What the does BPMP-FW do and why it don't run when it is loaded by Tboot-BPMP?

Autonomous Machines Jetson & Embedded Systems Jetson TX2
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The tx2 boot flow shows that Tboot-BPMP load BPMP-FW, but it is unhalted by u-boot after cpu running. Why this flow? What the BPMP-FW really do when it is running?