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[PW_SID:1077988] riscv: Add WFI to secondary hart spinwait loop #1735

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[PW_SID:1077988] riscv: Add WFI to secondary hart spinwait loop #1735

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9 changes: 8 additions & 1 deletion arch/riscv/kernel/head.S
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Original file line number Diff line number Diff line change
Expand Up @@ -385,7 +385,14 @@ SYM_CODE_START(_start_kernel)
* get far enough along the boot process that it should continue.
*/
.Lwait_for_cpu_up:
/* FIXME: We should WFI to save some energy here. */
/*
* Wait for the boot hart to populate the stack and task pointers.
* Use WFI to avoid burning power in a busy-wait loop. Per the
* RISC-V privileged spec, WFI wakes on a pending interrupt even
* with global interrupts disabled (e.g. SIE=0), and implementations
* are permitted to treat it as a NOP, so this is always safe.
*/
wfi
REG_L sp, (a1)
REG_L tp, (a2)
beqz sp, .Lwait_for_cpu_up
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