基於PowerPC的Linux內核之旅：__secondary_start(start_here)-下

在開始之前，先指出前面文章出現的一個錯誤，在對mmu_off函數中的RFI指令的介紹時，我簡單的認為是中斷返回，但後來想想，CPU初始化時，中斷還沒有使能，所以中斷返回的說法是不正確的，查了下資料，原來使用RFI指令也可以做程序跳轉使用，使用RFI進行程序跳轉的好處是，程序跳轉後將自動執行isync指令，以保證指令空間的同步，在Linux的初始化階段，使用RFI指令進行程序跳轉比較常見，這裡的RFI指令與中斷返回是沒有任何關系的。造成誤解請見諒。

另外，最近被工作上的事情困擾的有些力不從心了，感覺這篇對MMU的硬件初始化分析寫的極其的爛，待以後收拾了心情，我一定會加以改進的，當然，也希望高手能不吝指教，就當交個朋友吧。

先來看一下MMU_init_hw的詳細代碼(位於mm/ppc_mmu_32.c)：

1. void __init MMU_init_hw(void)
2. {
3. unsigned int hmask, mb, mb2;
4. unsigned int n_hpteg, lg_n_hpteg;
5. /*定義於hash_low_32.S，填充和清除Hash表*/
6. extern unsigned int hash_page_patch_A[];
7. extern unsigned int hash_page_patch_B[], hash_page_patch_C[];
8. extern unsigned int hash_page[];
9. extern unsigned int flush_hash_patch_A[], flush_hash_patch_B[];
11. if (!mmu_has_feature(MMU_FTR_HPTE_TABLE)) {
12. /*在hash_page的開始處放置blr指令,因為在603處理器中仍能接收到DSI(Data Storage Interrupt)異常*/
13. hash_page[0] = 0x4e800020;
14. flush_icache_range((unsigned long) &hash_page[0],
15. (unsigned long) &hash_page[1]); /*清空指令cache*/
16. return;
17. }
19. if ( ppc_md.progress ) ppc_md.progress("hash:enter", 0x105);
21. #define LG_HPTEG_SIZE 6 /* 每個PTEG為64個字節 */
22. #define SDR1_LOW_BITS ((n_hpteg - 1) >> 10)
23. #define MIN_N_HPTEG 1024 /* min 64kB hash table */
25. /*允許每頁內存都有一個HPTE*/
26. n_hpteg = total_memory / (PAGE_SIZE * 8);
27. if (n_hpteg < MIN_N_HPTEG)
28. n_hpteg = MIN_N_HPTEG;
29. lg_n_hpteg = __ilog2(n_hpteg);
30. if (n_hpteg & (n_hpteg - 1)) {
31. ++lg_n_hpteg; /* round up if not power of 2 */
32. n_hpteg = 1 << lg_n_hpteg;
33. }
34. Hash_size = n_hpteg << LG_HPTEG_SIZE;
36. /*為哈希表申請內存地址，這兩步就類似於malloc和memset*/
37. if ( ppc_md.progress ) ppc_md.progress("hash:find piece", 0x322);
38. Hash = __va(memblock_alloc_base(Hash_size, Hash_size,
39. __initial_memory_limit_addr));
40. cacheable_memzero(Hash, Hash_size);
41. _SDR1 = __pa(Hash) | SDR1_LOW_BITS;
43. Hash_end = (struct hash_pte *) ((unsigned long)Hash + Hash_size);
45. /*Patch up the instructions in hash_low_32.S:create_hpte*/
46. if ( ppc_md.progress ) ppc_md.progress("hash:patch", 0x345);
47. Hash_mask = n_hpteg - 1;
48. hmask = Hash_mask >> (16 - LG_HPTEG_SIZE);
49. mb2 = mb = 32 - LG_HPTEG_SIZE - lg_n_hpteg;
50. if (lg_n_hpteg > 16)
51. mb2 = 16 - LG_HPTEG_SIZE;
53. hash_page_patch_A[0] = (hash_page_patch_A[0] & ~0xffff)
54. | ((unsigned int)(Hash) >> 16);
55. hash_page_patch_A[1] = (hash_page_patch_A[1] & ~0x7c0) | (mb << 6);
56. hash_page_patch_A[2] = (hash_page_patch_A[2] & ~0x7c0) | (mb2 << 6);
57. hash_page_patch_B[0] = (hash_page_patch_B[0] & ~0xffff) | hmask;
58. hash_page_patch_C[0] = (hash_page_patch_C[0] & ~0xffff) | hmask;
60. /*確保patch過的地方是否從數據cache中保存，並清除指令cache*/
61. flush_icache_range((unsigned long) &hash_page_patch_A[0],
62. (unsigned long) &hash_page_patch_C[1]);
64. /*Patch up the instructions in hash_low_32.S:flush_hash_page*/
65. flush_hash_patch_A[0] = (flush_hash_patch_A[0] & ~0xffff)
66. | ((unsigned int)(Hash) >> 16);
67. flush_hash_patch_A[1] = (flush_hash_patch_A[1] & ~0x7c0) | (mb << 6);
68. flush_hash_patch_A[2] = (flush_hash_patch_A[2] & ~0x7c0) | (mb2 << 6);
69. flush_hash_patch_B[0] = (flush_hash_patch_B[0] & ~0xffff) | hmask;
70. flush_icache_range((unsigned long) &flush_hash_patch_A[0],
71. (unsigned long) &flush_hash_patch_B[1]);
73. if ( ppc_md.progress ) ppc_md.progress("hash:done", 0x205);
74. }