cpu.c

/*
 * Copyright (c) 2000, 2001, 2002, 2003, 2004, 2005, 2008, 2009
 *      The President and Fellows of Harvard College.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE UNIVERSITY AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE UNIVERSITY OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

/*
 * CPU control functions.
 */

#include <types.h>
#include <lib.h>
#include <mips/specialreg.h>
#include <mips/trapframe.h>
#include <platform/maxcpus.h>
#include <cpu.h>
#include <thread.h>

////////////////////////////////////////////////////////////

/*
 * Startup and exception-time stack hook.
 *
 * The MIPS lacks a good way to find the current CPU, current thread,
 * or current thread stack upon trap entry from user mode. To deal
 * with this, we store the CPU number (our number, not the hardware
 * number) in a nonessential field in the MMU, which is about the only
 * place possible, and then use that to index cpustacks[]. This gets
 * us the value to load as the stack pointer. We can then also load
 * curthread from cputhreads[] by parallel indexing.
 *
 * These arrays are also used to start up new CPUs, for roughly the
 * same reasons.
 */

vaddr_t cpustacks[MAXCPUS];
vaddr_t cputhreads[MAXCPUS];

/*
 * Do machine-dependent initialization of the cpu structure or things
 * associated with a new cpu. Note that we're not running on the new
 * cpu when this is called.
 */
void
cpu_machdep_init(struct cpu *c)
{
        vaddr_t stackpointer;

        KASSERT(c->c_number < MAXCPUS);

        if (c->c_curthread->t_stack == NULL) {
                /* boot cpu; don't need to do anything here */
        }
        else {
                /*
                 * Stick the stack in cpustacks[], and thread pointer
                 * in cputhreads[].
                 */

                /* stack base address */
                stackpointer = (vaddr_t) c->c_curthread->t_stack;
                /* since stacks grow down, get the top */
                stackpointer += STACK_SIZE;

                cpustacks[c->c_number] = stackpointer;
                cputhreads[c->c_number] = (vaddr_t)c->c_curthread;
        }
}

////////////////////////////////////////////////////////////

/*
 * Return the type name of the currently running CPU.
 */

const char *
cpu_identify(void)
{
        /* XXX Ought to be more sophisticated. */
        return "MIPS r3000";
}

////////////////////////////////////////////////////////////

/*
 * Interrupt control.
 *
 * While the mips actually has on-chip interrupt priority masking, in
 * the interests of simplicity, we don't use it. Instead we use
 * coprocessor 0 register 12 (the system coprocessor "status"
 * register) bit 0, IEc, which is the global interrupt enable flag.
 * (IEc stands for interrupt-enable-current.)
 */

/*
 * gcc inline assembly to get at the status register.
 *
 * Pipeline hazards:
 *    - there must be at least one cycle between GET_STATUS
 *      and SET_STATUS;
 *    - it may take up to three cycles after SET_STATUS for the
 *      interrupt state to really change.
 *
 * These considerations do not (currently) apply to System/161,
 * however.
 */
#define GET_STATUS(x) __asm volatile("mfc0 %0,$12" : "=r" (x))
#define SET_STATUS(x) __asm volatile("mtc0 %0,$12" :: "r" (x))

/*
 * Interrupts on.
 */
void
cpu_irqon(void)
{
        uint32_t x;

        GET_STATUS(x);
        x |= CST_IEc;
        SET_STATUS(x);
}

/*
 * Interrupts off.
 */
void
cpu_irqoff(void)
{
        uint32_t x;

        GET_STATUS(x);
        x &= ~(uint32_t)CST_IEc;
        SET_STATUS(x);
}

/*
 * Used below.
 */
static
void
cpu_irqonoff(void)
{
        uint32_t x, xon, xoff;

        GET_STATUS(x);
        xon = x | CST_IEc;
        xoff = x & ~(uint32_t)CST_IEc;
        SET_STATUS(xon);
        __asm volatile("nop; nop; nop; nop");
        SET_STATUS(xoff);
}

////////////////////////////////////////////////////////////

/*
 * Idling.
 */

/*
 * gcc inline assembly for the WAIT instruction.
 *
 * mips r2k/r3k has no idle instruction at all.
 *
 * However, to avoid completely overloading the computing cluster, we
 * appropriate the mips32 WAIT instruction.
 */

static
inline
void
wait(void)
{
        /*
         * The WAIT instruction goes into powersave mode until an
         * interrupt is trying to occur.
         *
         * Then switch interrupts on and off again, so we actually
         * take the interrupt.
         *
         * Note that the precise behavior of this instruction in the
         * System/161 simulator is partly guesswork. This code may not
         * work on a real mips.
         */
        __asm volatile(
                ".set push;"            /* save assembler mode */
                ".set mips32;"          /* allow MIPS32 instructions */
                ".set volatile;"        /* avoid unwanted optimization */
                "wait;"                 /* suspend until interrupted */
                ".set pop"              /* restore assembler mode */
              );
}

/*
 * Idle the processor until something happens.
 */
void 
cpu_idle(void)
{
        wait();
        cpu_irqonoff();
}

/*
 * Halt the CPU permanently.
 */
void
cpu_halt(void)
{
        cpu_irqoff();
        while (1) {
                wait();
        }
}