linux-kernel - Re: [PATCH] iommu/arm-smmu: Demote error messages to debug in shutdown callback

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Message-ID: <CAD=FV=VBM6cS1UmWTUJ2vrt0a2zn7xV3C53tpthBx58M2=1JPg@mail.gmail.com>
Date:   Wed, 22 Apr 2020 12:49:57 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
Cc:     Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
        Joerg Roedel <joro@...tes.org>,
        "list@....net:IOMMU DRIVERS , Joerg Roedel <joro@...tes.org>," 
        <iommu@...ts.linux-foundation.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] iommu/arm-smmu: Demote error messages to debug in
 shutdown callback

Hi,

On Tue, Mar 31, 2020 at 12:53 AM Sai Prakash Ranjan
<saiprakash.ranjan@...eaurora.org> wrote:
>
> Hi Will,
>
> On 2020-03-31 13:14, Will Deacon wrote:
> > On Tue, Mar 31, 2020 at 01:06:11PM +0530, Sai Prakash Ranjan wrote:
> >> On 2020-03-30 23:54, Doug Anderson wrote:
> >> > On Sat, Mar 28, 2020 at 12:35 AM Sai Prakash Ranjan
> >> > <saiprakash.ranjan@...eaurora.org> wrote:
> >> > >
> >> > > > Of course the fact that in practice we'll *always* see the warning
> >> > > > because there's no way to tear down the default DMA domains, and even
> >> > > > if all devices *have* been nicely quiesced there's no way to tell, is
> >> > > > certainly less than ideal. Like I say, it's not entirely clear-cut
> >> > > > either way...
> >> > > >
> >> > >
> >> > > Thanks for these examples, good to know these scenarios in case we
> >> > > come
> >> > > across these.
> >> > > However, if we see these error/warning messages appear everytime then
> >> > > what will be
> >> > > the credibility of these messages? We will just ignore these messages
> >> > > when
> >> > > these issues you mention actually appears because we see them
> >> > > everytime
> >> > > on
> >> > > reboot or shutdown.
> >> >
> >> > I would agree that if these messages are expected to be seen every
> >> > time, there's no way to fix them, and they're not indicative of any
> >> > problem then something should be done.  Seeing something printed at
> >> > "dev_error" level with an exclamation point (!) at the end makes me
> >> > feel like this is something that needs immediate action on my part.
> >> >
> >> > If we really can't do better but feel that the messages need to be
> >> > there, at least make them dev_info and less scary like:
> >> >
> >> >   arm-smmu 15000000.iommu: turning off; DMA should be quiesced before
> >> > now
> >> >
> >> > ...that would still give you a hint in the logs that if you saw a DMA
> >> > transaction after the message that it was a bug but also wouldn't
> >> > sound scary to someone who wasn't seeing any other problems.
> >> >
> >>
> >> We can do this if Robin is OK?
> >
> > It would be nice if you could figure out which domains are still live
> > when
> > the SMMU is being shut down in your case and verify that it *is* infact
> > benign before we start making the message more friendly. As Robin said
> > earlier, rogue DMA is a real nightmare to debug.
> >
>
> I could see this error message for all the clients of apps_smmu.
> I checked manually enabling bypass and removing iommus dt property
> for each client of apps_smmu.

Any update on the status here?  If I'm reading the conversation above,
Robin said: "we'll *always* see the warning because there's no way to
tear down the default DMA domains, and even if all devices *have* been
nicely quiesced there's no way to tell".  Did I understand that
properly?  If so, it seems like it's fully expected to see this
message on every reboot and it doesn't necessarily signify anything
bad.

-Doug