Re: [PATCH v6 0/7] iommu/arm-smmu, drm/msm: Fixes for stall-on-fault

[Will Deacon <will@kernel.org>]

Hi Connor,

On Thu, May 15, 2025 at 03:58:42PM -0400, Connor Abbott wrote:
> drm/msm uses the stall-on-fault model to record the GPU state on the
> first GPU page fault to help debugging. On systems where the GPU is
> paired with a MMU-500, there were two problems:
> 
> 1. The MMU-500 doesn't de-assert its interrupt line until the fault is
>    resumed, which led to a storm of interrupts until the fault handler
>    was called. If we got unlucky and the fault handler was on the same
>    CPU as the interrupt, there was a deadlock.
> 2. The GPU is capable of generating page faults much faster than we can
>    resume them. GMU (GPU Management Unit) shares the same context bank
>    as the GPU, so if there was a sudden spurt of page faults it would be
>    effectively starved and would trigger a watchdog reset, made even
>    worse because the GPU cannot be reset while there's a pending
>    transaction leaving the GPU permanently wedged.
> 
> Patches 1-2 and 4 fix the first problem by switching the IRQ to be a
> threaded IRQ and then making drm/msm do its devcoredump work
> synchronously in the threaded IRQ. Patch 4 is dependent on patches 1-2.
> Patch 6 fixes the second problem and is dependent on patch 3. Patch 5 is
> a cleanup for patch 4 and patch 7 is a subsequent further cleanup to get
> rid of the resume_fault() callback once we switch resuming to being done
> by the SMMU's fault handler.

Thanks for reworking this; I think it looks much better now from the
SMMU standpoint.

> I've organized the series in the order that it should be picked up:
> 
> - Patches 1-3 need to be applied to the iommu tree first.

Which kernel version did you base these on? I can't see to apply the
second patch, as you seem to have a stale copy of arm-smmu-qcom.c?

Will