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MMUv3 initialization sequence.

The code in the initialize_mmu macro sets up MMUv3 memory mapping
identically to MMUv2 fixed memory mapping. Depending on
CONFIG_INITIALIZE_XTENSA_MMU_INSIDE_VMLINUX symbol this code is
located in one of the following address ranges:

    0xF0000000..0xFFFFFFFF (will keep same address in MMU v2 layout;
    			 typically ROM)
    0x00000000..0x07FFFFFF (system RAM; this code is actually linked
    			 at 0xD0000000..0xD7FFFFFF [cached]
    			 or 0xD8000000..0xDFFFFFFF [uncached];
    			 in any case, initially runs elsewhere
    			 than linked, so have to be careful)

The code has the following assumptions:
  This code fragment is run only on an MMU v3.
  TLBs are in their reset state.
  ITLBCFG and DTLBCFG are zero (reset state).
  RASID is 0x04030201 (reset state).
  PS.RING is zero (reset state).
  LITBASE is zero (reset state, PC-relative literals); required to be PIC.

TLB setup proceeds along the following steps.

  Legend:
    VA = virtual address (two upper nibbles of it);
    PA = physical address (two upper nibbles of it);
    pc = physical range that contains this code;

After step 2, we jump to virtual address in 0x40000000..0x5fffffff
that corresponds to next instruction to execute in this code.
After step 4, we jump to intended (linked) address of this code.

    Step 0     Step1     Step 2     Step3     Step 4     Step5
 ============  =====  ============  =====  ============  =====
   VA      PA     PA    VA      PA     PA    VA      PA     PA
 ------    --     --  ------    --     --  ------    --     --
 E0..FF -> E0  -> E0  E0..FF -> E0         F0..FF -> F0  -> F0
 C0..DF -> C0  -> C0  C0..DF -> C0         E0..EF -> F0  -> F0
 A0..BF -> A0  -> A0  A0..BF -> A0         D8..DF -> 00  -> 00
 80..9F -> 80  -> 80  80..9F -> 80         D0..D7 -> 00  -> 00
 60..7F -> 60  -> 60  60..7F -> 60
 40..5F -> 40         40..5F -> pc  -> pc  40..5F -> pc
 20..3F -> 20  -> 20  20..3F -> 20
 00..1F -> 00  -> 00  00..1F -> 00

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