linux-kernel - Re: dma-debug: check_for

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Date:	Tue, 16 Jun 2009 14:41:43 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Cc:	joerg.roedel@....com, stefanr@...6.in-berlin.de,
	ben-linux@...ff.org, linux-kernel@...r.kernel.org
Subject: Re: dma-debug: check_for_stack background?

2009/6/16 FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>:
> On Mon, 15 Jun 2009 10:13:55 +0200
> Joerg Roedel <joerg.roedel@....com> wrote:
>
>> On Sun, Jun 14, 2009 at 11:43:42PM +0200, Stefan Richter wrote:
>> > Ben Dooks wrote:
>> >> On Sun, Jun 14, 2009 at 08:56:29PM +0200, Stefan Richter wrote:
>> >>> the DMA debug facility warns  about mappings of on-stack memory.
>> >>> What's the problem with that?
>> > ...
>> >> I'm not sure if it is possible on all architectures, however the
>> >> DMA mapping documentation prohibts the use of stack memory.
>> >
>> > Ah, it has all been documented in plain sight for ages.  Thanks for the
>> > pointer.
>>
>> Problem is that the stack might go away or could be otherwise reused
>> while the DMA is in-flight. This could cause really hard to debug
>> problems.
>
> Not true. Even if you use the stack buffer for DMA synchronously, it
> doesn't work (we had the bad code like starting dma and waiting for
> the completion).

Yes, there is cache alignement problem for stack buffer also.

>
> On some architectures, we can't use the stack buffer for DMA. That's
> why the DMA mapping documentation prohibits the use of stack memory
> for DMA.
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-- 
Lei Ming
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