c - 绕过 Nios II 处理器中的数据缓存

Question

我有以下 C 源文件，需要删除一些代码并添加一些代码以绕过 Nios_2_r2c 处理器上的数据缓存。我不知道如何做到这一点。

文件:switch.c

#include "system.h"
#include "pio_regs.h"
#include "bsu_macros.h"
#include "switches.h"

static struct pio_regs *SW = (struct pio_regs *)SWITCH_BASE;
static REGISTER SH_SW;

bits get_RUN ( void ) {
    SH_SW = SW->data;
    return getbit(SH_SW, 17);
}

文件:ledr.c

#include "system.h"
#include "pio_regs.h"
#include "bsu_macros.h"
#include "ledr.h"

static struct pio_regs *LEDR = (struct pio_regs *)LEDR_BASE;
static REGISTER SH_LEDR;

void LEDR_Init ( void ) {
    SH_LEDR = 0;
    LEDR->data = 0;
}

void show_RUN ( bits RUN ) {
    SH_LEDR = putbit (SH_LEDR, RUN, 12);
    LEDR->data = SH_LEDR;
}

使用 I/O 读写通过内联汇编获得它:

文件:switch.c

#include "system.h"
#include "pio_regs.h"
#include "bsu_macros.h"
#include "switches.h"

static struct pio_regs *SW = (struct pio_regs *)SWITCH_BASE;
static REGISTER SH_SW;

bits get_RUN ( void ) {
    //SH_SW = SW->data;
    __asm("ldwio %0, %1" : "=r"(SH_SW) : "m"(SW->data));
    return getbit(SH_SW, 17);
}

文件:ledr.c

#include "system.h"
#include "pio_regs.h"
#include "bsu_macros.h"
#include "ledr.h" 

static struct pio_regs *LEDR = (struct pio_regs *)LEDR_BASE;
static REGISTER SH_LEDR;

void LEDR_Init ( void ) {
    SH_LEDR = 0;
    //LEDR->data = 0;
    __asm("stwio %0, %1" : "=r"(SH_LEDR) : "m"(SW->data));
}

void show_RUN ( bits RUN ) {
    SH_LEDR = putbit (SH_LEDR, RUN, 12);
    //LEDR->data = SH_LEDR;
    __asm("stwio %0, %1" : "=r"(SH_SW) : "m"(SW->data));
}

Answer 1

您可以使用-mno-cache-volatile并将变量声明为 volatile 。事实上，通过读/写某些寄存器来实现的 MMIO必须始终是 volatile 的。

• -mno-cache-volatile

• -mcache-volatile

  Volatile memory access bypass the cache using the I/O variants of the load and store instructions. The default is not to bypass the cache.

https://gcc.gnu.org/onlinedocs/gcc/Nios-II-Options.html

另一个选项是始终通过 -mbypass-cache 绕过缓存

查看

• Memory mapped registers in C/C++
• Use of volatile for registers
• Why is volatile needed in C?
• Managing cache with memory mapped I/O
• Cache bypass in NIOS II