linux-kernel - Re: [PATCH] radeon: Do not directly dereference pointers to BIOS area.

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Message-Id: <20150320.132409.66093972510859656.davem@davemloft.net>
Date:	Fri, 20 Mar 2015 13:24:09 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	christian.koenig@....com
Cc:	alexander.deucher@....com, dri-devel@...ts.freedesktop.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] radeon: Do not directly dereference pointers to BIOS
 area.

From: Christian König <christian.koenig@....com>
Date: Fri, 20 Mar 2015 10:38:32 +0100

> On 19.03.2015 17:29, David Miller wrote:
>> From: Christian König <christian.koenig@....com>
>> Date: Thu, 19 Mar 2015 09:50:58 +0100
>>
>>> In general I would say yes, but for this particular hardware it's a
>>> bit questionable to do so.
>>>
>>> For radeon hardware to work correctly the CPU access to the PCIE BARs
>>> should work even without using the specialized IO macros/functions,
>>> otherwise mapping VRAM CPU accessible isn't really possible.
>>>
>>> What's the background of the change? Some problems on a certain CPU
>>> platform? or just general cleanups?
>> It's an _iomem_ pointer, it's not a virtual address.
>>
>> Therefore it is illegal to dereference the pointer.
>>
>> The value is opaque and has values that only make sense when used
>> with the readb() et al. interfaces.
>>
>> This code is relying upon the fact that on x86 it happens to be
>> a virtual address, but this won't work on many other architectures.
> 
> In this case I'm perfectly fine with it and the patch is Reviewed-by:
> Christian König <christian.koenig@....com>
> 
> Just wanted to make sure that you're not trying to get Radeon working
> on a platform which will never really support the necessary hardware
> features.

I would like this to get merged via the Radeon DRM maintainer, thanks.
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