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comments for timer setup

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This commit is contained in:
Robert Morris 2019-07-25 05:35:03 -04:00
parent aef3e0f5a4
commit 5d5e4e065f
4 changed files with 23 additions and 8 deletions
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5
kernel/kernelvec.S
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@ -86,6 +86,11 @@ kernelvec:
.globl machinevec
.align 4
machinevec:
# start.c has set up the memory that mscratch points to:
# scratch[0,8,16] : register save area.
# scratch[32] : address of CLINT's MTIMECMP register.
# scratch[40] : desired interval between interrupts.
csrrw a0, mscratch, a0
sd a1, 0(a0)
sd a2, 8(a0)

2
kernel/memlayout.h
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@ -29,7 +29,7 @@
// local interrupt controller, which contains the timer.
#define CLINT 0x2000000L
#define CLINT_MTIMECMP(hartid) (CLINT + 0x4000 + 8*(hartid))
#define CLINT_MTIME (CLINT + 0xBFF8)
#define CLINT_MTIME (CLINT + 0xBFF8) // cycles since boot.
// qemu puts programmable interrupt controller here.
#define PLIC 0x0c000000L

4
kernel/riscv.h
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@ -9,11 +9,11 @@ r_mhartid()
// Machine Status Register, mstatus
#define MSTATUS_MPP_MASK (3L << 11)
#define MSTATUS_MPP_MASK (3L << 11) // previous mode.
#define MSTATUS_MPP_M (3L << 11)
#define MSTATUS_MPP_S (1L << 11)
#define MSTATUS_MPP_U (0L << 11)
#define MSTATUS_MIE (1L << 3)
#define MSTATUS_MIE (1L << 3) // machine-mode interrupt enable.
static inline uint64
r_mstatus()

14
kernel/start.c
View file

@ -37,15 +37,25 @@ start()
w_mideleg(0xffff);
// set up to receive timer interrupts in machine mode,
// for pre-emptive switching and (on hart 0) to drive time.
// which arrive at machinevec in kernelvec.S,
// which turns them into software interrupts for
// devintr() in trap.c.
int id = r_mhartid();
uint64 *scratch = &mscratch0[32 * id];
// ask the CLINT for a timer interrupt 10,000 cycles from now.
*(uint64*)CLINT_MTIMECMP(id) = *(uint64*)CLINT_MTIME + 10000;
// prepare information in scratch[] for machinevec.
// scratch[0..3] : space for machinevec to save registers.
// scratch[4] : address of CLINT MTIMECMP register.
// scratch[5] : desired interval (in cycles) between timer interrupts.
uint64 *scratch = &mscratch0[32 * id];
scratch[4] = CLINT_MTIMECMP(id);
scratch[5] = 10000000;
w_mscratch((uint64)scratch);
// set the machine-mode trap handler.
w_mtvec((uint64)machinevec);
// enable machine-mode interrupts.
w_mstatus(r_mstatus() | MSTATUS_MIE);
// enable machine-mode timer interrupts.
w_mie(r_mie() | MIE_MTIE);
// keep each CPU's hartid in its tp register, for cpuid().