netdev - Re: [PATCH 14/17] iommu: remove DOMAIN_ATTR_DMA_USE_FLUSH

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Message-ID: <dff8eb80-8f74-972b-17e9-496c1fc0396f@arm.com>
Date:   Fri, 12 Mar 2021 16:18:24 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Christoph Hellwig <hch@....de>
Cc:     Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Li Yang <leoyang.li@....com>, freedreno@...ts.freedesktop.org,
        kvm@...r.kernel.org, Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev@...ts.ozlabs.org, dri-devel@...ts.freedesktop.org,
        virtualization@...ts.linux-foundation.org,
        iommu@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-arm-msm@...r.kernel.org,
        David Woodhouse <dwmw2@...radead.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 14/17] iommu: remove DOMAIN_ATTR_DMA_USE_FLUSH_QUEUE

On 2021-03-11 08:26, Christoph Hellwig wrote:
> On Wed, Mar 10, 2021 at 06:39:57PM +0000, Robin Murphy wrote:
>>> Actually... Just mirroring the iommu_dma_strict value into
>>> struct iommu_domain should solve all of that with very little
>>> boilerplate code.
>>
>> Yes, my initial thought was to directly replace the attribute with a
>> common flag at iommu_domain level, but since in all cases the behaviour
>> is effectively global rather than actually per-domain, it seemed
>> reasonable to take it a step further. This passes compile-testing for
>> arm64 and x86, what do you think?
> 
> It seems to miss a few bits, and also generally seems to be not actually
> apply to recent mainline or something like it due to different empty
> lines in a few places.

Yeah, that was sketched out on top of some other development patches, 
and in being so focused on not breaking any of the x86 behaviours I did 
indeed overlook fully converting the SMMU drivers... oops!

(my thought was to do the conversion for its own sake, then clean up the 
redundant attribute separately, but I guess it's fine either way)

> Let me know what you think of the version here:
> 
> http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/iommu-cleanup
> 
> I'll happily switch the patch to you as the author if you're fine with
> that as well.

I still have reservations about removing the attribute API entirely and 
pretending that io_pgtable_cfg is anything other than a SoC-specific 
private interface, but the reworked patch on its own looks reasonable to 
me, thanks! (I wasn't too convinced about the iommu_cmd_line wrappers 
either...) Just iommu_get_dma_strict() needs an export since the SMMU 
drivers can be modular - I consciously didn't add that myself since I 
was mistakenly thinking only iommu-dma would call it.

Robin.