linux-kernel - Re: Generic DMA

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Date:	Wed, 20 Jan 2010 12:18:04 +0100
From:	Michal Simek <monstr@...str.eu>
To:	Joerg Roedel <joerg.roedel@....com>
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
Subject: Re: Generic DMA - BUG_ON

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).

> 
> 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.

Let's look at code which is there.
dma_map_single_attrs - calls ops->map_page without any checking
The same for dma_map_sg_attrs, dma_map_page

The rest of functions is ok. If any checking will be there, I don't have 
any problem with it.

Michal




> 
> 	Joerg
> 
> 


-- 
Michal Simek, Ing. (M.Eng)
w: www.monstr.eu p: +42-0-721842854
Maintainer of Linux kernel 2.6 Microblaze Linux - http://www.monstr.eu/fdt/
Microblaze U-BOOT custodian
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