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

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Date:   Fri, 9 Jul 2021 14:56:47 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Joerg Roedel <joro@...tes.org>,
        Doug Anderson <dianders@...omium.org>
Cc:     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>,
        Joerg Roedel <joro@...tes.org>,
        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 2021-07-08 09:08, Joerg Roedel wrote:
> On Wed, Jul 07, 2021 at 01:00:13PM -0700, Doug Anderson wrote:
>> a) Nothing is inherently broken with my current approach.
>>
>> b) My current approach doesn't make anybody terribly upset even if
>> nobody is totally in love with it.
> 
> Well, no, sorry :)
> 
> I don't think it is a good idea to allow drivers to opt-out of the
> strict-setting. This is a platform or user decision, and the driver
> should accept whatever it gets.
> 
> So the real question is still why strict is the default setting and how
> to change that. Or document for the users that want performance how to
> change the setting, so that they can decide.

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

The "make this device go faster because I trust it" use-case is why we 
have the sysfs interface to switch between 2 and 4, so it's entirely 
logical to have the intermediate option as well for when 3 is "faster" 
enough while still giving a bit more peace of mind than full-on bypass.

Making it a platform-specific decision that's hidden in a driver - 
arm-smmu-qcom can be considered a dumping ground of detailed platform 
knowledge ;) - happens to work as a reasonable compromise for this 
specific case, but I concur that it could be viewed as setting a 
precedent for other cases which we definitely aren't as reasonable.

I've been thinking about the sysfs thing some more, and since it's 
Friday afternoon and I can't concentrate on what I'm supposed to be 
doing anyway, let's see how far I can get by Monday...

Robin.