Re: [PATCH 0/4] Add support for the TPM FF-A start method

[Sudeep Holla <sudeep.holla@arm.com>]

Hi Sumit,

On Fri, Feb 21, 2025 at 07:16:35PM +0530, Sumit Garg wrote:
> On Mon, Feb 17, 2025 at 10:56:58AM -0600, Stuart Yoder wrote:
> >
> > I don't see how changing TPM discovery to be via FF-A directly
> > would improve maintainability.
>
> You are considering ACPI at this point but when people want to use this
> TPM over FF-A on a platform using DT then it will require corresponding
> DT bindings. After that each platform has to enable TPM over FF-A in
> their corresponding ACPI/DT. All that won't be needed with auto
> discovery over FF-A.

I hear you and completely agree. However, someone thought it was a good idea
to align with other start methods and duplicate information in the TCG ACPI
specification. This is definitely a bad idea, as it may contradict the
firmware. All we needed was a simple flag to indicate whether FF-A is the
start method.

It sounds like a classic case of misalignment between specification authors
and practical implementation needs. Instead of a simple flag to indicate FF-A
as the start method, duplicating information in the TCG ACPI specification
seems unnecessary and potentially problematic—especially if it risks
conflicting with firmware behavior.

Anyway, I can't comment on how we ended up here, but this seems to be the reality.

--
Regards,
Sudeep