原文地址：https://blog.csdn.net/u011461299/article/details/10199989
在驱动程序的设计中，我们可能会用到flush_cache_all将ARM cache的内容刷新到RAM，这是因为ARM Linux中cache一般会被设定为write back的。而通常象DMA是访问不了cache，所以如果我们需要启动DMA将RAM中的内容写到Flash中或LCD framebuffer，那么我们就需要调用flush_cache_all将cache中最新的内容刷新到RAM中。如果不这样做在LCD中可能会出现花屏。本文主要分析在ARM平台上到底如何实现的。
它不仅仅flush所有的cache（包括ICache和DCache），也flush了Write Buffer。
1.1 flush_cache_all在ARM Linux中的实现
在include/asm-arm/cacheflush.h中：
#define flush_cache_all() __cpuc_flush_kern_all()//没有参数列表
#define __cpuc_flush_kern_all cpu_cache.flush_kern_all
在setup_processor():
list = lookup_processor_type(processor_id);
//根据processor id找到对应ARM CPU（常见的如ARM926）相关的信息，存在list中。如果想把事情彻底搞清楚，必然要问processor_id是怎么来。它是在Linux Kernel启动时候从ARM chip中读出来。如果以后有机会大家一起讨论ARM Linux的启动全过程，可以详细分析。
…
cpu_cache = *list->cache;
而lookup_processor_type定义在arch/arm/kernel/head-comman.S中：相应的assembler code如下：
.type __lookup_processor_type, %function
__lookup_processor_type:
adr r3, 3f
ldmda r3, {r5 - r7}
sub r3, r3, r7 @ get offset between virt&phys
add r5, r5, r3 @ convert virt addresses to
add r6, r6, r3 @ physical address space
1: ldmia r5, {r3, r4} @ value, mask
and r4, r4, r9 @ mask wanted bits
teq r3, r4
beq 2f
add r5, r5, #PROC_INFO_SZ @ sizeof(proc_info_list)
cmp r5, r6
blo 1b
mov r5, #0 @ unknown processor
2: mov pc, lr
/*
* This provides a C-API version of the above function.
*/
ENTRY(lookup_processor_type)
stmfd sp!, {r4 - r7, r9, lr}
mov r9, r0
bl __lookup_processor_type
mov r0, r5
ldmfd sp!, {r4 - r7, r9, pc}
/*
* Look in include/asm-arm/procinfo.h and arch/arm/kernel/arch.[ch] for
* more information about the __proc_info and __arch_info structures.
*/
.long __proc_info_begin
.long __proc_info_end
3: .long .
.long __arch_info_begin
.long __arch_info_end
它其实就是到__proc_info_begin开始的section中去找到对应当前SOC中用的CPU Cache相关的operation list
再由arch/arm/kernel/vmlinux.lds.S可以__proc_info_begin就是section *(.proc.info.init)的开始地址。
__proc_info_begin = .;
*(.proc.info.init)
__proc_info_end = .;
而我们知道我们所用是ARM926，所以其定义在arch/arm/mm/proc-arm926.S：
.section ".proc.info.init", #alloc, #execinstr
.type __arm926_proc_info,#object
__arm926_proc_info:
.long 0x41069260 @ ARM926EJ-S (v5TEJ)
.long 0xff0ffff0
.long PMD_TYPE_SECT | \
PMD_SECT_BUFFERABLE | \
PMD_SECT_CACHEABLE | \
PMD_BIT4 | \
PMD_SECT_AP_WRITE | \
PMD_SECT_AP_READ
.long PMD_TYPE_SECT | \
PMD_BIT4 | \
PMD_SECT_AP_WRITE | \
PMD_SECT_AP_READ
b __arm926_setup
.long cpu_arch_name
.long cpu_elf_name
.long HWCAP_SWP|HWCAP_HALF|HWCAP_THUMB|HWCAP_FAST_MULT|HWCAP_EDSP|HWCAP_JAVA
.long cpu_arm926_name
.long arm926_processor_functions
.long v4wbi_tlb_fns
.long v4wb_user_fns
.long arm926_cache_fns
.size __arm926_proc_info, . - __arm926_proc_info
arm926_cache_fns定义在同一个文件中，如下：
ENTRY(arm926_cache_fns)
.long arm926_flush_kern_cache_all
.long arm926_flush_user_cache_all
.long arm926_flush_user_cache_range
.long arm926_coherent_kern_range
.long arm926_coherent_user_range
.long arm926_flush_kern_dcache_page
.long arm926_dma_inv_range
.long arm926_dma_clean_range
.long arm926_dma_flush_range
它所对应的struct的定义：（include/asm-arm/cacheflush.h）
struct cpu_cache_fns {
void (*flush_kern_all)(void);
void (*flush_user_all)(void);
void (*flush_user_range)(unsigned long, unsigned long, unsigned int);
void (*coherent_kern_range)(unsigned long, unsigned long);
void (*coherent_user_range)(unsigned long, unsigned long);
void (*flush_kern_dcache_page)(void *);
void (*dma_inv_range)(const void *, const void *);
void (*dma_clean_range)(const void *, const void *);
void (*dma_flush_range)(const void *, const void *);
};
所以其实flush_cache_all在我们的项目中就是arm926_flush_kern_cache_all：其实现在同一个文件中：
/*
* flush_kern_cache_all()
* Clean and invalidate the entire cache.
*/
ENTRY(arm926_flush_kern_cache_all)
mov r2, #VM_EXEC
mov ip, #0
__flush_whole_cache:
#ifdef CONFIG_CPU_DCACHE_WRITETHROUGH
mcr p15, 0, ip, c7, c6, 0 @ invalidate D cache
#else
1: mrc p15, 0, r15, c7, c14, 3 @ test,clean,invalidate
bne 1b
#endif
tst r2, #VM_EXEC
mcrne p15, 0, ip, c7, c5, 0 @ invalidate I cache
mcrne p15, 0, ip, c7, c10, 4 @ drain WB
mov pc, lr