1Memory management for CRIS/MMU 2------------------------------ 3HISTORY: 4 5$Log: README.mm,v $ 6Revision 1.1 2001/12/17 13:59:27 bjornw 7Initial revision 8 9Revision 1.1 2000/07/10 16:25:21 bjornw 10Initial revision 11 12Revision 1.4 2000/01/17 02:31:59 bjornw 13Added discussion of paging and VM. 14 15Revision 1.3 1999/12/03 16:43:23 hp 16Blurb about that the 3.5G-limitation is not a MMU limitation 17 18Revision 1.2 1999/12/03 16:04:21 hp 19Picky comment about not mapping the first page 20 21Revision 1.1 1999/12/03 15:41:30 bjornw 22First version of CRIS/MMU memory layout specification. 23 24 25 26 27 28------------------------------ 29 30See the ETRAX-NG HSDD for reference. 31 32We use the page-size of 8 kbytes, as opposed to the i386 page-size of 4 kbytes. 33 34The MMU can, apart from the normal mapping of pages, also do a top-level 35segmentation of the kernel memory space. We use this feature to avoid having 36to use page-tables to map the physical memory into the kernel's address 37space. We also use it to keep the user-mode virtual mapping in the same 38map during kernel-mode, so that the kernel easily can access the corresponding 39user-mode process' data. 40 41As a comparision, the Linux/i386 2.0 puts the kernel and physical RAM at 42address 0, overlapping with the user-mode virtual space, so that descriptor 43registers are needed for each memory access to specify which MMU space to 44map through. That changed in 2.2, putting the kernel/physical RAM at 450xc0000000, to co-exist with the user-mode mapping. We will do something 46quite similar, but with the additional complexity of having to map the 47internal chip I/O registers and the flash memory area (including SRAM 48and peripherial chip-selets). 49 50The kernel-mode segmentation map: 51 52 ------------------------ ------------------------ 53FFFFFFFF| | => cached | | 54 | kernel seg_f | flash | | 55F0000000|______________________| | | 56EFFFFFFF| | => uncached | | 57 | kernel seg_e | flash | | 58E0000000|______________________| | DRAM | 59DFFFFFFF| | paged to any | Un-cached | 60 | kernel seg_d | =======> | | 61D0000000|______________________| | | 62CFFFFFFF| | | | 63 | kernel seg_c |==\ | | 64C0000000|______________________| \ |______________________| 65BFFFFFFF| | uncached | | 66 | kernel seg_b |=====\=========>| Registers | 67B0000000|______________________| \c |______________________| 68AFFFFFFF| | \a | | 69 | | \c | FLASH/SRAM/Peripheral| 70 | | \h |______________________| 71 | | \e | | 72 | | \d | | 73 | kernel seg_0 - seg_a | \==>| DRAM | 74 | | | Cached | 75 | | paged to any | | 76 | | =======> |______________________| 77 | | | | 78 | | | Illegal | 79 | | |______________________| 80 | | | | 81 | | | FLASH/SRAM/Peripheral| 8200000000|______________________| |______________________| 83 84In user-mode it looks the same except that only the space 0-AFFFFFFF is 85available. Therefore, in this model, the virtual address space per process 86is limited to 0xb0000000 bytes (minus 8192 bytes, since the first page, 870..8191, is never mapped, in order to trap NULL references). 88 89It also means that the total physical RAM that can be mapped is 256 MB 90(kseg_c above). More RAM can be mapped by choosing a different segmentation 91and shrinking the user-mode memory space. 92 93The MMU can map all 4 GB in user mode, but doing that would mean that a 94few extra instructions would be needed for each access to user mode 95memory. 96 97The kernel needs access to both cached and uncached flash. Uncached is 98necessary because of the special write/erase sequences. Also, the 99peripherial chip-selects are decoded from that region. 100 101The kernel also needs its own virtual memory space. That is kseg_d. It 102is used by the vmalloc() kernel function to allocate virtual contiguous 103chunks of memory not possible using the normal kmalloc physical RAM 104allocator. 105 106The setting of the actual MMU control registers to use this layout would 107be something like this: 108 109R_MMU_KSEG = ( ( seg_f, seg ) | // Flash cached 110 ( seg_e, seg ) | // Flash uncached 111 ( seg_d, page ) | // kernel vmalloc area 112 ( seg_c, seg ) | // kernel linear segment 113 ( seg_b, seg ) | // kernel linear segment 114 ( seg_a, page ) | 115 ( seg_9, page ) | 116 ( seg_8, page ) | 117 ( seg_7, page ) | 118 ( seg_6, page ) | 119 ( seg_5, page ) | 120 ( seg_4, page ) | 121 ( seg_3, page ) | 122 ( seg_2, page ) | 123 ( seg_1, page ) | 124 ( seg_0, page ) ); 125 126R_MMU_KBASE_HI = ( ( base_f, 0x0 ) | // flash/sram/periph cached 127 ( base_e, 0x8 ) | // flash/sram/periph uncached 128 ( base_d, 0x0 ) | // don't care 129 ( base_c, 0x4 ) | // physical RAM cached area 130 ( base_b, 0xb ) | // uncached on-chip registers 131 ( base_a, 0x0 ) | // don't care 132 ( base_9, 0x0 ) | // don't care 133 ( base_8, 0x0 ) ); // don't care 134 135R_MMU_KBASE_LO = ( ( base_7, 0x0 ) | // don't care 136 ( base_6, 0x0 ) | // don't care 137 ( base_5, 0x0 ) | // don't care 138 ( base_4, 0x0 ) | // don't care 139 ( base_3, 0x0 ) | // don't care 140 ( base_2, 0x0 ) | // don't care 141 ( base_1, 0x0 ) | // don't care 142 ( base_0, 0x0 ) ); // don't care 143 144NOTE: while setting up the MMU, we run in a non-mapped mode in the DRAM (0x40 145segment) and need to setup the seg_4 to a unity mapping, so that we don't get 146a fault before we have had time to jump into the real kernel segment (0xc0). This 147is done in head.S temporarily, but fixed by the kernel later in paging_init. 148 149 150Paging - PTE's, PMD's and PGD's 151------------------------------- 152 153[ References: asm/pgtable.h, asm/page.h, asm/mmu.h ] 154 155The paging mechanism uses virtual addresses to split a process memory-space into 156pages, a page being the smallest unit that can be freely remapped in memory. On 157Linux/CRIS, a page is 8192 bytes (for technical reasons not equal to 4096 as in 158most other 32-bit architectures). It would be inefficient to let a virtual memory 159mapping be controlled by a long table of page mappings, so it is broken down into 160a 2-level structure with a Page Directory containing pointers to Page Tables which 161each have maps of up to 2048 pages (8192 / sizeof(void *)). Linux can actually 162handle 3-level structures as well, with a Page Middle Directory in between, but 163in many cases, this is folded into a two-level structure by excluding the Middle 164Directory. 165 166We'll take a look at how an address is translated while we discuss how it's handled 167in the Linux kernel. 168 169The example address is 0xd004000c; in binary this is: 170 17131 23 15 7 0 17211010000 00000100 00000000 00001100 173 174|______| |__________||____________| 175 PGD PTE page offset 176 177Given the top-level Page Directory, the offset in that directory is calculated 178using the upper 8 bits: 179 180static inline pgd_t * pgd_offset(struct mm_struct * mm, unsigned long address) 181{ 182 return mm->pgd + (address >> PGDIR_SHIFT); 183} 184 185PGDIR_SHIFT is the log2 of the amount of memory an entry in the PGD can map; in our 186case it is 24, corresponding to 16 MB. This means that each entry in the PGD 187corresponds to 16 MB of virtual memory. 188 189The pgd_t from our example will therefore be the 208'th (0xd0) entry in mm->pgd. 190 191Since the Middle Directory does not exist, it is a unity mapping: 192 193static inline pmd_t * pmd_offset(pgd_t * dir, unsigned long address) 194{ 195 return (pmd_t *) dir; 196} 197 198The Page Table provides the final lookup by using bits 13 to 23 as index: 199 200static inline pte_t * pte_offset(pmd_t * dir, unsigned long address) 201{ 202 return (pte_t *) pmd_page(*dir) + ((address >> PAGE_SHIFT) & 203 (PTRS_PER_PTE - 1)); 204} 205 206PAGE_SHIFT is the log2 of the size of a page; 13 in our case. PTRS_PER_PTE is 207the number of pointers that fit in a Page Table and is used to mask off the 208PGD-part of the address. 209 210The so-far unused bits 0 to 12 are used to index inside a page linearily. 211 212The VM system 213------------- 214 215The kernels own page-directory is the swapper_pg_dir, cleared in paging_init, 216and contains the kernels virtual mappings (the kernel itself is not paged - it 217is mapped linearily using kseg_c as described above). Architectures without 218kernel segments like the i386, need to setup swapper_pg_dir directly in head.S 219to map the kernel itself. swapper_pg_dir is pointed to by init_mm.pgd as the 220init-task's PGD. 221 222To see what support functions are used to setup a page-table, let's look at the 223kernel's internal paged memory system, vmalloc/vfree. 224 225void * vmalloc(unsigned long size) 226 227The vmalloc-system keeps a paged segment in kernel-space at 0xd0000000. What 228happens first is that a virtual address chunk is allocated to the request using 229get_vm_area(size). After that, physical RAM pages are allocated and put into 230the kernel's page-table using alloc_area_pages(addr, size). 231 232static int alloc_area_pages(unsigned long address, unsigned long size) 233 234First the PGD entry is found using init_mm.pgd. This is passed to 235alloc_area_pmd (remember the 3->2 folding). It uses pte_alloc_kernel to 236check if the PGD entry points anywhere - if not, a page table page is 237allocated and the PGD entry updated. Then the alloc_area_pte function is 238used just like alloc_area_pmd to check which page table entry is desired, 239and a physical page is allocated and the table entry updated. All of this 240is repeated at the top-level until the entire address range specified has 241been mapped. 242 243 244 245