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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 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.