linux-kernel - Re: [PATCH] x86: Reserve legacy VGA MMIO area for x86_64 as well as x86

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Message-ID: <4BBFB07E.9020108@zytor.com>
Date:	Fri, 09 Apr 2010 15:55:58 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	Bjorn Helgaas <bjorn.helgaas@...com>,
	Yinghai <yinghai.lu@...cle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, linux-pci@...r.kernel.org,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	Andy Isaacson <adi@...apodia.org>,
	Thomas Renninger <trenn@...e.de>
Subject: Re: [PATCH] x86: Reserve legacy VGA MMIO area for x86_64 as well
 as x86_32

On 04/09/2010 03:51 PM, Alan Cox wrote:
>> Why is this different for 64-bit vs 32-bit?  Can you point me to any
>> references where I can learn about this?
> 
> IMHO its wrong for both
> 
> You can only reserve the region in question if you actually have a VGA
> device and mappings present.
> 
> It's wrong for non PCI systems
> It's wrong for legacy ISA systems with monochrome video/no video
> It's wrong for several embedded platforms.
> It's wrong if PCI isn't your root bridge
> 
> Basically the reservation is the wrong way to fix it. A much saner way to
> fix it would be to simply keep a list of address ranges not to use for
> PCI device relocation. For I/O ports of course we just fix up the PCI
> resources of the device to list them as we discover it (IDE legacy).
>
> You don't want to put anything at the VGA address that needs assigning an
> address. That is *totally* different to the question of whether you want
> to believe the space is 'reserved'. If something is at that address then
> presumably the firmware knows what it is doing. If a device driver wishes
> to reserve that address it's doing so with more information, later in
> boot so should be allowed to.

That's what I mean with reserved, I'm using the term in the
E820_RESERVED sense.  As in "don't put anything there without it being
an explicit assignment".

	-hpa
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