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/*
* This file contains the routines for handling the MMU on those
* PowerPC implementations where the MMU is not using the hash
* table, such as 8xx, 4xx, BookE's etc...
*
* Copyright 2008 Ben Herrenschmidt <benh@kernel.crashing.org>
* IBM Corp.
*
* Derived from previous arch/powerpc/mm/mmu_context.c
* and arch/powerpc/include/asm/mmu_context.h
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License
* as published by the Free Software Foundation; either version
* 2 of the License, or (at your option) any later version.
*
*/
#include <linux/mm.h>
#include <linux/init.h>
#include <asm/mmu_context.h>
#include <asm/tlbflush.h>
/*
* The MPC8xx has only 16 contexts. We rotate through them on each
* task switch. A better way would be to keep track of tasks that
* own contexts, and implement an LRU usage. That way very active
* tasks don't always have to pay the TLB reload overhead. The
* kernel pages are mapped shared, so the kernel can run on behalf
* of any task that makes a kernel entry. Shared does not mean they
* are not protected, just that the ASID comparison is not performed.
* -- Dan
*
* The IBM4xx has 256 contexts, so we can just rotate through these
* as a way of "switching" contexts. If the TID of the TLB is zero,
* the PID/TID comparison is disabled, so we can use a TID of zero
* to represent all kernel pages as shared among all contexts.
* -- Dan
*/
#ifdef CONFIG_8xx
#define NO_CONTEXT 16
#define LAST_CONTEXT 15
#define FIRST_CONTEXT 0
#elif defined(CONFIG_4xx)
#define NO_CONTEXT 256
#define LAST_CONTEXT 255
#define FIRST_CONTEXT 1
#elif defined(CONFIG_E200) || defined(CONFIG_E500)
#define NO_CONTEXT 256
#define LAST_CONTEXT 255
#define FIRST_CONTEXT 1
#else
#error Unsupported processor type
#endif
static unsigned long next_mmu_context;
static unsigned long context_map[LAST_CONTEXT / BITS_PER_LONG + 1];
static atomic_t nr_free_contexts;
static struct mm_struct *context_mm[LAST_CONTEXT+1];
static void steal_context(void);
/* Steal a context from a task that has one at the moment.
* This is only used on 8xx and 4xx and we presently assume that
* they don't do SMP. If they do then this will have to check
* whether the MM we steal is in use.
* We also assume that this is only used on systems that don't
* use an MMU hash table - this is true for 8xx and 4xx.
* This isn't an LRU system, it just frees up each context in
* turn (sort-of pseudo-random replacement :). This would be the
* place to implement an LRU scheme if anyone was motivated to do it.
* -- paulus
*/
static void steal_context(void)
{
struct mm_struct *mm;
/* free up context `next_mmu_context' */
/* if we shouldn't free context 0, don't... */
if (next_mmu_context < FIRST_CONTEXT)
next_mmu_context = FIRST_CONTEXT;
mm = context_mm[next_mmu_context];
flush_tlb_mm(mm);
destroy_context(mm);
}
/*
* Get a new mmu context for the address space described by `mm'.
*/
static inline void get_mmu_context(struct mm_struct *mm)
{
unsigned long ctx;
if (mm->context.id != NO_CONTEXT)
return;
while (atomic_dec_if_positive(&nr_free_contexts) < 0)
steal_context();
ctx = next_mmu_context;
while (test_and_set_bit(ctx, context_map)) {
ctx = find_next_zero_bit(context_map, LAST_CONTEXT+1, ctx);
if (ctx > LAST_CONTEXT)
ctx = 0;
}
next_mmu_context = (ctx + 1) & LAST_CONTEXT;
mm->context.id = ctx;
context_mm[ctx] = mm;
}
void switch_mmu_context(struct mm_struct *prev, struct mm_struct *next)
{
get_mmu_context(next);
set_context(next->context.id, next->pgd);
}
/*
* Set up the context for a new address space.
*/
int init_new_context(struct task_struct *t, struct mm_struct *mm)
{
mm->context.id = NO_CONTEXT;
return 0;
}
/*
* We're finished using the context for an address space.
*/
void destroy_context(struct mm_struct *mm)
{
preempt_disable();
if (mm->context.id != NO_CONTEXT) {
clear_bit(mm->context.id, context_map);
mm->context.id = NO_CONTEXT;
atomic_inc(&nr_free_contexts);
}
preempt_enable();
}
/*
* Initialize the context management stuff.
*/
void __init mmu_context_init(void)
{
/*
* Some processors have too few contexts to reserve one for
* init_mm, and require using context 0 for a normal task.
* Other processors reserve the use of context zero for the kernel.
* This code assumes FIRST_CONTEXT < 32.
*/
context_map[0] = (1 << FIRST_CONTEXT) - 1;
next_mmu_context = FIRST_CONTEXT;
atomic_set(&nr_free_contexts, LAST_CONTEXT - FIRST_CONTEXT + 1);
}
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