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。
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。
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