Linux下machine_desc結構體中的phys_io與io_pg_offst 的作用及使用方法

1. phys_io 與 io_pg_offst

我們在移植BSP的時候需要填充 machine_desc 結構體，其中有兩個字段 phys_io 和 io_pg_offst，如下紅色加粗部分：

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中已經沒有phys_io與io_pg_offs這兩個變量，後面的文章會分析這個問題，現在就來分析linux2.6.35中在machine_desc結構體中，有關phys_io和io_pg_offst變量的作用以及使用方法，在介紹phys_io和io_pg_offst變量的作用之前，我們先來熟悉一些machine_desc這結構體：

1. struct machine_desc {
2. /*
3. * Note! The first four elements are used
4. * by assembler code in head.S, head-common.S
5. */
6. unsigned int nr; /* architecture number */
7. unsigned int phys_io; /* start of physical io */
8. unsigned int io_pg_offst; /* byte offset for io
9. * page tabe entry */
11. const char *name; /* architecture name */
12. unsigned long boot_params; /* tagged list */
14. unsigned int video_start; /* start of video RAM */
15. unsigned int video_end; /* end of video RAM */
17. unsigned int reserve_lp0 :1; /* never has lp0 */
18. unsigned int reserve_lp1 :1; /* never has lp1 */
19. unsigned int reserve_lp2 :1; /* never has lp2 */
20. unsigned int soft_reboot :1; /* soft reboot */
21. void (*fixup)(struct machine_desc *,
22. struct tag *, char **,
23. struct meminfo *);
24. void (*map_io)(void);/* IO mapping function */
25. void (*init_irq)(void);
26. struct sys_timer *timer; /* system tick timer */
27. void (*init_machine)(void);
28. };

在行7和行8定義了這兩個變量，phys_io：物理IO的起始地址，io_pg_offst：IO頁表的偏移字節的地址（MMU頁表）。phys_io 用來保存 UART 的物理地址，io_pg_offst 用來保存 UART 的內核空間虛擬地址。兩者的映射關系在 arch/arm/kernel/head.S 中建立。這樣，在 kernel 沒有初始化完 MMU 時，就可以通過寫 io_pg_offst 向 UART 打印調試信息。這主要在 low level 的調試函數中使用，比如 printascii。

2. phys_io 與 io_pg_offst 的映射關系如何建立

現在可以進入arch\arm\kernel\head.S中分析這兩個變量的具體調用情況：

1. /*
2. * linux/arch/arm/kernel/head.S
3. *
4. * Copyright (C) 1994-2002 Russell King
5. * Copyright (c) 2003 ARM Limited
6. * All Rights Reserved
7. *
8. * This program is free software; you can redistribute it and/or modify
9. * it under the terms of the GNU General Public License version 2 as
10. * published by the Free Software Foundation.
11. *
12. * Kernel startup code for all 32-bit CPUs
13. */
14. ............
15. ............
16. ............
17. /*
18. * Kernel startup entry point.
19. * ---------------------------
20. *
21. * This is normally called from the decompressor code. The requirements
22. * are: MMU = off, D-cache = off, I-cache = dont care, r0 = 0,
23. * r1 = machine nr, r2 = atags pointer.
24. *
25. * This code is mostly position independent, so if you link the kernel at
26. * 0xc0008000, you call this at __pa(0xc0008000).
27. *
28. * See linux/arch/arm/tools/mach-types for the complete list of machine
29. * numbers for r1.
30. *
31. * We're trying to keep crap to a minimum; DO NOT add any machine specific
32. * crap here - that's what the boot loader (or in extreme, well justified
33. * circumstances, zImage) is for.
34. */
35. __HEAD
36. ENTRY(stext)
37. setmode PSR_F_BIT | PSR_I_BIT | SVC_MODE, r9 @ ensure svc mode
38. @ and irqs disabled
39. mrc p15, 0, r9, c0, c0 @ get processor id
40. bl __lookup_processor_type @ r5=procinfo r9=cpuid
41. movs r10, r5 @ invalid processor (r5=0)?
42. beq __error_p @ yes, error 'p'
43. bl __lookup_machine_type @ r5=machinfo
44. movs r8, r5 @ invalid machine (r5=0)?
45. beq __error_a @ yes, error 'a'
46. bl __vet_atags
47. bl __create_page_tables
49. /*
50. * The following calls CPU specific code in a position independent
51. * manner. See arch/arm/mm/proc-*.S for details. r10 = base of
52. * xxx_proc_info structure selected by __lookup_machine_type
53. * above. On return, the CPU will be ready for the MMU to be
54. * turned on, and r0 will hold the CPU control register value.
55. */
56. ldr r13, __switch_data @ address to jump to after
57. @ mmu has been enabled
58. adr lr, BSYM(__enable_mmu) @ return (PIC) address
59. ARM( add pc, r10, #PROCINFO_INITFUNC )
60. THUMB( add r12, r10, #PROCINFO_INITFUNC )
61. THUMB( mov pc, r12 )
62. ENDPROC(stext)
63. .....
64. .....
65. .....
66. /*
67. * Setup the initial page tables. We only setup the barest
68. * amount which are required to get the kernel running, which
69. * generally means mapping in the kernel code.
70. *
71. * r8 = machinfo
72. * r9 = cpuid
73. * r10 = procinfo
74. *
75. * Returns:
76. * r0, r3, r6, r7 corrupted
77. * r4 = physical page table address
78. */
79. __create_page_tables:
80. pgtbl r4 @ page table address
82. /*
83. * Clear the 16K level 1 swapper page table
84. */
85. mov r0, r4
86. mov r3, #0
87. add r6, r0, #0x4000
88. 1: str r3, [r0], #4
89. str r3, [r0], #4
90. str r3, [r0], #4
91. str r3, [r0], #4
92. teq r0, r6
93. bne 1b
95. ldr r7, [r10, #PROCINFO_MM_MMUFLAGS] @ mm_mmuflags
97. /*
98. * Create identity mapping for first MB of kernel to
99. * cater for the MMU enable. This identity mapping
100. * will be removed by paging_init(). We use our current program
101. * counter to determine corresponding section base address.
102. */
103. mov r6, pc
104. mov r6, r6, lsr #20 @ start of kernel section
105. orr r3, r7, r6, lsl #20 @ flags + kernel base
106. str r3, [r4, r6, lsl #2] @ identity mapping
108. /*
109. * Now setup the pagetables for our kernel direct
110. * mapped region.
111. */
112. add r0, r4, #(KERNEL_START & 0xff000000) >> 18
113. str r3, [r0, #(KERNEL_START & 0x00f00000) >> 18]!
114. ldr r6, =(KERNEL_END - 1)
115. add r0, r0, #4
116. add r6, r4, r6, lsr #18
117. 1: cmp r0, r6
118. add r3, r3, #1 << 20
119. strls r3, [r0], #4
120. bls 1b
122. #ifdef CONFIG_XIP_KERNEL
123. /*
124. * Map some ram to cover our .data and .bss areas.
125. */
126. orr r3, r7, #(KERNEL_RAM_PADDR & 0xff000000)
127. .if (KERNEL_RAM_PADDR & 0x00f00000)
128. orr r3, r3, #(KERNEL_RAM_PADDR & 0x00f00000)
129. .endif
130. add r0, r4, #(KERNEL_RAM_VADDR & 0xff000000) >> 18
131. str r3, [r0, #(KERNEL_RAM_VADDR & 0x00f00000) >> 18]!
132. ldr r6, =(_end - 1)
133. add r0, r0, #4
134. add r6, r4, r6, lsr #18
135. 1: cmp r0, r6
136. add r3, r3, #1 << 20
137. strls r3, [r0], #4
138. bls 1b
139. #endif
141. /*
142. * Then map first 1MB of ram in case it contains our boot params.
143. */
144. add r0, r4, #PAGE_OFFSET >> 18
145. orr r6, r7, #(PHYS_OFFSET & 0xff000000)
146. .if (PHYS_OFFSET & 0x00f00000)
147. orr r6, r6, #(PHYS_OFFSET & 0x00f00000)
148. .endif
149. str r6, [r0]
151. #ifdef CONFIG_DEBUG_LL
152. ldr r7, [r10, #PROCINFO_IO_MMUFLAGS] @ io_mmuflags
153. /*
154. * Map in IO space for serial debugging.
155. * This allows debug messages to be output
156. * via a serial console before paging_init.
157. */
158. ldr r3, [r8, #MACHINFO_PGOFFIO]
159. add r0, r4, r3
160. rsb r3, r3, #0x4000 @ PTRS_PER_PGD*sizeof(long)
161. cmp r3, #0x0800 @ limit to 512MB
162. movhi r3, #0x0800
163. add r6, r0, r3
164. ldr r3, [r8, #MACHINFO_PHYSIO]
165. orr r3, r3, r7
166. 1: str r3, [r0], #4
167. add r3, r3, #1 << 20
168. teq r0, r6
169. bne 1b
170. #if defined(CONFIG_ARCH_NETWINDER) || defined(CONFIG_ARCH_CATS)
171. /*
172. * If we're using the NetWinder or CATS, we also need to map
173. * in the 16550-type serial port for the debug messages
174. */
175. add r0, r4, #0xff000000 >> 18
176. orr r3, r7, #0x7c000000
177. str r3, [r0]
178. #endif
179. #ifdef CONFIG_ARCH_RPC
180. /*
181. * Map in screen at 0x02000000 & SCREEN2_BASE
182. * Similar reasons here - for debug. This is
183. * only for Acorn RiscPC architectures.
184. */
185. add r0, r4, #0x02000000 >> 18
186. orr r3, r7, #0x02000000
187. str r3, [r0]
188. add r0, r4, #0xd8000000 >> 18
189. str r3, [r0]
190. #endif
191. #endif
192. mov pc, lr
193. ENDPROC(__create_page_tables)

在47行中我們可以看到：bl __create_page_tables 就進入到了create_page_tables函數中去了，這個函數在上面的79行，將151行開始的代碼拿出來單獨分析：
#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 //這個循環把 phys_io 填充到 io_pg_offst 對應的 MMU 表項中
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

上面藍色加粗部分的代碼是在哪裡定義的呢？它是在arch\arm\kernel\asm-offsets.c定義的，請看下面加粗的代碼：

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;
}

通過以上的DEFINE宏定義取出phys_io與io_pg_offst分別賦給了MACHINE_PHYSIO和MACHINE_PGOFFIO，這樣， phys_io 和 io_pg_offst 就建立了映射關系。

3. printascii 與 uart
printascii 函數調用了一個 匯編宏 addruart。 這個宏在 arch/arm/mach-XXX/include/mach/debug-macro.S 中定義。它的代碼一般是這種形式：
.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

顯然，這裡用到了在 head.S 中建立的映射關系。這個函數有些芯片並沒有去實現。這個函數只用於low level 的調試函數。