linux-kernel - Re: Generic DMA

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Message-ID: <20100120105350.GA23394@flint.arm.linux.org.uk>
Date:	Wed, 20 Jan 2010 10:53:50 +0000
From:	Russell King <rmk+lkml@....linux.org.uk>
To:	monstr@...str.eu
Cc:	linux-kernel@...r.kernel.org, michal.simek@...alogix.com,
	arnd@...db.de, linux-arch@...r.kernel.org,
	fujita.tomonori@....ntt.co.jp, akpm@...ux-foundation.org,
	mingo@...e.hu, joerg.roedel@....com
Subject: Re: Generic DMA - BUG_ON

On Wed, Jan 20, 2010 at 11:08:30AM +0100, monstr@...str.eu wrote:
> Hi All,
> 
> I have this patch in my repo. I just added BUG_ON for dma ops.
> The reason for that is if driver not setup ops correctly than
> the system do bad access to any memory space without any visible reason.
> BUG_ON points to it and helps to solve where the problem is.

I have a question of principle to raise here.

If you have code which does:

	if (ops->foo)
		ops->foo();

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.

If you add a BUG_ON() to this, the only additional information you end
up with is (possibly) a nice friendly message which tells you the file
and line number - but you add additional run-time tests to the code.

The question is: is this worth it?  Is there some problem with the
original oops that makes the problem hard to find?  Should we be using
BUG_ON() to augment normal oopses in this way?

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:
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