Date: Mon, 4 May 2015 13:11:46 +0200
From: Joerg Roedel <joro@8bytes.org>
To: Borislav Petkov <bp@alien8.de>
Cc: =?iso-8859-1?Q?J=F6rg-Volker?= Peetz <jvpeetz@web.de>,
        linux-kernel@vger.kernel.org, Ingo Molnar <mingo@kernel.org>,
        tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com, x86@kernel.org,
        bhelgaas@google.com, Suravee.Suthikulpanit@amd.com
Subject: Re: [PATCH] x86, aperture: Check for GART before accessing GART
 registers
Message-ID: <20150504111146.GF15736@8bytes.org>
References: <551D661D.3030000@amd.com>
 <20150402160634.GB8045@gmail.com>
 <551D6CF5.1010604@amd.com>
 <20150402165356.GE4579@pd.tnic>
 <551D7695.2010509@amd.com>
 <20150402171705.GF4579@pd.tnic>
 <552CF6DF.3000002@web.de>
 <20150414113349.GA26075@pd.tnic>
 <552D5AF8.3070705@web.de>
 <20150414183643.GF14069@pd.tnic>
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On Tue, Apr 14, 2015 at 08:36:43PM +0200, Borislav Petkov wrote:
> 64MB out of how much? Does it even matter?
> 
> You could turn off CONFIG_GART_IOMMU and not build aperture_64.c at all.
> 
> I think then you'll fall back to swiotlb or whatever and that will most
> likely slow down DMA ... the story was something like that, at least,
> memory's hazy. Joerg's on CC, he'll correct me if I'm talking bull.

Using SWIOTLB will also eat 64MB of RAM, so it doesn't matter if you use
it instead of the GART driver. So whether your machine has GART or not,
it will cost you 64MB of RAM for DMA remapping (unless you have a real
IOMMU).


	Joerg

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