Hello Manish,

On 6/12/25 12:13, Manish Pandey2 wrote:

Thanks Ahmad for reaching out!

Thanks for the quick response.

There is a PR [1] on the Firmware Handoff specification that is still under discussion. Could you take a look and see if it could accommodate your use case?

Cool, thanks for pointing it out. I will check it out in detail.

To confirm my understanding, would the desired flow be something like:

1. *BL31 passes a read-only DT* to OP-TEE via the Transfer List (TL)

DT comes from BL2 originally, but yes, BL31 passes it along.

2. *OP-TEE consumes* the DT, adds memory reservations via a *DTO*, and appends this DTO to the TL

My original thought was that BL2 (via BL31) allocates some memory and passes it via transfer list for OP-TEE to fill out.

This allows reclaiming the space used by the overlay, which would otherwise not be possible:

- If OP-TEE uses arbitrary memory, it may turn out to be used otherwise

- If OP-TEE uses its own memory that it reserves, the space is never reclaimed as it doesn't know when it's no longer needed

3. *BL31 then forwards* the updated TL to BL33

Right.

Does that match what you had in mind?

Thanks, Ahmad

https://github.com/FirmwareHandoff/firmware_handoff/pull/74

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*From:* Ahmad Fatoum a.fatoum@pengutronix.de *Sent:* 11 June 2025 19:09 *To:* Manish Pandey2 Manish.Pandey2@arm.com *Cc:* op-tee@lists.trustedfirmware.org <op- tee@lists.trustedfirmware.org>; BAREBOX barebox@lists.infradead.org; rouven.czerwinski@linaro.org rouven.czerwinski@linaro.org; Michael Tretter m.tretter@pengutronix.de; kernel@pengutronix.de kernel@pengutronix.de *Subject:* Firmware Handoff for DT Overlay created by OP-TEE?   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 https:// www.youtube.com/watch?v=RxU798h8aiE

Cheers, Ahmad

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