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diff --git a/arch/cris/README.mm b/arch/cris/README.mm new file mode 100644 index 000000000..6de4e7077 --- /dev/null +++ b/arch/cris/README.mm @@ -0,0 +1,241 @@ +Memory management for CRIS/MMU +------------------------------ +HISTORY: + +$Log: README.mm,v $ +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 comparision, 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: + +extern 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: + +extern 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: + +extern 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. + + + |