LXR linux/arch/powerpc/mm/mmu_context

   1/*
   2 * This file contains the routines for handling the MMU on those
   3 * PowerPC implementations where the MMU substantially follows the
   4 * architecture specification.  This includes the 6xx, 7xx, 7xxx,
   5 * 8260, and POWER3 implementations but excludes the 8xx and 4xx.
   6 *  -- paulus
   7 *
   8 *  Derived from arch/ppc/mm/init.c:
   9 *    Copyright (C) 1995-1996 Gary Thomas (gdt@linuxppc.org)
  10 *
  11 *  Modifications by Paul Mackerras (PowerMac) (paulus@cs.anu.edu.au)
  12 *  and Cort Dougan (PReP) (cort@cs.nmt.edu)
  13 *    Copyright (C) 1996 Paul Mackerras
  14 *
  15 *  Derived from "arch/i386/mm/init.c"
  16 *    Copyright (C) 1991, 1992, 1993, 1994  Linus Torvalds
  17 *
  18 *  This program is free software; you can redistribute it and/or
  19 *  modify it under the terms of the GNU General Public License
  20 *  as published by the Free Software Foundation; either version
  21 *  2 of the License, or (at your option) any later version.
  22 *
  23 */
  24
  25#include <linux/mm.h>
  26#include <linux/init.h>
  27
  28#include <asm/mmu_context.h>
  29#include <asm/tlbflush.h>
  30
  31/*
  32 * On 32-bit PowerPC 6xx/7xx/7xxx CPUs, we use a set of 16 VSIDs
  33 * (virtual segment identifiers) for each context.  Although the
  34 * hardware supports 24-bit VSIDs, and thus >1 million contexts,
  35 * we only use 32,768 of them.  That is ample, since there can be
  36 * at most around 30,000 tasks in the system anyway, and it means
  37 * that we can use a bitmap to indicate which contexts are in use.
  38 * Using a bitmap means that we entirely avoid all of the problems
  39 * that we used to have when the context number overflowed,
  40 * particularly on SMP systems.
  41 *  -- paulus.
  42 */
  43#define NO_CONTEXT              ((unsigned long) -1)
  44#define LAST_CONTEXT            32767
  45#define FIRST_CONTEXT           1
  46
  47/*
  48 * This function defines the mapping from contexts to VSIDs (virtual
  49 * segment IDs).  We use a skew on both the context and the high 4 bits
  50 * of the 32-bit virtual address (the "effective segment ID") in order
  51 * to spread out the entries in the MMU hash table.  Note, if this
  52 * function is changed then arch/ppc/mm/hashtable.S will have to be
  53 * changed to correspond.
  54 *
  55 *
  56 * CTX_TO_VSID(ctx, va) (((ctx) * (897 * 16) + ((va) >> 28) * 0x111) \
  57 *                               & 0xffffff)
  58 */
  59
  60static unsigned long next_mmu_context;
  61static unsigned long context_map[LAST_CONTEXT / BITS_PER_LONG + 1];
  62
  63
  64/*
  65 * Set up the context for a new address space.
  66 */
  67int init_new_context(struct task_struct *t, struct mm_struct *mm)
  68{
  69        unsigned long ctx = next_mmu_context;
  70
  71        while (test_and_set_bit(ctx, context_map)) {
  72                ctx = find_next_zero_bit(context_map, LAST_CONTEXT+1, ctx);
  73                if (ctx > LAST_CONTEXT)
  74                        ctx = 0;
  75        }
  76        next_mmu_context = (ctx + 1) & LAST_CONTEXT;
  77        mm->context.id = ctx;
  78
  79        return 0;
  80}
  81
  82/*
  83 * We're finished using the context for an address space.
  84 */
  85void destroy_context(struct mm_struct *mm)
  86{
  87        preempt_disable();
  88        if (mm->context.id != NO_CONTEXT) {
  89                clear_bit(mm->context.id, context_map);
  90                mm->context.id = NO_CONTEXT;
  91        }
  92        preempt_enable();
  93}
  94
  95/*
  96 * Initialize the context management stuff.
  97 */
  98void __init mmu_context_init(void)
  99{
 100        /* Reserve context 0 for kernel use */
 101        context_map[0] = (1 << FIRST_CONTEXT) - 1;
 102        next_mmu_context = FIRST_CONTEXT;
 103}
 104