Re: [PATCH 1/2] Revert "arm64: tegra: Disable ISO SMMU for Tegra194"

[Thierry Reding]

On Tue, Feb 17, 2026 at 12:53:54PM +0900, Mikko Perttunen wrote:
> On Thursday, January 22, 2026 7:22 PM Mikko Perttunen wrote:
> > On Tuesday, December 9, 2025 1:21 PM Aaron Kling wrote:
> > > On Mon, Nov 3, 2025 at 12:05 PM Aaron Kling <webgeek1234@xxxxxxxxx> wrote:
> > > >
> > > > On Mon, Nov 3, 2025 at 5:07 AM Thierry Reding <thierry.reding@xxxxxxxxx> wrote:
> > > > >
> > > > > On Sat, Nov 01, 2025 at 06:13:26PM -0500, Aaron Kling wrote:
> > > > > > On Sat, Nov 1, 2025 at 6:01 PM Aaron Kling via B4 Relay
> > > > > > <devnull+webgeek1234.gmail.com@xxxxxxxxxx> wrote:
> > > > > > >
> > > > > > > From: Aaron Kling <webgeek1234@xxxxxxxxx>
> > > > > > >
> > > > > > > This reverts commit ebea268ea583ba4970df425dfef8c8e21d0a4e12.
> > > > > > >
> > > > > > > Mmu is now being enabled for the display controllers.
> > > > > > >
> > > > > > > Signed-off-by: Aaron Kling <webgeek1234@xxxxxxxxx>
> > > > > > > ---
> > > > > > >  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 <jbobek@xxxxxxxxxx>
> > 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 <jbobek@xxxxxxxxxx>
> >     Reviewed-by: Ashish Singhal <ashishsingha@xxxxxxxxxx>
> > 
> > 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.

The problem would happen if you enable some sort of early framebuffer
support, such as simple-drm or simple-framebuffer. Maybe even efifb. I
think it'd still be worth getting the iommu-addresses code into r35 if
for nothing else but to have a bit more of a safety buffer for the
future.

If we don't and for some reason decide that we want early framebuffer
support, it might be too late to get UEFI updated for Tegra194. I recall
that the UEFI code for Tegra194 is different from the one for Tegra234,
so it is probably not as trivial as a simple cherry-pick, but I'll try
to do some digging and find the code that does this for Xavier.

Thierry

Attachment: signature.asc
Description: PGP signature