Firmware Handoff for DT Overlay created by OP-TEE?

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

Hello Manish,

In your recent talk[1] at LinaroConnect about the Firmware Handoff
specification, you were asking about what other use cases it could
address for OP-TEE.

We ran into a possible use case that we would like to share:

BL2 can pass a DT to OP-TEE, which it would use to discover memory and
to fix up with memory reservations covering the memory it carves out for
itself.

OP-TEE also supports passing along the reservations as a device tree
overlay. AFAICS, there is no way currently to have both: A DT for OP-TEE
to probe hardware from and a way to pass the normal world a device tree
overlay in return.

I think this could be neatly addressed with Firmware Handoff and would
be a motivation for us to implement support in barebox BL2/BL33 to take
advantage of this. The current way is suboptimal as we would prefer the
DT passed to OP-TEE to be read-only. This avoids either having to copy
it or to pad it sufficiently in anticipation of it growing with the
addition of the new nodes.

[1]: https://www.youtube.com/watch?v=RxU798h8aiE

Cheers,
Ahmad

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