linux-kernel - Re: [PATCH 05/11] dt-bindings: document the Qualcomm TEE Shared Memory Bridge

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Message-ID: <CACMJSevO7sGZ5Yj_wBrs6kV+eo7iW_aLwBj68zjbU3dS7tJ-sA@mail.gmail.com>
Date:   Wed, 30 Aug 2023 16:39:54 +0200
From:   Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc:     Konrad Dybcio <konrad.dybcio@...aro.org>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Alex Elder <elder@...aro.org>,
        Srini Kandagatla <srinivas.kandagatla@...aro.org>,
        kernel@...cinc.com, linux-arm-msm@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 05/11] dt-bindings: document the Qualcomm TEE Shared
 Memory Bridge

On Wed, 30 Aug 2023 at 16:31, Krzysztof Kozlowski
<krzysztof.kozlowski@...aro.org> wrote:
>
> On 30/08/2023 15:48, Bartosz Golaszewski wrote:
> > On Tue, 29 Aug 2023 at 11:30, Konrad Dybcio <konrad.dybcio@...aro.org> wrote:
> >>
> >> On 29.08.2023 10:02, Krzysztof Kozlowski wrote:
> >>> On 28/08/2023 21:25, Bartosz Golaszewski wrote:
> >>>> Add Device Tree bindings for Qualcomm TEE Shared Memory Brige - a
> >>>> mechanism that allows sharing memory buffers between trustzone and the
> >>>> kernel.
> >>>
> >>> Subject prefix:
> >>> dt-bindings: firmware:
> >>>
> >>>
> >>>
> >>>>
> >>>> Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> >>>> ---
> >>>>  .../bindings/firmware/qcom,shm-bridge.yaml    | 36 +++++++++++++++++++
> >>>>  1 file changed, 36 insertions(+)
> >>>>  create mode 100644 Documentation/devicetree/bindings/firmware/qcom,shm-bridge.yaml
> >>>>
> >>>> diff --git a/Documentation/devicetree/bindings/firmware/qcom,shm-bridge.yaml b/Documentation/devicetree/bindings/firmware/qcom,shm-bridge.yaml
> >>>> new file mode 100644
> >>>> index 000000000000..f660962b7b86
> >>>> --- /dev/null
> >>>> +++ b/Documentation/devicetree/bindings/firmware/qcom,shm-bridge.yaml
> >>>> @@ -0,0 +1,36 @@
> >>>> +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> >>>> +%YAML 1.2
> >>>> +---
> >>>> +$id: http://devicetree.org/schemas/firmware/qcom,shm-bridge.yaml#
> >>>> +$schema: http://devicetree.org/meta-schemas/core.yaml#
> >>>> +
> >>>> +title: QCOM Shared Memory Bridge
> >>>> +
> >>>> +description: |
> >>>
> >>> Do not need '|' unless you need to preserve formatting.
> >>>
> >>>> +  Qualcomm TEE Shared Memory Bridge allows sharing limited areas of kernel's
> >>>> +  virtual memory with the trustzone in order to avoid mapping the entire RAM.
> >>>> +
> >>>> +maintainers:
> >>>> +  - Bjorn Andersson <andersson@...nel.org>
> >>>> +  - Konrad Dybcio <konrad.dybcio@...aro.org>
> >>>> +  - Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> >>>> +
> >>>> +properties:
> >>>> +  compatible:
> >>>> +    items:
> >>>> +      - enum:
> >>>> +          - qcom,shm-bridge-sa8775p
> >>>> +          - qcom,shm-bridge-sm8150
> >>>> +          - qcom,shm-bridge-sm8450
> >>>> +      - const: qcom,shm-bridge
> >>>> +
> >>>
> >>> Looks quite empty... Why this cannot be part of qcom,scm? IOW, why do
> >>> you need new binding if you do not have any resources here and the block
> >>> is essentially feature of qcom,scm firmware?
> >> Since it's "discoverable" (via retval of an scm call), I'd second the
> >> idea of probing this from within the SCM driver.
> >>
> >> Konrad
> >
> > Downstream has a bunch of DT switches that we don't support for now
> > upstream. I disagree about shoehorning this into the SCM driver. It
> > really is a layer on top of SCM but also SCM is a user of this
> > interface.
>
> Sure, for the driver makes sense, but it does not really explain why DT
> node is needed. It is not separate hardware. I doubt it is even separate
> firmware, but part of SCM.
>
> Best regards,
> Krzysztof
>

Because not all platforms support it and it's the simplest way of
marking the ones that do. Both SHM and SCM nodes sit on the firmware
node anyway. What do you recommend? A property of the SCM node? Like
'qcom,shm-bridge` or something?

Bart