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

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Message-ID: <e3555c49-2978-355f-93bb-dbfa7d09cab8@arm.com>
Date:   Tue, 13 Jul 2021 19:07:00 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Doug Anderson <dianders@...omium.org>,
        Joerg Roedel <joro@...tes.org>
Cc:     Will Deacon <will@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        John Garry <john.garry@...wei.com>,
        Rob Clark <robdclark@...omium.org>, quic_c_gdjako@...cinc.com,
        Saravana Kannan <saravanak@...gle.com>,
        Rajat Jain <rajatja@...gle.com>,
        Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>,
        Veerabhadrarao Badiganti <vbadigan@...eaurora.org>,
        Linux MMC List <linux-mmc@...r.kernel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        linux-pci@...r.kernel.org,
        "list@....net:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        Joerg Roedel <joro@...tes.org>,
        iommu@...ts.linux-foundation.org,
        Sonny Rao <sonnyrao@...omium.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        Jordan Crouse <jordan@...micpenguin.net>,
        Konrad Dybcio <konrad.dybcio@...ainline.org>,
        Krishna Reddy <vdumpa@...dia.com>,
        "Maciej W. Rozycki" <macro@...am.me.uk>,
        Nicolin Chen <nicoleotsuka@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 0/3] iommu: Enable non-strict DMA on QCom SD/MMC

On 2021-07-08 15:36, Doug Anderson wrote:
[...]
>> Or document for the users that want performance how to
>> change the setting, so that they can decide.
> 
> Pushing this to the users can make sense for a Linux distribution but
> probably less sense for an embedded platform. So I'm happy to make
> some way for a user to override this (like via kernel command line),
> but I also strongly believe there should be a default that users don't
> have to futz with that we think is correct.

FYI I did make progress on the "punt it to userspace" approach. I'm not 
posting it even as an RFC yet because I still need to set up a machine 
to try actually testing any of it (it's almost certainly broken 
somewhere), but in the end it comes out looking surprisingly not too bad 
overall. If you're curious to take a look in the meantime I put it here:

https://gitlab.arm.com/linux-arm/linux-rm/-/commits/iommu/fq

Cheers,
Robin.