linux-kernel - Re: [PATCH] x86, aperture: Check for GART before accessing GART registers

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Date:	Tue, 7 Apr 2015 09:46:26 -0500
From:	Aravind Gopalakrishnan <aravind.gopalakrishnan@....com>
To:	Borislav Petkov <bp@...e.de>
CC:	Ingo Molnar <mingo@...nel.org>, <tglx@...utronix.de>,
	<mingo@...hat.com>, <hpa@...or.com>, <x86@...nel.org>,
	<bhelgaas@...gle.com>, <linux-kernel@...r.kernel.org>,
	<Suravee.Suthikulpanit@....com>, <joro@...tes.org>
Subject: Re: [PATCH] x86, aperture: Check for GART before accessing GART registers

On 4/7/2015 7:34 AM, Borislav Petkov wrote:
> On Mon, Apr 06, 2015 at 06:10:22PM -0500, Aravind Gopalakrishnan wrote:
>>> So what happens if !early_pci_allowed() but the GART is present? We'll
>>> set amd_northbridges.flags |= AMD_NB_GART, but won't run any of the
>>> setup code in aperture_64.c, right? Is that a valid setup?
>> It might be a valid setup. But it would work correctly only if BIOS did the
>> right thing with setting up aperture space.
>>
>> If !early_pci_allowed() and BIOS did not setup aperture correctly, that
>> would have caused problems already.
>> But it has not been an issue so far right?
> I think the right question to ask is:
>
> What are we doing when there's no GART?
>
> So what you probably want to do is:
>
> 	if (!amd_gart_present())
> 		return -ENODEV;
>
> *before* the compound check.
>
> Now, if we have detected a GART, i.e., amd_gart_present() is TRUE, the
> code will behave as it used to behave before.
>
> This should be the least intrusive change going forward IMHO.
>

Okay. I'll do that and correct the typos Ingo pointed out earlier and 
resend.

Thanks,
-Aravind.

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