linux-kernel - Re: [PATCH 4/4] dt-bindings: firmware: Add Qualcomm UEFI Secure Application client

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Date:   Tue, 26 Jul 2022 13:15:25 +0200
From:   Maximilian Luz <luzmaximilian@...il.com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Ard Biesheuvel <ardb@...nel.org>
Cc:     Konrad Dybcio <konrad.dybcio@...ainline.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Steev Klimaszewski <steev@...i.org>,
        Shawn Guo <shawn.guo@...aro.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Cristian Marussi <cristian.marussi@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-arm-msm@...r.kernel.org, linux-efi@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] dt-bindings: firmware: Add Qualcomm UEFI Secure
 Application client

Hi,

On 7/26/22 12:17, Krzysztof Kozlowski wrote:
> On 24/07/2022 00:49, Maximilian Luz wrote:
>> Add bindings for the Qualcomm Trusted Execution Environment (TrEE) UEFI
>> Secure application (uefisecapp) client.
>>
>> Signed-off-by: Maximilian Luz <luzmaximilian@...il.com>
>> ---
>>   .../firmware/qcom,tee-uefisecapp.yaml         | 38 +++++++++++++++++++
>>   MAINTAINERS                                   |  1 +
>>   2 files changed, 39 insertions(+)
>>   create mode 100644 Documentation/devicetree/bindings/firmware/qcom,tee-uefisecapp.yaml
>>
>> diff --git a/Documentation/devicetree/bindings/firmware/qcom,tee-uefisecapp.yaml b/Documentation/devicetree/bindings/firmware/qcom,tee-uefisecapp.yaml
>> new file mode 100644
>> index 000000000000..9e5de1005d5c
>> --- /dev/null
>> +++ b/Documentation/devicetree/bindings/firmware/qcom,tee-uefisecapp.yaml
>> @@ -0,0 +1,38 @@
>> +# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
>> +%YAML 1.2
>> +---
>> +$id: http://devicetree.org/schemas/soc/qcom/qcom,rpmh-rsc.yaml#
> 
> Does not look like you tested the bindings. Please run `make
> dt_binding_check` (see
> Documentation/devicetree/bindings/writing-schema.rst for instructions).

Sorry, first time submitting a schema. Already saw the warning of Rob's
bot and Will fix this in v2.

>> +$schema: http://devicetree.org/meta-schemas/core.yaml#
>> +
>> +title: Qualcomm Trusted Execution Environment UEFI Secure Application
>> +
>> +maintainers:
>> +  - Maximilian Luz <luzmaximilian@...il.com>
>> +
>> +description: |
>> +  Various Qualcomm SoCs do not allow direct access to UEFI variables. Instead,
>> +  these need to be accessed via the UEFI Secure Application (uefisecapp),
>> +  residing in the Trusted Execution Environment (TrEE). These bindings mark the
>> +  presence of uefisecapp and allow the respective client driver to load and
>> +  install efivar operations, providing the kernel with access to UEFI
>> +  variables.
>> +
>> +properties:
>> +  compatible:
>> +    const: qcom,tee-uefisecapp
> 
> Isn't this SoC-specific device? Generic compatibles are usually not
> expected.

This is essentially software (kernel driver) talking to software (in the
TrustZone), so I don't expect there to be anything SoC specific about it.

>> +
>> +required:
>> +  - compatible
>> +
>> +additionalProperties: false
>> +
>> +examples:
>> +  - |
>> +    firmware {
>> +        scm {
>> +            compatible = "qcom,scm-sc8180x", "qcom,scm";
>> +        };
>> +        tee-uefisecapp {
>> +            compatible = "qcom,tee-uefisecapp";
> 
> You did not model here any dependency on SCM. This is not full
> description of the firmware/hardware

How would I do that? A lot of other stuff also depends on SCM being
present (e.g. qcom_q6v5_pas for loading mdt files) and I don't see them
declare this in the device tree. As far as I can tell, SCM is pretty
much expected to be there at all times (i.e. can't be unloaded) and
drivers check for it when probing via qcom_scm_is_available(),
deferring probe if not.

Don't take this as an excuse as in "I want to leave that out", it's just
that I don't know how one would declare such a dependency explicitly. If
you can tell me how to fix it, I'll include that for v2.

Regards,
Max