linux/arch/arc/include/asm/mmu_context.h

/*
 * Copyright (C) 2004, 2007-2010, 2011-2012 Synopsys, Inc. (www.synopsys.com)
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation.
 *
 * vineetg: May 2011
 *  -Refactored get_new_mmu_context( ) to only handle live-mm.
 *   retiring-mm handled in other hooks
 *
 * Vineetg: March 25th, 2008: Bug #92690
 *  -Major rewrite of Core ASID allocation routine get_new_mmu_context
 *
 * Amit Bhor, Sameer Dhavale: Codito Technologies 2004
 */

#ifndef _ASM_ARC_MMU_CONTEXT_H
#define _ASM_ARC_MMU_CONTEXT_H

#include <asm/arcregs.h>
#include <asm/tlb.h>

#include <asm-generic/mm_hooks.h>

/*		ARC700 ASID Management
 *
 * ARC MMU provides 8-bit ASID (0..255) to TAG TLB entries, allowing entries
 * with same vaddr (different tasks) to co-exit. This provides for
 * "Fast Context Switch" i.e. no TLB flush on ctxt-switch
 *
 * Linux assigns each task a unique ASID. A simple round-robin allocation
 * of H/w ASID is done using software tracker @asid_cache.
 * When it reaches max 255, the allocation cycle starts afresh by flushing
 * the entire TLB and wrapping ASID back to zero.
 *
 * A new allocation cycle, post rollover, could potentially reassign an ASID
 * to a different task. Thus the rule is to refresh the ASID in a new cycle.
 * The 32 bit @asid_cache (and mm->asid) have 8 bits MMU PID and rest 24 bits
 * serve as cycle/generation indicator and natural 32 bit unsigned math
 * automagically increments the generation when lower 8 bits rollover.
 */

#define MM_CTXT_ASID_MASK	0x000000ff /* MMU PID reg :8 bit PID */
#define MM_CTXT_CYCLE_MASK	(~MM_CTXT_ASID_MASK)

#define MM_CTXT_FIRST_CYCLE	(MM_CTXT_ASID_MASK + 1)
#define MM_CTXT_NO_ASID		0UL

#define hw_pid(mm)		(mm->context.asid & MM_CTXT_ASID_MASK)

extern unsigned int asid_cache;

/*
 * Get a new ASID if task doesn't have a valid one (unalloc or from prev cycle)
 * Also set the MMU PID register to existing/updated ASID
 */
static inline void get_new_mmu_context(struct mm_struct *mm)
{
	unsigned long flags;

	local_irq_save(flags);

	/*
	 * Move to new ASID if it was not from current alloc-cycle/generation.
	 * This is done by ensuring that the generation bits in both mm->ASID
	 * and cpu's ASID counter are exactly same.
	 *
	 * Note: Callers needing new ASID unconditionally, independent of
	 * 	 generation, e.g. local_flush_tlb_mm() for forking  parent,
	 * 	 first need to destroy the context, setting it to invalid
	 * 	 value.
	 */
	if (!((mm->context.asid ^ asid_cache) & MM_CTXT_CYCLE_MASK))
		goto set_hw;

	/* move to new ASID and handle rollover */
	if (unlikely(!(++asid_cache & MM_CTXT_ASID_MASK))) {

		flush_tlb_all();

		/*
		 * Above checke for rollover of 8 bit ASID in 32 bit container.
		 * If the container itself wrapped around, set it to a non zero
		 * "generation" to distinguish from no context
		 */
		if (!asid_cache)
			asid_cache = MM_CTXT_FIRST_CYCLE;
	}

	/* Assign new ASID to tsk */
	mm->context.asid = asid_cache;

set_hw:
	write_aux_reg(ARC_REG_PID, hw_pid(mm) | MMU_ENABLE);

	local_irq_restore(flags);
}

/*
 * Initialize the context related info for a new mm_struct
 * instance.
 */
static inline int
init_new_context(struct task_struct *tsk, struct mm_struct *mm)
{
	mm->context.asid = MM_CTXT_NO_ASID;
	return 0;
}

/* Prepare the MMU for task: setup PID reg with allocated ASID
    If task doesn't have an ASID (never alloc or stolen, get a new ASID)
*/
static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
			     struct task_struct *tsk)
{
#ifndef CONFIG_SMP
	/* PGD cached in MMU reg to avoid 3 mem lookups: task->mm->pgd */
	write_aux_reg(ARC_REG_SCRATCH_DATA0, next->pgd);
#endif

	get_new_mmu_context(next);
}

/*
 * Called at the time of execve() to get a new ASID
 * Note the subtlety here: get_new_mmu_context() behaves differently here
 * vs. in switch_mm(). Here it always returns a new ASID, because mm has
 * an unallocated "initial" value, while in latter, it moves to a new ASID,
 * only if it was unallocated
 */
#define activate_mm(prev, next)		switch_mm(prev, next, NULL)

static inline void destroy_context(struct mm_struct *mm)
{
	mm->context.asid = MM_CTXT_NO_ASID;
}

/* it seemed that deactivate_mm( ) is a reasonable place to do book-keeping
 * for retiring-mm. However destroy_context( ) still needs to do that because
 * between mm_release( ) = >deactive_mm( ) and
 * mmput => .. => __mmdrop( ) => destroy_context( )
 * there is a good chance that task gets sched-out/in, making it's ASID valid
 * again (this teased me for a whole day).
 */
#define deactivate_mm(tsk, mm)   do { } while (0)

#define enter_lazy_tlb(mm, tsk)

#endif /* __ASM_ARC_MMU_CONTEXT_H */