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Diffstat (limited to 'arch/cris/arch-v10/README.mm')
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diff --git a/arch/cris/arch-v10/README.mm b/arch/cris/arch-v10/README.mm deleted file mode 100644 index 67731d7..0000000 --- a/arch/cris/arch-v10/README.mm +++ /dev/null @@ -1,244 +0,0 @@ -Memory management for CRIS/MMU ------------------------------- -HISTORY: - -$Log: README.mm,v $ -Revision 1.1 2001/12/17 13:59:27 bjornw -Initial revision - -Revision 1.1 2000/07/10 16:25:21 bjornw -Initial revision - -Revision 1.4 2000/01/17 02:31:59 bjornw -Added discussion of paging and VM. - -Revision 1.3 1999/12/03 16:43:23 hp -Blurb about that the 3.5G-limitation is not a MMU limitation - -Revision 1.2 1999/12/03 16:04:21 hp -Picky comment about not mapping the first page - -Revision 1.1 1999/12/03 15:41:30 bjornw -First version of CRIS/MMU memory layout specification. - - - - - ------------------------------- - -See the ETRAX-NG HSDD for reference. - -We use the page-size of 8 kbytes, as opposed to the i386 page-size of 4 kbytes. - -The MMU can, apart from the normal mapping of pages, also do a top-level -segmentation of the kernel memory space. We use this feature to avoid having -to use page-tables to map the physical memory into the kernel's address -space. We also use it to keep the user-mode virtual mapping in the same -map during kernel-mode, so that the kernel easily can access the corresponding -user-mode process' data. - -As a comparison, the Linux/i386 2.0 puts the kernel and physical RAM at -address 0, overlapping with the user-mode virtual space, so that descriptor -registers are needed for each memory access to specify which MMU space to -map through. That changed in 2.2, putting the kernel/physical RAM at -0xc0000000, to co-exist with the user-mode mapping. We will do something -quite similar, but with the additional complexity of having to map the -internal chip I/O registers and the flash memory area (including SRAM -and peripherial chip-selets). - -The kernel-mode segmentation map: - - ------------------------ ------------------------ -FFFFFFFF| | => cached | | - | kernel seg_f | flash | | -F0000000|______________________| | | -EFFFFFFF| | => uncached | | - | kernel seg_e | flash | | -E0000000|______________________| | DRAM | -DFFFFFFF| | paged to any | Un-cached | - | kernel seg_d | =======> | | -D0000000|______________________| | | -CFFFFFFF| | | | - | kernel seg_c |==\ | | -C0000000|______________________| \ |______________________| -BFFFFFFF| | uncached | | - | kernel seg_b |=====\=========>| Registers | -B0000000|______________________| \c |______________________| -AFFFFFFF| | \a | | - | | \c | FLASH/SRAM/Peripheral| - | | \h |______________________| - | | \e | | - | | \d | | - | kernel seg_0 - seg_a | \==>| DRAM | - | | | Cached | - | | paged to any | | - | | =======> |______________________| - | | | | - | | | Illegal | - | | |______________________| - | | | | - | | | FLASH/SRAM/Peripheral| -00000000|______________________| |______________________| - -In user-mode it looks the same except that only the space 0-AFFFFFFF is -available. Therefore, in this model, the virtual address space per process -is limited to 0xb0000000 bytes (minus 8192 bytes, since the first page, -0..8191, is never mapped, in order to trap NULL references). - -It also means that the total physical RAM that can be mapped is 256 MB -(kseg_c above). More RAM can be mapped by choosing a different segmentation -and shrinking the user-mode memory space. - -The MMU can map all 4 GB in user mode, but doing that would mean that a -few extra instructions would be needed for each access to user mode -memory. - -The kernel needs access to both cached and uncached flash. Uncached is -necessary because of the special write/erase sequences. Also, the -peripherial chip-selects are decoded from that region. - -The kernel also needs its own virtual memory space. That is kseg_d. It -is used by the vmalloc() kernel function to allocate virtual contiguous -chunks of memory not possible using the normal kmalloc physical RAM -allocator. - -The setting of the actual MMU control registers to use this layout would -be something like this: - -R_MMU_KSEG = ( ( seg_f, seg ) | // Flash cached - ( seg_e, seg ) | // Flash uncached - ( seg_d, page ) | // kernel vmalloc area - ( seg_c, seg ) | // kernel linear segment - ( seg_b, seg ) | // kernel linear segment - ( seg_a, page ) | - ( seg_9, page ) | - ( seg_8, page ) | - ( seg_7, page ) | - ( seg_6, page ) | - ( seg_5, page ) | - ( seg_4, page ) | - ( seg_3, page ) | - ( seg_2, page ) | - ( seg_1, page ) | - ( seg_0, page ) ); - -R_MMU_KBASE_HI = ( ( base_f, 0x0 ) | // flash/sram/periph cached - ( base_e, 0x8 ) | // flash/sram/periph uncached - ( base_d, 0x0 ) | // don't care - ( base_c, 0x4 ) | // physical RAM cached area - ( base_b, 0xb ) | // uncached on-chip registers - ( base_a, 0x0 ) | // don't care - ( base_9, 0x0 ) | // don't care - ( base_8, 0x0 ) ); // don't care - -R_MMU_KBASE_LO = ( ( base_7, 0x0 ) | // don't care - ( base_6, 0x0 ) | // don't care - ( base_5, 0x0 ) | // don't care - ( base_4, 0x0 ) | // don't care - ( base_3, 0x0 ) | // don't care - ( base_2, 0x0 ) | // don't care - ( base_1, 0x0 ) | // don't care - ( base_0, 0x0 ) ); // don't care - -NOTE: while setting up the MMU, we run in a non-mapped mode in the DRAM (0x40 -segment) and need to setup the seg_4 to a unity mapping, so that we don't get -a fault before we have had time to jump into the real kernel segment (0xc0). This -is done in head.S temporarily, but fixed by the kernel later in paging_init. - - -Paging - PTE's, PMD's and PGD's -------------------------------- - -[ References: asm/pgtable.h, asm/page.h, asm/mmu.h ] - -The paging mechanism uses virtual addresses to split a process memory-space into -pages, a page being the smallest unit that can be freely remapped in memory. On -Linux/CRIS, a page is 8192 bytes (for technical reasons not equal to 4096 as in -most other 32-bit architectures). It would be inefficient to let a virtual memory -mapping be controlled by a long table of page mappings, so it is broken down into -a 2-level structure with a Page Directory containing pointers to Page Tables which -each have maps of up to 2048 pages (8192 / sizeof(void *)). Linux can actually -handle 3-level structures as well, with a Page Middle Directory in between, but -in many cases, this is folded into a two-level structure by excluding the Middle -Directory. - -We'll take a look at how an address is translated while we discuss how it's handled -in the Linux kernel. - -The example address is 0xd004000c; in binary this is: - -31 23 15 7 0 -11010000 00000100 00000000 00001100 - -|______| |__________||____________| - PGD PTE page offset - -Given the top-level Page Directory, the offset in that directory is calculated -using the upper 8 bits: - -static inline pgd_t * pgd_offset(struct mm_struct * mm, unsigned long address) -{ - return mm->pgd + (address >> PGDIR_SHIFT); -} - -PGDIR_SHIFT is the log2 of the amount of memory an entry in the PGD can map; in our -case it is 24, corresponding to 16 MB. This means that each entry in the PGD -corresponds to 16 MB of virtual memory. - -The pgd_t from our example will therefore be the 208'th (0xd0) entry in mm->pgd. - -Since the Middle Directory does not exist, it is a unity mapping: - -static inline pmd_t * pmd_offset(pgd_t * dir, unsigned long address) -{ - return (pmd_t *) dir; -} - -The Page Table provides the final lookup by using bits 13 to 23 as index: - -static inline pte_t * pte_offset(pmd_t * dir, unsigned long address) -{ - return (pte_t *) pmd_page(*dir) + ((address >> PAGE_SHIFT) & - (PTRS_PER_PTE - 1)); -} - -PAGE_SHIFT is the log2 of the size of a page; 13 in our case. PTRS_PER_PTE is -the number of pointers that fit in a Page Table and is used to mask off the -PGD-part of the address. - -The so-far unused bits 0 to 12 are used to index inside a page linearily. - -The VM system -------------- - -The kernels own page-directory is the swapper_pg_dir, cleared in paging_init, -and contains the kernels virtual mappings (the kernel itself is not paged - it -is mapped linearily using kseg_c as described above). Architectures without -kernel segments like the i386, need to setup swapper_pg_dir directly in head.S -to map the kernel itself. swapper_pg_dir is pointed to by init_mm.pgd as the -init-task's PGD. - -To see what support functions are used to setup a page-table, let's look at the -kernel's internal paged memory system, vmalloc/vfree. - -void * vmalloc(unsigned long size) - -The vmalloc-system keeps a paged segment in kernel-space at 0xd0000000. What -happens first is that a virtual address chunk is allocated to the request using -get_vm_area(size). After that, physical RAM pages are allocated and put into -the kernel's page-table using alloc_area_pages(addr, size). - -static int alloc_area_pages(unsigned long address, unsigned long size) - -First the PGD entry is found using init_mm.pgd. This is passed to -alloc_area_pmd (remember the 3->2 folding). It uses pte_alloc_kernel to -check if the PGD entry points anywhere - if not, a page table page is -allocated and the PGD entry updated. Then the alloc_area_pte function is -used just like alloc_area_pmd to check which page table entry is desired, -and a physical page is allocated and the table entry updated. All of this -is repeated at the top-level until the entire address range specified has -been mapped. - - - |
