We added this hack to AM62Ax so that unread mailbox messages between DM-R5F and C7 did not prevent the processor from entering low power mode: https://git.ti.com/cgit/ti-linux-kernel/ti-linux-kernel/commit/arch/arm64/boot/dts/ti/k3-am62a7-sk.dts?h=ti-linux-6.1.y-cicd&id=4186fbe25a8c2f68565bd95002dc619fed256ae3

A similar hack needs to be added to AM62x, as well as any other device that we plan to add low power mode for in the future (AM62P?)

This has already impacted multiple customers, including:
https://e2e.ti.com/support/processors-group/processors/f/processors-forum/1299511/processor-sdk-am62x-am62x-suspend-resume-not-working-without-m4-fw-deepsleep-mode-entry-reboots-the-device

The workaround given those customers was to simply disable all mailboxes in the Linux devicetree, but the approach in the kernel patch linked above is more flexible and elegant.

We could potentially test by switching the IPC_Echo example that is loaded into the remote cores with non-IPC Hello World examples and seeing if low power mode transitions are now possible. If so, that would confirm that unread mailboxes from the IPC_Echo example to the non-initialized remote core were the cause of the error.