RISCV基础开发(十一)

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2021年8月21日11:28:38 评论 1,304

QEMU裸机开发之M模式定时器中断

定时器相关的寄存器也是定义在clint中的,再来看下寄存器映射,如下所示。

RISCV基础开发(十一)

同样的我们只关心第一个核的寄存器,和定时器相关的总共就两个寄存器,“mtime”寄存器相当于计数器,从系统上电开始就一直增加,“mtimecmp”是比较寄存器,当mtime的值增加到和其一样时就可以产生中断。我们在“clint.c”中添加两个接口操作定时器,如下所示。

#include "address.h"

// core local interruptor (CLINT), which contains the timer.
#define CLINT_MSIP             (CLINT_REG_BASE + 0x0)
#define CLINT_MTIMECMP         (CLINT_REG_BASE + 0x4000)
#define CLINT_MTIME            (CLINT_REG_BASE + 0xBFF8) // cycles since boot.

void msoftint_make(void)
{
    *(volatile unsigned int*)CLINT_MSIP = 1;
}

void msoftint_clear(void)
{
    *(volatile unsigned int*)CLINT_MSIP = 0;
}

unsigned long timer_get(void)
{
    return (*(volatile unsigned long*)CLINT_MTIME);
}

void timer_set(unsigned long interval)
{
    *(volatile unsigned long*)CLINT_MTIMECMP = interval;
}

对定时器操作需要知道其输入时钟频率,QEMU中定时器的工作频率大概是10MHz,所以我们在“clint.h”中定义这个频率宏,如下所示。

#define TIMER_CLK_RATE  (10 * 1000 * 1000) //10MHz

在“start.c”中添加设置比较器的代码,如下所示。

void start(void)
{
    printf("%s %d.\r\n", __func__, __LINE__);

    machine_trap_init();

    //msoftint_make();

    timer_set(timer_get() + TIMER_CLK_RATE);
}

将当前时间加上定时器的时钟频率,也就是在1s后产生中断的意思,同样的,在中断处理中,我们需要更新比较寄存器,以让定时器持续每隔1s产生一个中断,如下所示。

void machine_trap(void)
{ 
    unsigned long cause = mcause_get();
    unsigned long mepc  = mepc_get();
    unsigned long tval  = mtval_get();

    int is_int = (cause & (1l << 63l)) ? 1 : 0;
    int mcode = cause & 0xff;

    if (mcode >= 16) {
        printf("%s : %s.\r\n", is_int ? "Interrupt" : "Exception", "Unknown code");
        return;
    }

    if (is_int) {
        printf("Interrupt : %s.\r\n", interrupt_cause[mcode]); 
        switch (mcode) {
        case M_SOFT_INT:
            msoftint_clear();
            break;
        case M_TIMER_INT:
            timer_set(timer_get() + TIMER_CLK_RATE);
            break;
        }
    } else {
        printf("Exception : %s.\r\n", exception_cause[mcode]); 
        switch (mcode) {
        case ILLEGAL_INSTRUCTION:
            printf("tval = %p\r\n", tval);
            printf("mepc = %p\r\n", mepc);
            break;
        case ECALL_FROM_SMODE:
            break;
        }

        mepc_set(mepc + 4);
    }

    return;
}

在命令行执行“make qemu”,可以看到每隔1s打印一条定时器中断信息,如下所示。

gewenbin@gewenbin-virtual-machine:~/Desktop/qemu_test/lesson6$ make qemu
riscv64-unknown-elf-gcc    -c -o entry.o entry.S
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 start.o start.c
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 uart.o uart.c
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-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 clint.o clint.c
riscv64-unknown-elf-ld -z max-page-size=4096 -T kernel.ld -o kernelimage entry.o start.o uart.o trap.o clint.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 none -kernel kernelimage -m 128M -smp 1 -nographic
start 28.
Interrupt : Machine timer interrupt.
Interrupt : Machine timer interrupt.
Interrupt : Machine timer interrupt.
Interrupt : Machine timer interrupt.
Interrupt : Machine timer interrupt.

工程源码:链接:https://pan.baidu.com/s/1TnTYr7mywdKj5bxpdmWnyA,提取码:q772,见lesson6。

gewenbin
  • 本文由 发表于 2021年8月21日11:28:38
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