On Thursday, January 22, 2026 7:22 PM Mikko Perttunen wrote:

On Tuesday, December 9, 2025 1:21 PM Aaron Kling wrote:

quoted

On Mon, Nov 3, 2025 at 12:05 PM Aaron Kling [off-list ref] wrote:

quoted

On Mon, Nov 3, 2025 at 5:07 AM Thierry Reding [off-list ref] wrote:

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On Sat, Nov 01, 2025 at 06:13:26PM -0500, Aaron Kling wrote:

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On Sat, Nov 1, 2025 at 6:01 PM Aaron Kling via B4 Relay
[off-list ref] wrote:

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From: Aaron Kling <redacted>

This reverts commit ebea268ea583ba4970df425dfef8c8e21d0a4e12.

Mmu is now being enabled for the display controllers.

Signed-off-by: Aaron Kling <redacted>
---
 arch/arm64/boot/dts/nvidia/tegra194.dtsi | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/boot/dts/nvidia/tegra194.dtsi b/arch/arm64/boot/dts/nvidia/tegra194.dtsi
index 1399342f23e1c4f73b278adc66dfb948fc30d326..854ed6d46aa1d8eedcdfbae1fdde1374adf40337 100644
--- a/arch/arm64/boot/dts/nvidia/tegra194.dtsi
+++ b/arch/arm64/boot/dts/nvidia/tegra194.dtsi

@@ -1807,7 +1807,7 @@ iommu@10000000 {
                        #iommu-cells = <1>;

                        nvidia,memory-controller = <&mc>;
-                       status = "disabled";
+                       status = "okay";
                };

                smmu: iommu@12000000 {

--

2.51.0

Question for Jon as the author of the commit being reverted. The
commit message states "we do not have a way to pass frame-buffer
memory from the bootloader to the kernel". If I understand this
correctly, this is talking about seamless handoff. What does this have
to do with enabling mmu on the display controllers? Seamless does not
work on any tegra arch as far as I'm aware, but Tegra194 is the only
one that doesn't have mmu enabled for the dc's. But enabling mmu
allows for better and faster memory allocation. My initial attempts to
enable this didn't work because I tried to attach them to the main mmu
unit, see the related freedesktop issue [0]. After noticing in the
downstream dt that the dc's are on a separate unit, I made it work.
And so far, it seems to work just as well as Tegra186. Then when I was
packaging up the change to submit, I found that this had been
explicitly disabled. But I'm not seeing why. Am I missing some
additional factors?

This isn't seamless handoff to the Tegra DRM driver for display, but
rather to simple-framebuffer. While this does technically work, it also
causes a spew of SMMU faults during early boot because the firmware does
not properly pass the SMMU mapping information to the kernel.

In a nutshell what happens is that the firmware sets up the display
controller to scan out from a reserved memory region, but it does so
without involving the SMMU, so it uses physical addresses directly. When
the kernel boots and the SMMU is enabled the continued accesses from
display hardware cause SMMU faults (because there is no mapping for the
framebuffer addresses).

That said, we did solve these issues and this may not be happening
anymore with the most recent L4T releases, so it may be okay to revert
this now. We should find out exactly which release includes all the
needed changes so that it can be referenced in the commit message. I
want to avoid people running new kernels with an old L4T release and
then seeing these errors without any reference as to why that might
suddenly happen.

For reference, I have rolled back my Android usecase to use the L4T
r32.7.6 bootloaders on T194 for a variety of reasons. So I am using
cboot as the final bootloader and not edk2 as in L4T r34/r35. I have a
pending cboot patch to support simple-framebuffer handoff, but haven't
fully verified it as tegra-drm is currently unable to takeover from
simplefb like openrm does for t234. But all that to say that since I
no longer use r35 for t194 I don't have the setup to easily verify
which point release works here and what doesn't.

Any further thoughts on this patch?

Aaron

FWIW,

looks like the edk2 patch to update iommu-addresses --

commit 6071946461389221d2314cbbae0377610b5b1f6a
Author: Jan Bobek [off-list ref]
Date:   Tue Mar 21 00:15:27 2023 +0000

    feat(NvDisplayControllerDxe): update FDT with framebuffer info
    
    On ready-to-boot and whenever FDT is installed, update FDT with
    framebuffer mode information, base address and size.
    
    Signed-off-by: Jan Bobek [off-list ref]
    Reviewed-by: Ashish Singhal [off-list ref]

is in since r36.2

$ git tag --contains 6071946461389221d2314cbbae0377610b5b1f6a | grep "^r"                                                                 
r36.2
r36.3.0
r36.4.0
r36.4.3
r36.4.4
r36.4.5
r38.2
r38.4

Not so good for T194 since r36 only supports Orin.

I'll look into getting this cherry-picked to r35.

Mikko

I looked into this and it appears a version of this is in r35, but it only supports T234. However, I also found that at one point, L4T bootloader configuration has been modified to place the display controllers into SMMU bypass until otherwise configured by the kernel -- which the kernel does in tegra_mc_probe_device.

I think that means there is still potential for an issue where the display continues to be on between tegra_mc_probe_device and tegradrm reconfiguring it. However, I cannot reproduce that happening -- most likely the display is being turned off before that because of a clock or power domain being turned off.

In any case, this means that we no longer need to pass the framebuffer's information to the kernel. I think it would be good to have some clarity to ensure the issue described above cannot happen, but otherwise we should be able to enable IOMMU.

Mikko