From: Arnd Bergmann <arnd@arndb.de>
To: monstr@monstr.eu
Subject: Re: Generic DMA - BUG_ON
Date: Wed, 20 Jan 2010 20:03:12 +0100
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Cc: Joerg Roedel <joerg.roedel@amd.com>, linux-kernel@vger.kernel.org,
       michal.simek@petalogix.com, linux-arch@vger.kernel.org,
       fujita.tomonori@lab.ntt.co.jp, akpm@linux-foundation.org, mingo@elte.hu
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On Wednesday 20 January 2010, Michal Simek wrote:
> Joerg Roedel wrote:
> > On Wed, Jan 20, 2010 at 10:53:50AM +0000, Russell King wrote:
> >> and ops is NULL, then this code will oops; you will get a full register
> >> dump and backtrace.  You can use this information along with markup_oops.pl
> >> to find out where the problem is.
> > 
> > You can't rely on the oops if the code runs in process context. The
> > process may have address 0 mapped which would result in a security hole.
> > We had two of these bugs last year.
> 
> That's the same problem which I had some days ago and Microblaze misses 
> valuable backtrace (because we don't have FP or constant frame size).

You can do what x86 does and just print anything in the stack that looks
like part of a kernel function.

> > But I don't see any point in checking for dma_ops != NULL too. Any
> > developer would mention such a bug long before init is started.
> 
> I agree that checking adds extra cycles to every dma-api call.
> 
> I like as wrote Russel to check if ops exists or not.

If you are worried about the overhead, you could just add the BUG_ON
to the map calls but not to unmap and sync, which are rather unlikely
(and incorrect) to be called before a map.

	Arnd
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