QEMU OpenSBI 裸机开发之定时器中断
1. 中断处理准备
在s模式下的中断设置和处理逻辑和原来的基本一致，首先在entry.S中添加中断处理汇编入口，如下所示：
#
# supervisor trap entry.
#
.globl supervisor_trap_entry
.align 4
supervisor_trap_entry:
# sscratch holds smode trap stack address,
# swap current sp and smode trap stack address
csrrw sp, sscratch, sp
// call the C trap handler in trap.c
call supervisor_trap
# restore old sp value
csrrw sp, sscratch, sp
sret
这里就是设置好中断栈，然后跳转到c函数处理，supervisor_trap和之前的也一样，定义在trap.c中，如下所示。
#include "csr.h"
#include "sbi.h"
#include "timer.h"
#include "printf.h"
#define S_SOFT_INT (1)
#define M_SOFT_INT (3)
#define S_TIMER_INT (5)
#define M_TIMER_INT (7)
#define S_EXT_INT (9)
#define M_EXT_INT (11)
#define INSTRUCTION_ADDR_MISALIGNED (0)
#define INSTRUCTION_ACCESS_FAULT (1)
#define ILLEGAL_INSTRUCTION (2)
#define BREAK_POINT (3)
#define LOAD_ADDR_MISALIGNED (4)
#define LOAD_ACCESS_FAULT (5)
#define STORE_ADDR_MISALIGNED (6)
#define STORE_ACCESS_FAULT (7)
#define ECALL_FROM_UMODE (8)
#define ECALL_FROM_SMODE (9)
#define ECALL_FROM_MMODE (11)
#define INSTRUCTION_PAGE_FAULT (12)
#define LOAD_PAGE_FAULT (13)
#define STORE_PAGE_FAULT (15)
static char *interrupt_cause[] = {
"Reserved",
"Supervisor software interrupt",
"Reserved",
"Machine software interrupt",
"Reserved",
"Supervisor timer interrupt",
"Reserved",
"Machine timer interrupt",
"Reserved",
"Supervisor external interrupt",
"Reserved",
"Machine external interrupt",
"Reserved",
"Reserved",
"Reserved",
"Reserved"
};
static char *exception_cause[] = {
"Instruction address misaligned",
"Instruction access fault",
"Illegal instruction",
"Breakpoint",
"Load address misaligned",
"Load access fault",
"Store/AMO address misaligned",
"Store/AMO access fault",
"Environment call from U-mode",
"Environment call from S-mode",
"Reserved",
"Environment call from M-mode",
"Instruction page fault",
"Load page fault",
"Reserved",
"Store/AMO page fault"
};
void supervisor_trap(void)
{
unsigned long tval = stval_get();
unsigned long sepc = sepc_get();
unsigned long cause = scause_get();
int is_int = (cause & (1l << 63l)) ? 1 : 0;
int scode = cause & 0xff;
if (scode >= 16) {
printf("%s : %s.\r\n", is_int ? "Interrupt" : "Exception", "Unknown code");
return;
}
if (is_int) {
printf("Interrupt : %s.\r\n", interrupt_cause[scode]);
switch (scode) {
case S_SOFT_INT:
// acknowledge the software interrupt by clearing
// the SSIP bit in sip.
sip_set(sip_get() & (~SIP_SSIP));
break;
case S_TIMER_INT:
// acknowledge the timer interrupt by set new val.
sbi_set_timer(mtime_get() + TIMER_CLK_RATE);
break;
}
} else {
printf("Exception : %s.\r\n", exception_cause[scode]);
switch (scode) {
case ILLEGAL_INSTRUCTION:
printf("tval = %p\r\n", tval);
printf("sepc = %p\r\n", sepc);
break;
}
sepc_set(sepc + 4);
}
return;
}
在使用中断之前需要设置好中断相关寄存器，这是在start.c中处理的，逻辑和原来的基本一样，如下所示。
static void supervisor_trap_init(void)
{
//this stack is used for supervisor_trap_entry in entry.S
sscratch_set((unsigned long)(__stack_start + 4096 * 2));
// set the supervisor trap handler.
stvec_set((unsigned long)supervisor_trap_entry);
// enable supervisor interrupts.
sstatus_set(sstatus_get() | SSTATUS_SIE);
// enable supervisor timer and soft interrupts.
sie_set(sie_get() | SIE_STIE | SIE_SSIE);
}
2. 设置定时器
在start.c中，通过SBI接口来设置定时器比较器的值，如下所示。
void start(void)
{
printf("%s %d.\r\n", __func__, __LINE__);
supervisor_trap_init();
sbi_set_timer(mtime_get() + TIMER_CLK_RATE);
}
这里我们读取定时器的当前值用了一个新的接口mtime_get，定时器的值可以通过MMIO映射的寄存器来访问，也可以通过csr指令来访问，用csr指令访问的汇编代码如下。
static inline unsigned long mtime_get(void)
{
unsigned long val;
asm volatile("csrr %0, time" : "=r" (val) );
return val;
}
sbi_set_timer这个函数的参数是要设置的比较器新值，所以需要将当前值先读取出来然后加上一个间隔值，这里加上的是定时器工作频率值，也就是设置1s产生1个中断。
在定时器中断处理中我们需要再次调用sbi_set_timer来设置新的值，这样定时器就可以每隔1s产生一个中断，如下所示。
case S_TIMER_INT:
// acknowledge the timer interrupt by set new val.
sbi_set_timer(mtime_get() + TIMER_CLK_RATE);
break;
在RISCV中，定时器的中断是通过设置新的值来清除的，如果不想下次产生中断，可以设置一个比当前定时器值小的值来实现。
3. 测试
在命令行执行“make qemu”，可以每隔1s打印一条s模式定时器中断信息，如下所示。
gewenbin@gewenbin-virtual-machine:~/Desktop/qemu_test/lesson14$ make qemu
riscv64-unknown-elf-gcc -Wall -Werror -O -fno-omit-frame-pointer -ggdb -mcmodel=medany -ffreestanding -fno-common -nostdlib -mno-relax -I. -fno-stack-protector -fno-pie -no-pie -c -o trap.o trap.c
riscv64-unknown-elf-ld -z max-page-size=4096 -T kernel.ld -o kernelimage entry.o start.o printf.o trap.o
riscv64-unknown-elf-objdump -S kernelimage > kernel.asm
riscv64-unknown-elf-objdump -t kernelimage | sed '1,/SYMBOL TABLE/d; s/ .* / /; /^$/d' > kernel.sym
qemu-system-riscv64 -machine virt -bios fw_jump.elf -kernel kernelimage -m 128M -smp 1 -nographic
OpenSBI v0.8-81-g7dcb1e1
____ _____ ____ _____
/ __ \ / ____| _ \_ _|
| | | |_ __ ___ _ __ | (___ | |_) || |
| | | | '_ \ / _ \ '_ \ \___ \| _ < | |
| |__| | |_) | __/ | | |____) | |_) || |_
\____/| .__/ \___|_| |_|_____/|____/_____|
| |
|_|
Platform Name : riscv-virtio,qemu
Platform Features : timer,mfdeleg
Platform HART Count : 1
Firmware Base : 0x80000000
Firmware Size : 112 KB
Runtime SBI Version : 0.2
Domain0 Name : root
Domain0 Boot HART : 0
Domain0 HARTs : 0*
Domain0 Region00 : 0x0000000080000000-0x000000008001ffff ()
Domain0 Region01 : 0x0000000000000000-0xffffffffffffffff (R,W,X)
Domain0 Next Address : 0x0000000080200000
Domain0 Next Arg1 : 0x0000000082200000
Domain0 Next Mode : S-mode
Domain0 SysReset : yes
Boot HART ID : 0
Boot HART Domain : root
Boot HART ISA : rv64imafdcsu
Boot HART Features : scounteren,mcounteren,time
Boot HART PMP Count : 16
Boot HART PMP Granularity : 4
Boot HART PMP Address Bits: 54
Boot HART MHPM Count : 0
Boot HART MHPM Count : 0
Boot HART MIDELEG : 0x0000000000000222
Boot HART MEDELEG : 0x000000000000b109
start 29.
Interrupt : Supervisor timer interrupt.
Interrupt : Supervisor timer interrupt.
Interrupt : Supervisor timer interrupt.
Interrupt : Supervisor timer interrupt.
Interrupt : Supervisor timer interrupt.
4. 工程源码
工程源码：链接：https://pan.baidu.com/s/1TnTYr7mywdKj5bxpdmWnyA，提取码：q772，见lesson14。
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