linux-kernel - Re: [PATCH] iommu/dma-iommu: properly respect configured address space size

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Message-ID: <9d239c5c-a4ed-f177-319f-7c467a6059ba@arm.com>
Date:   Thu, 10 Nov 2016 16:17:30 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     Marek Szyprowski <m.szyprowski@...sung.com>,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] iommu/dma-iommu: properly respect configured address
 space size

On 10/11/16 15:59, Joerg Roedel wrote:
> On Tue, Nov 08, 2016 at 11:37:23AM +0000, Robin Murphy wrote:
>> TBH I've been pondering ripping the size stuff out of dma-iommu, as it
>> all stems from me originally failing to understand what dma_32bit_pfn is
>> actually for.
> 
> The point of dma_32bit_pfn is to allocate dma-address below 4G by
> default. This is a performance optimization so that even devices capable
> of 64bit DMA are using SAC by default instead of DAC.
> 
> Since it is the goal to share a dma-iommu implemenation between
> architectures, I would rather prefer not to rip this stuff out.

Oh, I didn't mean rip it out entirely, just get rid of the bogus
assumption that it's the "size" of the domain, especially when given a
>32-bit DMA mask, since that defeats the very optimisation I do now
understand (although it might still be OK for platform devices where
SAC/DAC doesn't apply, to avoid the rb_last() overhead every time).

>From the patch I've started, "rip it out" turns out to actually be
mostly "rewrite the comments" anyway - I'll post something soon.

Robin.