linux-kernel - Re: [PATCH 1/2] arm64: dts: qcom: agatti: Fix IOMMU DT properties

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Message-ID: <5a8e3805-588b-4e2b-bb84-35bdc71521fa@oss.qualcomm.com>
Date: Tue, 30 Dec 2025 18:16:05 +0530
From: Akhil P Oommen <akhilpo@....qualcomm.com>
To: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>,
        Sumit Garg <sumit.garg@...nel.org>
Cc: linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
        andersson@...nel.org, konradybcio@...nel.org, robh@...nel.org,
        krzk+dt@...nel.org, conor+dt@...nel.org,
        vikash.garodia@....qualcomm.com, dikshita.agarwal@....qualcomm.com,
        robin.clark@....qualcomm.com, lumag@...nel.org,
        linux-kernel@...r.kernel.org, Sumit Garg <sumit.garg@....qualcomm.com>,
        Prakash Gupta <guptap@....qualcomm.com>
Subject: Re: [PATCH 1/2] arm64: dts: qcom: agatti: Fix IOMMU DT properties

On 12/30/2025 12:35 PM, Dmitry Baryshkov wrote:
> On Tue, Dec 30, 2025 at 10:45:38AM +0530, Sumit Garg wrote:
>> On Mon, Dec 29, 2025 at 10:00:02PM +0200, Dmitry Baryshkov wrote:
>>> On Mon, Dec 29, 2025 at 12:42:57PM +0530, Sumit Garg wrote:
>>>> From: Sumit Garg <sumit.garg@....qualcomm.com>
>>>>
>>>> Fix IOMMU DT propeties for GPU, display and video peripherals via
>>>> dropping SMMU stream IDs which relates to secure context bank.
>>>>
>>>> This problem only surfaced when the Gunyah based firmware stack is
>>>> ported on Agatti replacing the legacy QHEE based firmware stack. Assigning
>>>> Linux kernel (HLOS) VMID to secure context bank stream IDs is treated
>>>> as a fault by Gunyah hypervisor which were previously ignored by QHEE
>>>> hypervisor.
>>>
>>> We are porting the underlaying layer. Can we make it to behave in a way
>>> similar to the _existing_ software? If DT was a part of the firmware, it
>>> would have been fine to perform such updates. But as it is not, we
>>> really should try not to break the interface boundary between firmware
>>> and the kernel.
>>
>> I support your viewpoint. But in this current case Linux kernel VMID mapping
>> for secure context bank stream IDs is incorrect. As Konrad mentioned in the
>> other thread that for secure media streaming use-cases, it rather requires
>> proper representation via iommu-map such that appropriate VMID can be mapped.
> 
> Yes, I understand this part. I'm basically suggesting that Gunyah should
> ignore this SID when programming the actual SMMU (probably like QHEE
> did).

But the existing DT description for Agatti GPU is buggy. SID 2 is for
secure access from GPU and it should not be mixed with non-secure
access. We haven't hit any issue so far because the KMD doesn't have
support for secure usecase yet.

At least for the GPU property, I feel that the right thing to do is to
correct the buggy description.

-Akhil

> 
>>
>>>
>>> I'm fine with the change, but I think we should make sure that Gunyah
>>> doesn't break existing device trees.
>>
>> Sure, this change isn't breaking any DT ABI but brings compliance to
>> usage of stream IDs.
> 
> Actually, it does, as you can see from Rob's email. You didn't perform
> make dtbs_check while submitting this patch.
>