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

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Date:   Wed, 14 Jul 2021 08:14:24 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Joerg Roedel <joro@...tes.org>, 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>,
        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

Hi,

On Tue, Jul 13, 2021 at 11:07 AM Robin Murphy <robin.murphy@....com> wrote:
>
> 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

Being able to change this at runtime through sysfs sounds great and it
fills all the needs I'm aware of, thanks! In Chrome OS we can just use
this with some udev rules and get everything we need. I'm happy to
give this a spin when you're ready for extra testing.

-Doug