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+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 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.
+
+
+