TX2 BootROM and redundant boot slots

[graycat]

First of all, thanks for all the hard work you've all been doing here! It's been extremely valuble for a TX2i based product we've been developing at my company.

Speaking of which, we've recently switched over to using a cboot-only A/B redundant boot flow using RAUC with tegra-boot-tools, which (once I gave up on the idea of trying to keep u-boot in the chain, for now at least) was very straightforward to setup and is working great (we're using the kirkstone-l4t-r32.7.x branch). I've been digging through Nvidia docs and spelunking the meta-tegra and tegra-boot-tools source to make sure I fully understand the redundant boot flow, and there's one piece I'm not entirely sure on yet:

From what I can tell the BootROM always looks at the BCT partition only (ignoring SMD), and it always works its way through the bootloader entries in the table sequentially. So unless the MB1 partition gets corrupted, it's always going to jump to MB1 and not MB1_b (being the second entry in the BCT). Then from there, the running MB1 reads its corresponding (i.e. *_b if MB1_b is running) MB1-BCT for init settings and SMD to populate the scratch register with the slot info, then jumps to the TBoot-BPMP in MB2 or MB2_b based on the contents of its SMD. In other words, the MB1, MB1-BCT, and SMD partitions used are always chosen by the BootROM based entirely off verifying MB1 against what's stored in the BCT entry, and the actual A/B slot booting process starts at the MB1->MB2 handoff. Is this an accurate understanding?

Oh and follow-up question: I'm sure I can find the answer with a bit more digging, but when running tegra-bootloader-update on the t186 to write a new bup payload to the inactive slot, does BCT get updated with a new table order so the new MB1 (if it's changed and has beeen updated) is the first entry? Or will the BL entry order in BCT always remain the same?

[madisongh]

Your description of those early boot stages sounds right to me. When updating, the BCT payload is written to each of the three BCT slots, and the mb1 payload is written to both the mb1 and mb1_b partitions. The order should always be the same for the mb1 stage, as far as I know - mb1 always primary, with mb1_b only used as a backup.

[graycat]

OK great, thanks for confirming.