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 comparison, 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.
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