RISC-V: Add cpu_ops and modify default booting method

Currently, all non-booting harts start booting after the booting hart
updates the per-hart stack pointer. This is done in a way that, it's
difficult to implement any other booting method without breaking the
backward compatibility.

Define a cpu_ops method that allows to introduce other booting methods
in future. Modify the current booting method to be compatible with
cpu_ops.
Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/include/asm/cpu_ops.h

+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (c) 2020 Western Digital Corporation or its affiliates.
+ * Based on arch/arm64/include/asm/cpu_ops.h
+ */
+#ifndef __ASM_CPU_OPS_H
+#define __ASM_CPU_OPS_H
+
+#include <linux/init.h>
+#include <linux/sched.h>
+#include <linux/threads.h>
+
+/**
+ * struct cpu_operations - Callback operations for hotplugging CPUs.
+ *
+ * @name:		Name of the boot protocol.
+ * @cpu_prepare:	Early one-time preparation step for a cpu. If there
+ *			is a mechanism for doing so, tests whether it is
+ *			possible to boot the given HART.
+ * @cpu_start:		Boots a cpu into the kernel.
+ */
+struct cpu_operations {
+	const char	*name;
+	int		(*cpu_prepare)(unsigned int cpu);
+	int		(*cpu_start)(unsigned int cpu,
+				     struct task_struct *tidle);
+};
+
+extern const struct cpu_operations *cpu_ops[NR_CPUS];
+void __init cpu_set_ops(int cpu);
+void cpu_update_secondary_bootdata(unsigned int cpuid,
+				   struct task_struct *tidle);
+
+#endif /* ifndef __ASM_CPU_OPS_H */

arch/riscv/kernel/Makefile

@@ -34,6 +34,8 @@ obj-$(CONFIG_RISCV_M_MODE)	+= clint.o
 obj-$(CONFIG_FPU)		+= fpu.o
 obj-$(CONFIG_SMP)		+= smpboot.o
 obj-$(CONFIG_SMP)		+= smp.o
+obj-$(CONFIG_SMP)		+= cpu_ops.o
+obj-$(CONFIG_SMP)		+= cpu_ops_spinwait.o
 obj-$(CONFIG_MODULES)		+= module.o
 obj-$(CONFIG_MODULE_SECTIONS)	+= module-sections.o
 

arch/riscv/kernel/cpu_ops.c

+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (c) 2020 Western Digital Corporation or its affiliates.
+ */
+
+#include <linux/errno.h>
+#include <linux/mm.h>
+#include <linux/of.h>
+#include <linux/string.h>
+#include <linux/sched.h>
+#include <linux/sched/task_stack.h>
+#include <asm/cpu_ops.h>
+#include <asm/sbi.h>
+#include <asm/smp.h>
+
+const struct cpu_operations *cpu_ops[NR_CPUS] __ro_after_init;
+
+void *__cpu_up_stack_pointer[NR_CPUS];
+void *__cpu_up_task_pointer[NR_CPUS];
+
+extern const struct cpu_operations cpu_ops_spinwait;
+
+void cpu_update_secondary_bootdata(unsigned int cpuid,
+				   struct task_struct *tidle)
+{
+	int hartid = cpuid_to_hartid_map(cpuid);
+
+	/* Make sure tidle is updated */
+	smp_mb();
+	WRITE_ONCE(__cpu_up_stack_pointer[hartid],
+		   task_stack_page(tidle) + THREAD_SIZE);
+	WRITE_ONCE(__cpu_up_task_pointer[hartid], tidle);
+}
+
+void __init cpu_set_ops(int cpuid)
+{
+	cpu_ops[cpuid] = &cpu_ops_spinwait;
+}

arch/riscv/kernel/cpu_ops_spinwait.c

+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (c) 2020 Western Digital Corporation or its affiliates.
+ */
+
+#include <linux/errno.h>
+#include <linux/of.h>
+#include <linux/string.h>
+#include <asm/cpu_ops.h>
+#include <asm/sbi.h>
+#include <asm/smp.h>
+
+const struct cpu_operations cpu_ops_spinwait;
+
+static int spinwait_cpu_prepare(unsigned int cpuid)
+{
+	if (!cpu_ops_spinwait.cpu_start) {
+		pr_err("cpu start method not defined for CPU [%d]\n", cpuid);
+		return -ENODEV;
+	}
+	return 0;
+}
+
+static int spinwait_cpu_start(unsigned int cpuid, struct task_struct *tidle)
+{
+	/*
+	 * In this protocol, all cpus boot on their own accord.  _start
+	 * selects the first cpu to boot the kernel and causes the remainder
+	 * of the cpus to spin in a loop waiting for their stack pointer to be
+	 * setup by that main cpu.  Writing to bootdata
+	 * (i.e __cpu_up_stack_pointer) signals to the spinning cpus that they
+	 * can continue the boot process.
+	 */
+	cpu_update_secondary_bootdata(cpuid, tidle);
+
+	return 0;
+}
+
+const struct cpu_operations cpu_ops_spinwait = {
+	.name		= "spinwait",
+	.cpu_prepare	= spinwait_cpu_prepare,
+	.cpu_start	= spinwait_cpu_start,
+};

arch/riscv/kernel/smpboot.c

@@ -25,6 +25,7 @@
 #include <linux/sched/task_stack.h>
 #include <linux/sched/mm.h>
 #include <asm/clint.h>
+#include <asm/cpu_ops.h>
 #include <asm/irq.h>
 #include <asm/mmu_context.h>
 #include <asm/tlbflush.h>
@@ -34,8 +35,6 @@
 
 #include "head.h"
 
-void *__cpu_up_stack_pointer[NR_CPUS];
-void *__cpu_up_task_pointer[NR_CPUS];
 static DECLARE_COMPLETION(cpu_running);
 
 void __init smp_prepare_boot_cpu(void)
@@ -46,6 +45,7 @@ void __init smp_prepare_boot_cpu(void)
 void __init smp_prepare_cpus(unsigned int max_cpus)
 {
 	int cpuid;
+	int ret;
 
 	/* This covers non-smp usecase mandated by "nosmp" option */
 	if (max_cpus == 0)
@@ -54,6 +54,11 @@ void __init smp_prepare_cpus(unsigned int max_cpus)
 	for_each_possible_cpu(cpuid) {
 		if (cpuid == smp_processor_id())
 			continue;
+		if (cpu_ops[cpuid]->cpu_prepare) {
+			ret = cpu_ops[cpuid]->cpu_prepare(cpuid);
+			if (ret)
+				continue;
+		}
 		set_cpu_present(cpuid, true);
 	}
 }
@@ -65,6 +70,8 @@ void __init setup_smp(void)
 	bool found_boot_cpu = false;
 	int cpuid = 1;
 
+	cpu_set_ops(0);
+
 	for_each_of_cpu_node(dn) {
 		hart = riscv_of_processor_hartid(dn);
 		if (hart < 0)
@@ -92,36 +99,38 @@ void __init setup_smp(void)
 			cpuid, nr_cpu_ids);
 
 	for (cpuid = 1; cpuid < nr_cpu_ids; cpuid++) {
-		if (cpuid_to_hartid_map(cpuid) != INVALID_HARTID)
+		if (cpuid_to_hartid_map(cpuid) != INVALID_HARTID) {
+			cpu_set_ops(cpuid);
 			set_cpu_possible(cpuid, true);
+		}
 	}
 }
 
+int start_secondary_cpu(int cpu, struct task_struct *tidle)
+{
+	if (cpu_ops[cpu]->cpu_start)
+		return cpu_ops[cpu]->cpu_start(cpu, tidle);
+
+	return -EOPNOTSUPP;
+}
+
 int __cpu_up(unsigned int cpu, struct task_struct *tidle)
 {
 	int ret = 0;
-	int hartid = cpuid_to_hartid_map(cpu);
 	tidle->thread_info.cpu = cpu;
 
-	/*
-	 * On RISC-V systems, all harts boot on their own accord.  Our _start
-	 * selects the first hart to boot the kernel and causes the remainder
-	 * of the harts to spin in a loop waiting for their stack pointer to be
-	 * setup by that main hart.  Writing __cpu_up_stack_pointer signals to
-	 * the spinning harts that they can continue the boot process.
-	 */
-	smp_mb();
-	WRITE_ONCE(__cpu_up_stack_pointer[hartid],
-		  task_stack_page(tidle) + THREAD_SIZE);
-	WRITE_ONCE(__cpu_up_task_pointer[hartid], tidle);
-
-	lockdep_assert_held(&cpu_running);
-	wait_for_completion_timeout(&cpu_running,
+	ret = start_secondary_cpu(cpu, tidle);
+	if (!ret) {
+		lockdep_assert_held(&cpu_running);
+		wait_for_completion_timeout(&cpu_running,
 					    msecs_to_jiffies(1000));
 
-	if (!cpu_online(cpu)) {
-		pr_crit("CPU%u: failed to come online\n", cpu);
-		ret = -EIO;
+		if (!cpu_online(cpu)) {
+			pr_crit("CPU%u: failed to come online\n", cpu);
+			ret = -EIO;
+		}
+	} else {
+		pr_crit("CPU%u: failed to start\n", cpu);
 	}
 
 	return ret;