xref: /linux/arch/riscv/kernel/cpu_ops_spinwait.c (revision a4eb44a6435d6d8f9e642407a4a06f65eb90ca04)
1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3  * Copyright (c) 2020 Western Digital Corporation or its affiliates.
4  */
5 
6 #include <linux/errno.h>
7 #include <linux/of.h>
8 #include <linux/string.h>
9 #include <linux/sched/task_stack.h>
10 #include <asm/cpu_ops.h>
11 #include <asm/sbi.h>
12 #include <asm/smp.h>
13 
14 const struct cpu_operations cpu_ops_spinwait;
15 void *__cpu_spinwait_stack_pointer[NR_CPUS] __section(".data");
16 void *__cpu_spinwait_task_pointer[NR_CPUS] __section(".data");
17 
18 static void cpu_update_secondary_bootdata(unsigned int cpuid,
19 				   struct task_struct *tidle)
20 {
21 	int hartid = cpuid_to_hartid_map(cpuid);
22 
23 	/*
24 	 * The hartid must be less than NR_CPUS to avoid out-of-bound access
25 	 * errors for __cpu_spinwait_stack/task_pointer. That is not always possible
26 	 * for platforms with discontiguous hartid numbering scheme. That's why
27 	 * spinwait booting is not the recommended approach for any platforms
28 	 * booting Linux in S-mode and can be disabled in the future.
29 	 */
30 	if (hartid == INVALID_HARTID || hartid >= NR_CPUS)
31 		return;
32 
33 	/* Make sure tidle is updated */
34 	smp_mb();
35 	WRITE_ONCE(__cpu_spinwait_stack_pointer[hartid],
36 		   task_stack_page(tidle) + THREAD_SIZE);
37 	WRITE_ONCE(__cpu_spinwait_task_pointer[hartid], tidle);
38 }
39 
40 static int spinwait_cpu_prepare(unsigned int cpuid)
41 {
42 	if (!cpu_ops_spinwait.cpu_start) {
43 		pr_err("cpu start method not defined for CPU [%d]\n", cpuid);
44 		return -ENODEV;
45 	}
46 	return 0;
47 }
48 
49 static int spinwait_cpu_start(unsigned int cpuid, struct task_struct *tidle)
50 {
51 	/*
52 	 * In this protocol, all cpus boot on their own accord.  _start
53 	 * selects the first cpu to boot the kernel and causes the remainder
54 	 * of the cpus to spin in a loop waiting for their stack pointer to be
55 	 * setup by that main cpu.  Writing to bootdata
56 	 * (i.e __cpu_spinwait_stack_pointer) signals to the spinning cpus that they
57 	 * can continue the boot process.
58 	 */
59 	cpu_update_secondary_bootdata(cpuid, tidle);
60 
61 	return 0;
62 }
63 
64 const struct cpu_operations cpu_ops_spinwait = {
65 	.name		= "spinwait",
66 	.cpu_prepare	= spinwait_cpu_prepare,
67 	.cpu_start	= spinwait_cpu_start,
68 };
69