What's the difference between __raw_readl/__raw_writel and readl/writel in linux kernel?
- raw denotes native byte ordering, non-raw means little-endian
- the __ prefix alternatives don't include memory barriers
#define writel(v,c) ({ __iowmb(); writel_relaxed(v,c); })//io写内存屏障，确保之前的io写都完成，再执行后面的写。 byteswapping
#define writel_relaxed(v,c) __raw_writel((__force u32) cpu_to_le32(v),c)//cpu native format--->little endian format
#define __raw_writel(v,a) (__chk_io_ptr(a), *(volatile unsigned int __force *)(a) = (v))//无内存屏障，no byteswapping