RISC-V: Move spinwait booting method to its own config

The spinwait booting method should only be used for platforms with older
firmware without SBI HSM extension or M-mode firmware because spinwait
method can't support cpu hotplug, kexec or sparse hartid. It is better
to move the entire spinwait implementation to its own config which can
be disabled if required. It is enabled by default to maintain backward
compatibility and M-mode Linux.

Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/Kconfig

@@ -374,6 +374,20 @@ config RISCV_SBI_V01
 	  This config allows kernel to use SBI v0.1 APIs. This will be
 	  deprecated in future once legacy M-mode software are no longer in use.
 
+config RISCV_BOOT_SPINWAIT
+	bool "Spinwait booting method"
+	depends on SMP
+	default y
+	help
+	  This enables support for booting Linux via spinwait method. In the
+	  spinwait method, all cores randomly jump to Linux. One of the cores
+	  gets chosen via lottery and all other keep spinning on a percpu
+	  variable. This method cannot support CPU hotplug and sparse hartid
+	  scheme. It should be only enabled for M-mode Linux or platforms relying
+	  on older firmware without SBI HSM extension. All other platforms should
+	  rely on ordered booting via SBI HSM extension which gets chosen
+	  dynamically at runtime if the firmware supports it.
+
 config KEXEC
 	bool "Kexec system call"
 	select KEXEC_CORE

arch/riscv/kernel/Makefile

@@ -43,7 +43,8 @@ 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_RISCV_BOOT_SPINWAIT) += cpu_ops_spinwait.o
 obj-$(CONFIG_MODULES)		+= module.o
 obj-$(CONFIG_MODULE_SECTIONS)	+= module-sections.o
 

arch/riscv/kernel/cpu_ops.c

@@ -15,7 +15,15 @@
 const struct cpu_operations *cpu_ops[NR_CPUS] __ro_after_init;
 
 extern const struct cpu_operations cpu_ops_sbi;
+#ifdef CONFIG_RISCV_BOOT_SPINWAIT
 extern const struct cpu_operations cpu_ops_spinwait;
+#else
+const struct cpu_operations cpu_ops_spinwait = {
+	.name		= "",
+	.cpu_prepare	= NULL,
+	.cpu_start	= NULL,
+};
+#endif
 
 void __init cpu_set_ops(int cpuid)
 {

arch/riscv/kernel/head.S

@@ -259,7 +259,7 @@ pmp_done:
 	li t0, SR_FS
 	csrc CSR_STATUS, t0
 
-#ifdef CONFIG_SMP
+#ifdef CONFIG_RISCV_BOOT_SPINWAIT
 	li t0, CONFIG_NR_CPUS
 	blt a0, t0, .Lgood_cores
 	tail .Lsecondary_park
@@ -285,7 +285,7 @@ pmp_done:
 	beq t0, t1, .Lsecondary_start
 
 #endif /* CONFIG_XIP */
-#endif /* CONFIG_SMP */
+#endif /* CONFIG_RISCV_BOOT_SPINWAIT */
 
 #ifdef CONFIG_XIP_KERNEL
 	la sp, _end + THREAD_SIZE
@@ -344,7 +344,7 @@ clear_bss_done:
 	call soc_early_init
 	tail start_kernel
 
-#ifdef CONFIG_SMP
+#if CONFIG_RISCV_BOOT_SPINWAIT
 .Lsecondary_start:
 	/* Set trap vector to spin forever to help debug */
 	la a3, .Lsecondary_park
@@ -371,7 +371,7 @@ clear_bss_done:
 	fence
 
 	tail .Lsecondary_start_common
-#endif
+#endif /* CONFIG_RISCV_BOOT_SPINWAIT */
 
 END(_start_kernel)
 

arch/riscv/kernel/head.h

@@ -16,7 +16,9 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa);
 asmlinkage void __init __copy_data(void);
 #endif
 
+#ifdef CONFIG_RISCV_BOOT_SPINWAIT
 extern void *__cpu_spinwait_stack_pointer[];
 extern void *__cpu_spinwait_task_pointer[];
+#endif
 
 #endif /* __ASM_HEAD_H */