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

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Message-ID: <CALHNRZ_QrQHCmF7f1z29tAmuNR-=rG1SgYJ1sssK3VXiQqURYg@mail.gmail.com>
Date: Sat, 1 Nov 2025 18:13:26 -0500
From: Aaron Kling <webgeek1234@...il.com>
To: webgeek1234@...il.com
Cc: Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>, 
	Conor Dooley <conor+dt@...nel.org>, Thierry Reding <thierry.reding@...il.com>, 
	Jonathan Hunter <jonathanh@...dia.com>, devicetree@...r.kernel.org, 
	linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] Revert "arm64: tegra: Disable ISO SMMU for Tegra194"

On Sat, Nov 1, 2025 at 6:01 PM Aaron Kling via B4 Relay
<devnull+webgeek1234.gmail.com@...nel.org> wrote:
>
> From: Aaron Kling <webgeek1234@...il.com>
>
> This reverts commit ebea268ea583ba4970df425dfef8c8e21d0a4e12.
>
> Mmu is now being enabled for the display controllers.
>
> Signed-off-by: Aaron Kling <webgeek1234@...il.com>
> ---
>  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@...00000 {
>                         #iommu-cells = <1>;
>
>                         nvidia,memory-controller = <&mc>;
> -                       status = "disabled";
> +                       status = "okay";
>                 };
>
>                 smmu: iommu@...00000 {
>
> --
> 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?

Aaron

[0] https://gitlab.freedesktop.org/drm/tegra/-/issues/5