From: Robin Murphy <robin.murphy@arm.com>
To: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
Cc: Will Deacon <will@kernel.org>, Joerg Roedel <joro@8bytes.org>,
iommu@lists.linux-foundation.org,
Douglas Anderson <dianders@chromium.org>,
linux-arm-msm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] iommu/arm-smmu: Demote error messages to debug in shutdown callback
Date: Fri, 27 Mar 2020 19:02:58 +0000 [thread overview]
Message-ID: <5858bdac-b7f9-ac26-0c0d-c9653cef841d@arm.com> (raw)
In-Reply-To: <37db9a4d524aa4d7529ae47a8065c9e0@codeaurora.org>
On 2020-03-27 3:09 pm, Sai Prakash Ranjan wrote:
> Hi Robin,
>
> Thanks for taking a look at this.
>
> On 2020-03-27 19:42, Robin Murphy wrote:
>> On 2020-03-27 1:28 pm, Sai Prakash Ranjan wrote:
>>> Currently on reboot/shutdown, the following messages are
>>> displayed on the console as error messages before the
>>> system reboots/shutdown.
>>>
>>> On SC7180:
>>>
>>> arm-smmu 15000000.iommu: removing device with active domains!
>>> arm-smmu 5040000.iommu: removing device with active domains!
>>>
>>> Demote the log level to debug since it does not offer much
>>> help in identifying/fixing any issue as the system is anyways
>>> going down and reduce spamming the kernel log.
>>
>> I've gone back and forth on this pretty much ever since we added the
>> shutdown hook - on the other hand, if any devices *are* still running
>> in those domains at this point, then once we turn off the SMMU and let
>> those IOVAs go out on the bus as physical addresses, all manner of
>> weirdness may ensue. Thus there is an argument for *some* indication
>> that this may happen, although IMO it could be downgraded to at least
>> dev_warn().
>>
>
> Any pointers to the weirdness here after SMMU is turned off?
> Because if we look at the call sites, device_shutdown is called
> from kernel_restart_prepare or kernel_shutdown_prepare which would
> mean system is going down anyways, so do we really care about these
> error messages or warnings from SMMU?
>
> arm_smmu_device_shutdown
> platform_drv_shutdown
> device_shutdown
> kernel_restart_prepare
> kernel_restart
Imagine your network driver doesn't implement a .shutdown method (so the
hardware is still active regardless of device links), happens to have an
Rx buffer or descriptor ring DMA-mapped at an IOVA that looks like the
physical address of the memory containing some part of the kernel text
lower down that call stack, and the MAC receives a broadcast IP packet
at about the point arm_smmu_device_shutdown() is returning. Enjoy
debugging that ;)
And if coincidental memory corruption seems too far-fetched for your
liking, other fun alternatives might include "display tries to scan out
from powered-off device, deadlocks interconnect and prevents anything
else making progress", or "access to TZC-protected physical address
triggers interrupt and over-eager Secure firmware resets system before
orderly poweroff has a chance to finish".
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...
Robin.
next prev parent reply other threads:[~2020-03-27 19:03 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-27 13:28 [PATCH] iommu/arm-smmu: Demote error messages to debug in shutdown callback Sai Prakash Ranjan
2020-03-27 14:12 ` Robin Murphy
2020-03-27 15:09 ` Sai Prakash Ranjan
2020-03-27 16:17 ` Rob Clark
2020-03-27 18:18 ` Sai Prakash Ranjan
2020-03-27 19:02 ` Robin Murphy [this message]
2020-03-28 7:35 ` Sai Prakash Ranjan
2020-03-30 18:24 ` Doug Anderson
2020-03-31 7:36 ` Sai Prakash Ranjan
2020-03-31 7:44 ` Will Deacon
2020-03-31 7:53 ` Sai Prakash Ranjan
2020-04-22 19:49 ` Doug Anderson
2020-04-23 8:17 ` Sai Prakash Ranjan
2020-04-23 9:28 ` Robin Murphy
2020-04-23 9:41 ` Sai Prakash Ranjan
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