arch/riscv/kernel/head.S

/* SPDX-License-Identifier: GPL-2.0-only */
/*
 * Copyright (C) 2012 Regents of the University of California
 */

#include <asm/thread_info.h>
#include <asm/asm-offsets.h>
#include <asm/asm.h>
#include <linux/init.h>
#include <linux/linkage.h>
#include <asm/thread_info.h>
#include <asm/page.h>
#include <asm/csr.h>
#include <asm/image.h>

__INIT
ENTRY(_start)
	/*
	 * Image header expected by Linux boot-loaders. The image header data
	 * structure is described in asm/image.h.
	 * Do not modify it without modifying the structure and all bootloaders
	 * that expects this header format!!
	 */
	/* jump to start kernel */
	j _start_kernel
	/* reserved */
	.word 0
	.balign 8
#if __riscv_xlen == 64
	/* Image load offset(2MB) from start of RAM */
	.dword 0x200000
#else
	/* Image load offset(4MB) from start of RAM */
	.dword 0x400000
#endif
	/* Effective size of kernel image */
	.dword _end - _start
	.dword __HEAD_FLAGS
	.word RISCV_HEADER_VERSION
	.word 0
	.dword 0
	.ascii RISCV_IMAGE_MAGIC
	.balign 4
	.ascii RISCV_IMAGE_MAGIC2
	.word 0

.global _start_kernel
_start_kernel:
	/* Mask all interrupts */
	csrw CSR_SIE, zero
	csrw CSR_SIP, zero

	/* Load the global pointer */
.option push
.option norelax
	la gp, __global_pointer$
.option pop

	/*
	 * Disable FPU to detect illegal usage of
	 * floating point in kernel space
	 */
	li t0, SR_FS
	csrc CSR_SSTATUS, t0

#ifdef CONFIG_SMP
	li t0, CONFIG_NR_CPUS
	bgeu a0, t0, .Lsecondary_park
#endif

	/* Pick one hart to run the main boot sequence */
	la a3, hart_lottery
	li a2, 1
	amoadd.w a3, a2, (a3)
	bnez a3, .Lsecondary_start

	/* Clear BSS for flat non-ELF images */
	la a3, __bss_start
	la a4, __bss_stop
	ble a4, a3, clear_bss_done
clear_bss:
	REG_S zero, (a3)
	add a3, a3, RISCV_SZPTR
	blt a3, a4, clear_bss
clear_bss_done:

	/* Save hart ID and DTB physical address */
	mv s0, a0
	mv s1, a1
	la a2, boot_cpu_hartid
	REG_S a0, (a2)

	/* Initialize page tables and relocate to virtual addresses */
	la sp, init_thread_union + THREAD_SIZE
	mv a0, s1
	call setup_vm
	la a0, early_pg_dir
	call relocate

	/* Restore C environment */
	la tp, init_task
	sw zero, TASK_TI_CPU(tp)
	la sp, init_thread_union + THREAD_SIZE

	/* Start the kernel */
	call parse_dtb
	tail start_kernel

relocate:
	/* Relocate return address */
	li a1, PAGE_OFFSET
	la a2, _start
	sub a1, a1, a2
	add ra, ra, a1

	/* Point stvec to virtual address of intruction after satp write */
	la a2, 1f
	add a2, a2, a1
	csrw CSR_STVEC, a2

	/* Compute satp for kernel page tables, but don't load it yet */
	srl a2, a0, PAGE_SHIFT
	li a1, SATP_MODE
	or a2, a2, a1

	/*
	 * Load trampoline page directory, which will cause us to trap to
	 * stvec if VA != PA, or simply fall through if VA == PA.  We need a
	 * full fence here because setup_vm() just wrote these PTEs and we need
	 * to ensure the new translations are in use.
	 */
	la a0, trampoline_pg_dir
	srl a0, a0, PAGE_SHIFT
	or a0, a0, a1
	sfence.vma
	csrw CSR_SATP, a0
.align 2
1:
	/* Set trap vector to spin forever to help debug */
	la a0, .Lsecondary_park
	csrw CSR_STVEC, a0

	/* Reload the global pointer */
.option push
.option norelax
	la gp, __global_pointer$
.option pop

	/*
	 * Switch to kernel page tables.  A full fence is necessary in order to
	 * avoid using the trampoline translations, which are only correct for
	 * the first superpage.  Fetching the fence is guarnteed to work
	 * because that first superpage is translated the same way.
	 */
	csrw CSR_SATP, a2
	sfence.vma

	ret

.Lsecondary_start:
#ifdef CONFIG_SMP
	/* Set trap vector to spin forever to help debug */
	la a3, .Lsecondary_park
	csrw CSR_STVEC, a3

	slli a3, a0, LGREG
	la a1, __cpu_up_stack_pointer
	la a2, __cpu_up_task_pointer
	add a1, a3, a1
	add a2, a3, a2

	/*
	 * This hart didn't win the lottery, so we wait for the winning hart to
	 * get far enough along the boot process that it should continue.
	 */
.Lwait_for_cpu_up:
	/* FIXME: We should WFI to save some energy here. */
	REG_L sp, (a1)
	REG_L tp, (a2)
	beqz sp, .Lwait_for_cpu_up
	beqz tp, .Lwait_for_cpu_up
	fence

	/* Enable virtual memory and relocate to virtual address */
	la a0, swapper_pg_dir
	call relocate

	tail smp_callin
#endif

.align 2
.Lsecondary_park:
	/* We lack SMP support or have too many harts, so park this hart */
	wfi
	j .Lsecondary_park
END(_start)

__PAGE_ALIGNED_BSS
	/* Empty zero page */
	.balign PAGE_SIZE