cpu 访问外设的方法

目前我知道有两种：Memory-mapped I/O and port-mapped I/O 。参考 wiki [1].

但我又参考了另外一片文章[2]：

内存映射 有些体系结构的 CPU （如，PowerPC 、m68k 等）通常只实现一个物理地址空间（ RAM ）。在这种情况下，外设 I/O 端口的物理地址就被映射到 CPU 的单一物理地址空间中，而成为存储空间的一部分。此时，CPU 可以象访问一个内存单元那样访问外设 I/O 端口，而不需要 设立专门的外设 I/O 指令。这就是所谓的“存储空间映射方式”（ Memory － mapped ）。ARM 体系的 CPU 均采用这一模式.

为了验证这篇文章的说法“CPU 可以象访问一个内存单元那样访问外设 I/O 端口，而不需要 设立专门的外设 I/O 指令”。我查了了一下 linux 驱动的代码。发现使用 memory-mapped I/O 的驱动访问外设是使用 readb/readw/readl 这样的接口（ write 类似）。代码定义在 arch/arm/include/asm/io.h (与平台无关）

/*
 *  Memory access primitives
 *  ------------------------
 *
 * These perform PCI memory accesses via an ioremap region.  They don't
 * take an address as such, but a cookie.
 *
 * Again, these are defined to perform little endian accesses.  See the
 * IO port primitives for more information.
 */
#ifndef readl
#define readb_relaxed(c) ({ u8  __r = __raw_readb(c); __r; })
#define readw_relaxed(c) ({ u16 __r = le16_to_cpu((__force __le16) \
					__raw_readw(c)); __r; })
#define readl_relaxed(c) ({ u32 __r = le32_to_cpu((__force __le32) \
					__raw_readl(c)); __r; })

#define writeb_relaxed(v,c)	__raw_writeb(v,c)
#define writew_relaxed(v,c)	__raw_writew((__force u16) cpu_to_le16(v),c)
#define writel_relaxed(v,c)	__raw_writel((__force u32) cpu_to_le32(v),c)

#define readb(c)		({ u8  __v = readb_relaxed(c); __iormb(); __v; })
#define readw(c)		({ u16 __v = readw_relaxed(c); __iormb(); __v; })
#define readl(c)		({ u32 __v = readl_relaxed(c); __iormb(); __v; })

拿 readb 分析，发现它最终会调用__raw_readb 。__raw_readb 应该是与平台实现有关的，比如

(1)arm

定义在 arch/arm/include/asm/io.h

static inline u8 __raw_readb(const volatile void __iomem *addr)
{
	u8 val;
	asm volatile("ldrb %0, %1"
		     : "=r" (val)
		     : "Qo" (*(volatile u8 __force *)addr));
	return val;
}

这里使用了汇编指令 ldrb （ Load Register Byte (register)）来读值。如此看来，在 arm 平台用 mmio 访问是不是有自己的指令呢？

(2)powerpc

定义在 arch/powerpc/include/asm/io.h 。 不管是有没有使用 Indirect IO address tokens ，还是直接访问传过来的地址。都没有像 arm 那样通过一条汇编指令来访问。powerpc 看起来是符合这篇文章的说法的。

*
 * When CONFIG_PPC_INDIRECT_MMIO is set, the platform can provide hooks
 * on all MMIOs. (Note that this is all 64 bits only for now)
 *
 * To help platforms who may need to differentiate MMIO addresses in
 * their hooks, a bitfield is reserved for use by the platform near the
 * top of MMIO addresses (not PIO, those have to cope the hard way).
 *
 * The highest address in the kernel virtual space are:
 *
 *  d0003fffffffffff	# with Hash MMU
 *  c00fffffffffffff	# with Radix MMU
 *
 * The top 4 bits are reserved as the region ID on hash, leaving us 8 bits
 * that can be used for the field.
 *
 * The direct IO mapping operations will then mask off those bits
 * before doing the actual access, though that only happen when
 * CONFIG_PPC_INDIRECT_MMIO is set, thus be careful when you use that
 * mechanism
 *
 * For PIO, there is a separate CONFIG_PPC_INDIRECT_PIO which makes
 * all PIO functions call through a hook.
 */

#ifdef CONFIG_PPC_INDIRECT_MMIO
#define PCI_IO_IND_TOKEN_SHIFT	52
#define PCI_IO_IND_TOKEN_MASK	(0xfful << PCI_IO_IND_TOKEN_SHIFT)
#define PCI_FIX_ADDR(addr)						\
	((PCI_IO_ADDR)(((unsigned long)(addr)) & ~PCI_IO_IND_TOKEN_MASK))
#define PCI_GET_ADDR_TOKEN(addr)					\
	(((unsigned long)(addr) & PCI_IO_IND_TOKEN_MASK) >> 		\
		PCI_IO_IND_TOKEN_SHIFT)
#define PCI_SET_ADDR_TOKEN(addr, token) 				\
do {									\
	unsigned long __a = (unsigned long)(addr);			\
	__a &= ~PCI_IO_IND_TOKEN_MASK;					\
	__a |= ((unsigned long)(token)) << PCI_IO_IND_TOKEN_SHIFT;	\
	(addr) = (void __iomem *)__a;					\
} while(0)
#else
#define PCI_FIX_ADDR(addr) (addr)
#endif


/*
 * Non ordered and non-swapping "raw" accessors
 */

static inline unsigned char __raw_readb(const volatile void __iomem *addr)
{
	return *(volatile unsigned char __force *)PCI_FIX_ADDR(addr);
}

请问 v 友怎么看待这个问题呢？难道是某块代码我看错或者理解错了？

[1]： https://en.wikipedia.org/wiki/Memory-mapped_I/O_and_port-mapped_I/O

[2]： http://blog.chinaunix.net/uid-30035173-id-4714589.html