immediately esoteric?
>>
>> On the other hand, I would immediately agree if the exhaustion
>> occurred on an atypical setup.
>
> How much memory do you have in this machine? We could probably make the
> number of pre-allocated entries dependent on the memory available in the
> machine like Ingo suggested some time ago to avoid such problems.

I have 4GB. Since this change is specific to x86, I guess the only
corner case we need to protect from this change is developers on small
x86 embedded systems such as MIDs, so lowering the allocated size on
<1GB systems makes sense.
-- 
Daniel J Blueman
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