linux下machine_desc结构体中的phys_io与io_pg_offst 的作用及使用方法

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作者：EasyWave                                                                                 时间：2012.02.12

类别：linux驱动开发                                                                           声明：转载，请保留链接

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1. phys_io 与 io_pg_offst

我们在移植BSP的时候需要填充 machine_desc 结构体，其中有两个字段 phys_io 和 io_pg_offst，如下红色加粗部分：

MACHINE_START(W90P950EVB, "W90P950EVB")
.phys_io = W90X900_PA_UART,
.io_pg_offst = (((u32)W90X900_VA_UART) >> 18) & 0xfffc,
.boot_params =  0x100,
.map_io  = nuc950evb_map_io,
.init_irq = nuc900_init_irq,
.init_machine = nuc950evb_init,
.timer  = &nuc900_timer,
MACHINE_END

在linux2.6.38中已经没有phys_io与io_pg_offs这两个变量，后面的文章会分析这个问题，现在就来分析linux2.6.35中在machine_desc结构体中，有关phys_io和io_pg_offst变量的作用以及使用方法，在介绍phys_io和io_pg_offst变量的作用之前，我们先来熟悉一些machine_desc这结构体：

struct machine_desc { /*  * Note! The first four elements are used  * by assembler code in head.S, head-common.S  */ unsigned int  nr;  /* architecture number */ unsigned int  phys_io; /* start of physical io */ unsigned int  io_pg_offst; /* byte offset for io        * page tabe entry */ const char  *name;  /* architecture name */ unsigned long  boot_params; /* tagged list  */ unsigned int  video_start; /* start of video RAM */ unsigned int  video_end; /* end of video RAM */ unsigned int  reserve_lp0 :1; /* never has lp0 */ unsigned int  reserve_lp1 :1; /* never has lp1 */ unsigned int  reserve_lp2 :1; /* never has lp2 */ unsigned int  soft_reboot :1; /* soft reboot  */ void   (*fixup)(struct machine_desc *,      struct tag *, char **,      struct meminfo *); void   (*map_io)(void);/* IO mapping function */ void   (*init_irq)(void); struct sys_timer *timer;  /* system tick timer */ void   (*init_machine)(void);};

在行7和行8定义了这两个变量，phys_io：物理IO的起始地址，io_pg_offst：IO页表的偏移字节的地址（MMU页表）。phys_io 用来保存 UART 的物理地址，io_pg_offst 用来保存 UART 的内核空间虚拟地址。两者的映射关系在 arch/arm/kernel/head.S 中建立。这样，在 kernel 没有初始化完 MMU 时，就可以通过写 io_pg_offst 向 UART 打印调试信息。这主要在 low level 的调试函数中使用，比如 printascii。

2. phys_io 与 io_pg_offst 的映射关系如何建立

现在可以进入arch\arm\kernel\head.S中分析这两个变量的具体调用情况：

/* *  linux/arch/arm/kernel/head.S * *  Copyright (C) 1994-2002 Russell King *  Copyright (c) 2003 ARM Limited *  All Rights Reserved * * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License version 2 as * published by the Free Software Foundation. * *  Kernel startup code for all 32-bit CPUs */..................................../* * Kernel startup entry point. * --------------------------- * * This is normally called from the decompressor code.  The requirements * are: MMU = off, D-cache = off, I-cache = dont care, r0 = 0, * r1 = machine nr, r2 = atags pointer. * * This code is mostly position independent, so if you link the kernel at * 0xc0008000, you call this at __pa(0xc0008000). * * See linux/arch/arm/tools/mach-types for the complete list of machine * numbers for r1. * * We're trying to keep crap to a minimum; DO NOT add any machine specific * crap here - that's what the boot loader (or in extreme, well justified * circumstances, zImage) is for. */__HEADENTRY(stext)setmodePSR_F_BIT | PSR_I_BIT | SVC_MODE, r9 @ ensure svc mode@ and irqs disabledmrcp15, 0, r9, c0, c0@ get processor idbl__lookup_processor_type@ r5=procinfo r9=cpuidmovsr10, r5@ invalid processor (r5=0)?beq__error_p@ yes, error 'p'bl__lookup_machine_type@ r5=machinfomovsr8, r5@ invalid machine (r5=0)?beq__error_a@ yes, error 'a'bl__vet_atagsbl__create_page_tables/* * The following calls CPU specific code in a position independent * manner.  See arch/arm/mm/proc-*.S for details.  r10 = base of * xxx_proc_info structure selected by __lookup_machine_type * above.  On return, the CPU will be ready for the MMU to be * turned on, and r0 will hold the CPU control register value. */ldrr13, __switch_data@ address to jump to after@ mmu has been enabledadrlr, BSYM(__enable_mmu)@ return (PIC) address ARM(addpc, r10, #PROCINFO_INITFUNC) THUMB(addr12, r10, #PROCINFO_INITFUNC) THUMB(movpc, r12)ENDPROC(stext).............../* * Setup the initial page tables.  We only setup the barest * amount which are required to get the kernel running, which * generally means mapping in the kernel code. * * r8  = machinfo * r9  = cpuid * r10 = procinfo * * Returns: *  r0, r3, r6, r7 corrupted *  r4 = physical page table address */__create_page_tables:pgtblr4@ page table address/* * Clear the 16K level 1 swapper page table */movr0, r4movr3, #0addr6, r0, #0x40001:strr3, [r0], #4strr3, [r0], #4strr3, [r0], #4strr3, [r0], #4teqr0, r6bne1bldrr7, [r10, #PROCINFO_MM_MMUFLAGS] @ mm_mmuflags/* * Create identity mapping for first MB of kernel to * cater for the MMU enable.  This identity mapping * will be removed by paging_init().  We use our current program * counter to determine corresponding section base address. */movr6, pcmovr6, r6, lsr #20@ start of kernel sectionorrr3, r7, r6, lsl #20@ flags + kernel basestrr3, [r4, r6, lsl #2]@ identity mapping/* * Now setup the pagetables for our kernel direct * mapped region. */addr0, r4,  #(KERNEL_START & 0xff000000) >> 18strr3, [r0, #(KERNEL_START & 0x00f00000) >> 18]!ldrr6, =(KERNEL_END - 1)addr0, r0, #4addr6, r4, r6, lsr #181:cmpr0, r6addr3, r3, #1 << 20strlsr3, [r0], #4bls1b#ifdef CONFIG_XIP_KERNEL/* * Map some ram to cover our .data and .bss areas. */orrr3, r7, #(KERNEL_RAM_PADDR & 0xff000000).if(KERNEL_RAM_PADDR & 0x00f00000)orrr3, r3, #(KERNEL_RAM_PADDR & 0x00f00000).endifaddr0, r4,  #(KERNEL_RAM_VADDR & 0xff000000) >> 18strr3, [r0, #(KERNEL_RAM_VADDR & 0x00f00000) >> 18]!ldrr6, =(_end - 1)addr0, r0, #4addr6, r4, r6, lsr #181:cmpr0, r6addr3, r3, #1 << 20strlsr3, [r0], #4bls1b#endif/* * Then map first 1MB of ram in case it contains our boot params. */addr0, r4, #PAGE_OFFSET >> 18orrr6, r7, #(PHYS_OFFSET & 0xff000000).if(PHYS_OFFSET & 0x00f00000)orrr6, r6, #(PHYS_OFFSET & 0x00f00000).endifstrr6, [r0]#ifdef CONFIG_DEBUG_LLldrr7, [r10, #PROCINFO_IO_MMUFLAGS] @ io_mmuflags/* * Map in IO space for serial debugging. * This allows debug messages to be output * via a serial console before paging_init. */ldrr3, [r8, #MACHINFO_PGOFFIO]addr0, r4, r3rsbr3, r3, #0x4000@ PTRS_PER_PGD*sizeof(long)cmpr3, #0x0800@ limit to 512MBmovhir3, #0x0800addr6, r0, r3ldrr3, [r8, #MACHINFO_PHYSIO]orrr3, r3, r71:strr3, [r0], #4addr3, r3, #1 << 20teqr0, r6bne1b#if defined(CONFIG_ARCH_NETWINDER) || defined(CONFIG_ARCH_CATS)/* * If we're using the NetWinder or CATS, we also need to map * in the 16550-type serial port for the debug messages */addr0, r4, #0xff000000 >> 18orrr3, r7, #0x7c000000strr3, [r0]#endif#ifdef CONFIG_ARCH_RPC/* * Map in screen at 0x02000000 & SCREEN2_BASE * Similar reasons here - for debug.  This is * only for Acorn RiscPC architectures. */addr0, r4, #0x02000000 >> 18orrr3, r7, #0x02000000strr3, [r0]addr0, r4, #0xd8000000 >> 18strr3, [r0]#endif#endifmovpc, lrENDPROC(__create_page_tables)

在47行中我们可以看到：bl  __create_page_tables  就进入到了create_page_tables函数中去了，这个函数在上面的79行，将151行开始的代码拿出来单独分析：
#ifdef CONFIG_DEBUG_LL
 ldr r7, [r10, #PROCINFO_IO_MMUFLAGS] @ io_mmuflags
 /*
  * Map in IO space for serial debugging.
  * This allows debug messages to be output
  * via a serial console before paging_init.
  */
 ldr r3, [r8, #MACHINFO_PGOFFIO]
 add r0, r4, r3
 rsb r3, r3, #0x4000   @ PTRS_PER_PGD*sizeof(long)
 cmp r3, #0x0800   @ limit to 512MB
 movhi r3, #0x0800
 add r6, r0, r3
 ldr r3, [r8, #MACHINFO_PHYSIO]
 orr r3, r3, r7
1: str r3, [r0], #4   //这个循环把 phys_io 填充到 io_pg_offst 对应的 MMU 表项中
 add r3, r3, #1 << 20
 teq r0, r6
 bne 1b
#if defined(CONFIG_ARCH_NETWINDER) || defined(CONFIG_ARCH_CATS)
 /*
  * If we're using the NetWinder or CATS, we also need to map
  * in the 16550-type serial port for the debug messages
  */
 add r0, r4, #0xff000000 >> 18
 orr r3, r7, #0x7c000000
 str r3, [r0]
#endif
#ifdef CONFIG_ARCH_RPC
 /*
  * Map in screen at 0x02000000 & SCREEN2_BASE
  * Similar reasons here - for debug.  This is
  * only for Acorn RiscPC architectures.
  */
 add r0, r4, #0x02000000 >> 18
 orr r3, r7, #0x02000000
 str r3, [r0]
 add r0, r4, #0xd8000000 >> 18
 str r3, [r0]
#endif
#endif

上面蓝色加粗部分的代码是在哪里定义的呢？它是在arch\arm\kernel\asm-offsets.c定义的，请看下面加粗的代码：

int main(void)
{
  DEFINE(TSK_ACTIVE_MM,  offsetof(struct task_struct, active_mm));
  BLANK();
  DEFINE(TI_FLAGS,  offsetof(struct thread_info, flags));
  DEFINE(TI_PREEMPT,  offsetof(struct thread_info, preempt_count));
  ........
  ........

  DEFINE(SYS_ERROR0,  0x9f0000);
  BLANK();
  DEFINE(SIZEOF_MACHINE_DESC, sizeof(struct machine_desc));
  DEFINE(MACHINFO_TYPE,  offsetof(struct machine_desc, nr));
  DEFINE(MACHINFO_NAME,  offsetof(struct machine_desc, name));
  DEFINE(MACHINFO_PHYSIO, offsetof(struct machine_desc, phys_io));
  DEFINE(MACHINFO_PGOFFIO, offsetof(struct machine_desc, io_pg_offst));
  BLANK();
  ........
  .........

  return 0;
}

通过以上的DEFINE宏定义取出phys_io与io_pg_offst分别赋给了MACHINE_PHYSIO和MACHINE_PGOFFIO，这样， phys_io 和 io_pg_offst 就建立了映射关系。

3. printascii 与 uart
printascii 函数调用了一个 汇编宏 addruart。 这个宏在 arch/arm/mach-XXX/include/mach/debug-macro.S 中定义。它的代码一般是这种形式：
    .macro    addruart,rx
    @ see if the MMU is enabled and select appropriate base address
    mrc    p15, 0, \rx, c1, c0
    tst    \rx, #1
    ldreq    \rx, =SUART_BASE_PA
    ldrne    \rx, =SUART_BASE_UA
    .endm

显然，这里用到了在 head.S 中建立的映射关系。这个函数有些芯片并没有去实现。这个函数只用于low level 的调试函数。