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

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Date:   Wed, 27 Jul 2022 15:03:49 +0200
From:   Maximilian Luz <luzmaximilian@...il.com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        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 7/27/22 13:38, Krzysztof Kozlowski wrote:
> On 26/07/2022 19:01, Maximilian Luz wrote:
>> On 7/26/22 17:41, Sudeep Holla wrote:
>>> On Tue, Jul 26, 2022 at 05:15:41PM +0200, Maximilian Luz wrote:
>>>>
>>>> So ultimately I think it's better to add a DT entry for it.
>>>
>>> I disagree for the reason that once you discover more apps running on the
>>> secure side, you want to add more entries and update DT on the platform
>>> every time you discover some new firmware entity and you wish to interact
>>> with it from the non-secure side.
>>
>> Just as you'll have to add a driver to the kernel and update whatever is
>> probing the TrEE interface and add those strings to that interface. If
>> you then start doing SoC-specific lists, I think you'd be pretty much
>> re-implementing a DT in the kernel driver...
> 
> But you don't have any of these names in the DT either. Your DT node
> only indicates the presence of your driver, but does not hold any
> additional information like these IDs.

Because the compatible implicates the ID-string which implicates the driver
interface. If the ID-string for uefisecapp would be different we'd very likely
need a different driver for that as well, meaning a new compatible too. I
thought it would be superfluous to put that in the DT.

> Basically we start modelling firmware components in devicetree. :/

Is there really a good way around it? As far as I can see the
alternative (especially for the apps that need to be loaded manually) is
hard-coding everything in the driver. Which IMHO just spreads device
specific information everywhere.

Also: Let's use the TPM app as example. If that would be a SPI or I2C
device, you'd model it in the DT. Just because it's a hardware device
that's accessible via SCM/firmware you now don't?

If I were absolutely certain that there is a reliable mechanism to
detect these apps, I'd agree with having a driver to instantiate those
devices. But I am not.

Regards,
Max