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

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Date:   Tue, 26 Jul 2022 17:15:41 +0200
From:   Maximilian Luz <luzmaximilian@...il.com>
To:     Sudeep Holla <sudeep.holla@....com>
Cc:     Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        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>,
        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

On 7/26/22 16:30, Sudeep Holla wrote:
> On Sun, Jul 24, 2022 at 12:49:49AM +0200, Maximilian Luz wrote:
>> Add bindings for the Qualcomm Trusted Execution Environment (TrEE) UEFI
>> Secure application (uefisecapp) client.
>>

[...]

>> +examples:
>> +  - |
>> +    firmware {
>> +        scm {
>> +            compatible = "qcom,scm-sc8180x", "qcom,scm";
>> +        };
>> +        tee-uefisecapp {
>> +            compatible = "qcom,tee-uefisecapp";
>> +        };
> 
> Do you expect some issues using the scm driver APIs without the
> any additions in the DT ? I mean can't you auto-discover by using the
> APIs. I haven't looked at the driver or any other patches in the series,
> but I would like to know if we can avoid adding any new bindings if it
> can be discovered via those SCM driver APIs.

Not at scale, at least as far as I can tell.

Part of the setup-process of this driver is to query an "application ID"
from a unique string identifying the application (in this case
"qcom.tz.uefisecapp"). If that call fails, we know the app is not there.

But: If we'd want to support more than just "uefisecapp" we'd have to
query each app in some predefined list. As far as I can tell, there's no
method to enumerate all present/loaded ones. The Windows driver seems to
use a hard-coded list of apps that are present on some specific SoC.

It might be possible that there exists such a method, but if it does, the
Windows driver doesn't seem to use it and I don't know about it.

Also, there would need to be at least some type of compatible to
indicate the presence of that TrEE / Secure Application interface used by
uefisecapp. Unless you want to send some potentially unsupported SCM
commands on every platform with qcom,scm and see what comes back.

So ultimately I think it's better to add a DT entry for it. That also
(hopefully) ensures that someone tested and (at least in some way)
validated this. Again, It's a reverse engineered driver.

Regards,
Max