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

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Date:   Tue, 31 Mar 2020 13:23:33 +0530
From:   Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
To:     Will Deacon <will@...nel.org>
Cc:     Doug Anderson <dianders@...omium.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 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.

Thanks,
Sai

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