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

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Date:   Thu, 23 Apr 2020 13:47:03 +0530
From:   Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
To:     Doug Anderson <dianders@...omium.org>,
        Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>
Cc:     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>,
        linux-arm-msm-owner@...r.kernel.org
Subject: Re: [PATCH] iommu/arm-smmu: Demote error messages to debug in
 shutdown callback

Hi,

On 2020-04-23 01:19, Doug Anderson wrote:
> 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.
> 

Understanding is the same, waiting for Will and Robin to check if its OK
to make the message more friendly.

-Sai
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