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aspeed: Add boot stub for smp booting
This is a boot stub that is similar to the code u-boot runs, allowing the kernel to boot the secondary CPU. u-boot works as follows: 1. Initialises the SMP mailbox area in the SCU at 0x1e6e2180 with default values 2. Copies a stub named 'mailbox_insn' from flash to the SCU, just above the mailbox area 3. Sets AST_SMP_MBOX_FIELD_READY to a magic value to indicate the secondary can begin execution from the stub 4. The stub waits until the AST_SMP_MBOX_FIELD_GOSIGN register is set to a magic value 5. Jumps to the address in AST_SMP_MBOX_FIELD_ENTRY, starting Linux Linux indicates it is ready by writing the address of its entrypoint function to AST_SMP_MBOX_FIELD_ENTRY and the 'go' magic number to AST_SMP_MBOX_FIELD_GOSIGN. The secondary CPU sees this at step 4 and breaks out of it's loop. To be compatible, a fixed qemu stub is loaded into the mailbox area. As qemu can ensure the stub is loaded before execution starts, we do not need to emulate the AST_SMP_MBOX_FIELD_READY behaviour of u-boot. The secondary CPU's program counter points to the beginning of the stub, allowing qemu to start secondaries at step four. Reboot behaviour is preserved by resetting AST_SMP_MBOX_FIELD_GOSIGN when the secondaries are reset. This is only configured when the system is booted with -kernel and qemu does not execute u-boot first. Reviewed-by: Cédric Le Goater <clg@kaod.org> Tested-by: Cédric Le Goater <clg@kaod.org> Signed-off-by: Joel Stanley <joel@jms.id.au> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
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