LDD-LinuxDeviceDrivers

函数	描述	底层函数
phys_addr_t virt_to_phys(volatile void *address)	将内核的虚拟地址转换为物理地址	__pa或者 __virt_to_phys
void *phys_to_virt(phys_addr_t x)	将物理地址转换为虚拟内核	__va或者__phys_to_virt

其中phy_addr_t用以表示一个内存地址, 定义在include/linux/types.h中

#ifdef CONFIG_PHYS_ADDR_T_64BIT
	typedef u64 phys_addr_t;
#else
	typedef u32 phys_addr_t;
#endif

可以通过搜索我们内核的.config文件搜索CONFIG_PHYS_ADDR_T_64BIT这个配置参数

cat /usr/src/linux-headers-4.2.0-36-generic/.config | grep CONFIG_PHYS_ADDR_T_64BIT

#2 virt_to_phys/phys_to_virt实现

##2.1 不同架构的实现

架构	定义
标准	include/asm-generic/io.h, line 724
arm	arch/arm/include/asm/memory.h, line 272
arm64	arch/arm64/include/asm/memory.h
x86	arch/x86/include/asm/io.h, line 118

virt_to_phys函数在内核中是通过__pa或者__virt_to_phys函数实现的
phys_to_virt函数在内核中是通过__va或者__phys_to_virt函数实现的

##2.2 arm64上的实现

不同架构的实现有细微差别, 但是其基本思路是一致的

下面我们就以arm64上的实现为例进行讲解, 定义在arch/arm64/include/asm/memory.h

/*
 * These are *only* valid on the kernel direct mapped RAM memory.
 * Note: Drivers should NOT use these.  They are the wrong
 * translation for translating DMA addresses.  Use the driver
 * DMA support - see dma-mapping.h.
 */
#define virt_to_phys virt_to_phys
static inline phys_addr_t virt_to_phys(const volatile void *x)
{
    return __virt_to_phys((unsigned long)(x));
}

#define phys_to_virt phys_to_virt
static inline void *phys_to_virt(phys_addr_t x)
{
    return (void *)__phys_to_virt(x);
}

/*
 * Drivers should NOT use these either.
 */
#define __pa(x)         __virt_to_phys((unsigned long)(x))
#define __va(x)         ((void *)__phys_to_virt((phys_addr_t)(x)))
#define pfn_to_kaddr(pfn)       __va((phys_addr_t)(pfn) << PAGE_SHIFT)

可以看到真是的实现是__virt_to_phys和__phys_to_virt函数实现的

##2.3 __virt_to_phys函数

__virt_to_phys函数定义在arch/arm64/include/asm/memory.h, line 109

/*
 * Physical vs virtual RAM address space conversion.  These are
 * private definitions which should NOT be used outside memory.h
 * files.  Use virt_to_phys/phys_to_virt/__pa/__va instead.
 */
#define __virt_to_phys(x) ({                        \
    phys_addr_t __x = (phys_addr_t)(x);                 \
    __x & BIT(VA_BITS - 1) ? (__x & ~PAGE_OFFSET) + PHYS_OFFSET :   \
                 (__x - kimage_voffset); })

其中__x中间变量是内核的一点小技巧, 通过宏实现的函数没有检查, 比如传入一个x++

其主要操作就是

__x & BIT(VA_BITS - 1) ? (__x & ~PAGE_OFFSET) + PHYS_OFFSET : (__x - kimage_voffset); })

##2.4 __phys_to_virt函数

__phys_to_virt函数定义在arch/arm64/include/asm/memory.h, line 114

#define __phys_to_virt(x)       ((unsigned long)((x) - PHYS_OFFSET) | PAGE_OFFSET)

##2.5 __phys_to_kimg函数

#define __phys_to_kimg(x)       ((unsigned long)((x) + kimage_voffset))

##2.6 page_to_phys和phys_to_page函数

/*
 * Convert a page to/from a physical address
 */
#define page_to_phys(page)      (__pfn_to_phys(page_to_pfn(page)))
#define phys_to_page(phys)      (pfn_to_page(__phys_to_pfn(phys)))