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

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Message-ID: <550BEA98.8010609@amd.com>
Date:	Fri, 20 Mar 2015 10:38:32 +0100
From:	Christian König <christian.koenig@....com>
To:	David Miller <davem@...emloft.net>
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.

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.

Regards,
Christian.
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