On 2025-06-25 9:42 am, Marek Szyprowski wrote:
On 25.06.2025 09:39, Kaustabh Chakraborty wrote:
On 2025-06-24 17:12, Robin Murphy wrote:
On 2025-06-18 3:02 pm, Kaustabh Chakraborty wrote:
Since bcb81ac6ae3c (iommu: Get DT/ACPI parsing into the proper probe
path),
The Samsung Exynos 7870 DECON device (with patches [1], [2], and
[3]) seems
to not work anymore. Upon closer inspection, I observe that there is an
IOMMU crash.
[ 2.918189] exynos-sysmmu 14860000.sysmmu: 14830000.decon: [READ]
PAGE FAULT occurred at 0x6715b3e0
[ 2.918199] exynos-sysmmu 14860000.sysmmu: Page table base:
0x0000000044a14000
[ 2.918243] exynos-drm exynos-drm: bound 14830000.decon (ops
decon_component_ops)
[ 2.922868] exynos-sysmmu 14860000.sysmmu: Lv1 entry: 0x4205001
[ 2.922877] Kernel panic - not syncing: Unrecoverable System MMU
Fault!
[ 2.922885] CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted
6.16.0-rc2-exynos7870 #722 PREEMPT
[ 2.995312] Hardware name: Samsung Galaxy J7 Prime (DT)
[ 3.000509] Call trace:
[ 3.002938] show_stack+0x18/0x24 (C)
[ 3.006582] dump_stack_lvl+0x60/0x80
[ 3.010224] dump_stack+0x18/0x24
[ 3.013521] panic+0x168/0x360
[ 3.016558] exynos_sysmmu_irq+0x224/0x2ac
[ ...]
[ 3.108786] ---[ end Kernel panic - not syncing: Unrecoverable
System MMU Fault! ]---
For starters, what if you just remove this panic() from the IOMMU
driver? Frankly it seems a bit excessive anyway...
I've tried that, sysmmu repeatedly keeps issuing interrupts (yes, even
after clearing the interrupt bit) indefinitely.
Right, this is because decon device is still accessing system memory in
a loop trying to display the splash screen. That panic is indeed a bit
excessive, but what else IOMMU driver can do if no page fault handle is
registered?
Report the unhandled fault and continue, like most drivers already do.
If there's another fault, then that can get reported as well. It's kind
of the point that if a misbehaving device has been prevented from
accessing memory then it has *not* adversely affected the rest of the
system.
I suppose if one wanted to be really clever then a driver could
implement its own backoff mechanism where if it detects a sustained high
rate of unhandled faults then it disables its interrupt for a bit, to
mitigate the physical interrupt storm as well as avoid flooding the
kernel log more than is useful.
From the logs below it seems there is apparently unexpected traffic
already going through the IOMMU when it wakes up. Is this the DRM
drivers doing something sketchy, or has the bootloader left the
display running for a splash screen? However in the latter case I
don't obviously see why delaying the IOMMU probe should make much
difference, given that the decon driver should still be waiting for
it either way.
The display is initialized by the bootloader for splash yes, but I reckon
it doesn't use the IOMMU as it's accessible from a framebuffer region.
Right, bootloader configured decon device to display splash screen, what
means that decon device is constantly reading splash screen pixel data
from system memory. There is no such thing as a 'framebuffer region', it
is just a system memory, which exynos sysmmu protects when enabled. So
far this issue of splash screen from bootloader has not yet been solved
in mainline. On other Exynos based supported boards this works only
because there are also power domain drivers enabled, which are
instantiated before the display related device and respective sysmmu
device. That power domain driver shuts down power effectively disabling
the display before the sysmmu gets probbed.
And presumably the sysmmu device itself doesn't need to depend on that
power domain? OK, that at least makes sense.
Long time ago I've pointed this issue and proposed some simple solution
like a special initial identity mapping for the memory range used for
splash screen, but that proposal is no longer applicable for the current
code.
As a workaround I would suggest to shutdown display in the decon device
before starting the kernel (i.e. from the 'kernel loading mid-stage
bootloader' if you have such).
We do now have the tools to handle this properly - if the bootloader can
be updated to add the appropriate "iommu-addresses" property[1] to the
framebuffer reservation, then it's a case of hooking up support in
exynos-iommu via of_iommu_get_resv_regions().
For a short-term kernel-side hack you could probably implement
.def_domain_type to force IDENTITY for decon devices, as long as you can
then convince the DRM driver to pick another device to grab a DMA ops
domain from.
Thanks,
Robin.
