ARM: Start/Wakeup/Bringup the other CPU cores/APs and pass execution start address?

Question

I\'ve been banging my head with this for the last 3-4 days and I can\'t find a DECENT explanatory documentation (from ARM or unofficial) to help me. I\'ve got an ODR

Answer 1

For clarity and future reference, there's a subtle piece of information missing here thanks to the lack of proper documentation of the Exynos boot protocol (n.b. this question should really be marked "Exynos 5" rather than "Cortex-A15" - it's a SoC-specific thing and what ARM says is only a general recommendation). From cold boot, the secondary cores aren't in WFI, they're still powered off.

The simpler minimal solution (based on what Linux's hotplug does), which I worked out in the process of writing a boot shim to get a hypervisor running on the XU, takes two steps:

1. First write the entry point address to the Exynos holding pen (0x02073000 + 0x1c)
2. Then poke the power controller to switch on the relevant core(s): That way, they drop out of the secure boot path into the holding pen to find the entry point waiting for them, skipping the WFI loop and obviating the need to even touch the GIC at all.

Unless you're planning a full-on CPU hotplug implementation you can skip checking the cluster ID - if we're booting, we're on cluster 0 and nowhere else (the check for pre-production chips with backwards cluster registers should be unnecessary on the Odroid too - certainly was for me).

From my investigation, firing up the A7s is a little more involved. Judging from the Exynos big.LITTLE switcher driver, it seems you need to poke a separate set of power controller registers to enable cluster 1 first (and you may need to mess around with the CCI too, especially to have the MMUs and caches on) - I didn't get further since by that point it was more "having fun" than "doing real work"...

As an aside, Samsung's mainline patch for CPU hotplug on the 5410 makes the core power control stuff rather clearer than the mess in their downstream code, IMO.