linux-kernel - Re: [PATCH 2/2] x86 amd_gart_64: Verify we can perform the remapping requested

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Date:	Fri, 11 Nov 2011 10:59:33 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Joerg Roedel <joro@...tes.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] x86 amd_gart_64: Verify we can perform the remapping requested

Joerg Roedel <joro@...tes.org> writes:

> On Mon, Oct 24, 2011 at 12:07:36PM +0200, Joerg Roedel wrote:
>> On Mon, Oct 17, 2011 at 02:20:15PM -0700, Eric W. Biederman wrote:
>> > 
>> > Recently I had a driver try with a peculiar 2G dma memory limit.
>> > The driver failed in weird and strange ways because the GART remapping
>> > apperture had been allocated above 2G where the driver cound not reach,
>> > and no error was reported when the mappings were setup.
>> > 
>> > Implement gart_dma_supported to test for this problem case and to return
>> > and error if we can not support the remapping.
>> 
>> You do basically the same as for swiotlb, so it must be good :)
>
> Okay, thinking more about this, I am not so sure anymore it is a really
> good idea to fix it this way. It basically signals the driver that DMA
> is not possible with the device if the aperture does not fit into
> the dma_mask. But DMA within the ZONE_DMA might still work, no?
>
> So I think the right way to fix it is to return a dma-mapping error if
> and only if the address needs to be remapped by the aperture (of swiotlb or
> gart).

Perhaps I am mistaken but since the method is iommu type specific if we
don't actually use the iommu we should not call the method so I think
this is patch is already implementing what you want.

> This still allows the driver to use GFP_DMA allocations.

I definitely agree on that.

Eric
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