linux-kernel - Re: [PATCH v2 0/3] iommu: Enable non-strict DMA on QCom SD/MMC

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Message-ID: <255adda2-3b5f-b080-4da1-f3c5d5a4f7a6@arm.com>
Date:   Wed, 14 Jul 2021 11:29:08 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     Doug Anderson <dianders@...omium.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-pci@...r.kernel.org,
        Konrad Dybcio <konrad.dybcio@...ainline.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Rajat Jain <rajatja@...gle.com>, Will Deacon <will@...nel.org>,
        Rob Clark <robdclark@...omium.org>,
        Saravana Kannan <saravanak@...gle.com>,
        Jonathan Corbet <corbet@....net>, quic_c_gdjako@...cinc.com,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Veerabhadrarao Badiganti <vbadigan@...eaurora.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Sonny Rao <sonnyrao@...omium.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Randy Dunlap <rdunlap@...radead.org>,
        Linux MMC List <linux-mmc@...r.kernel.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "list@....net:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Maciej W. Rozycki" <macro@...am.me.uk>
Subject: Re: [PATCH v2 0/3] iommu: Enable non-strict DMA on QCom SD/MMC

On 2021-07-14 11:15, Joerg Roedel wrote:
> Hi Robin,
> 
> On Fri, Jul 09, 2021 at 02:56:47PM +0100, Robin Murphy wrote:
>> As I mentioned before, conceptually I think this very much belongs in sysfs
>> as a user decision. We essentially have 4 levels of "strictness":
>>
>> 1: DMA domain with bounce pages
>> 2: DMA domain
>> 3: DMA domain with flush queue
>> 4: Identity domain
> 
> Together with reasonable defaults (influenced by compile-time
> options) it seems to be a good thing to configure at runtime via
> sysfs.
> 
> We already have CONFIG_IOMMU_DEFAULT_PASSTHROUGH, which can probably be
> extended to be an option list:
> 
> 	- CONFIG_IOMMU_DEFAULT_PASSTHROUGH: Trusted devices are identity
> 					    mapped
> 
> 	- CONFIG_IOMMU_DEFAULT_DMA_STRICT: Trusted devices are DMA
> 					   mapped with strict flush
> 					   behavior on unmap
> 
> 	- CONFIG_IOMMU_DEFAULT_DMA_LAZY: Trusted devices are DMA mapped
> 					 with flush queues for performance

Indeed, I got focused on the sysfs angle, but rearranging the Kconfig 
default that way to match makes a lot of sense, and is another thing 
which should fall out really easily from my domain type rework, so I'll 
add that to my branch now before I forget again.

> Untrusted devices always get into the DMA domain with bounce pages by
> default.
> 
> The defaults can be changed at runtime via sysfs. We already have basic
> support for runtime switching of the default domain, so that can be
> re-used.

As mentioned yesterday, already done! I'm hoping to be able to post the 
patches next week after some testing :)

Cheers,
Robin.