Branch data     Line data    Source code

       1                 :            : // SPDX-License-Identifier: GPL-2.0
       2                 :            : /*
       3                 :            :  * Emulate a local clock event device via a pseudo clock device.
       4                 :            :  */
       5                 :            : #include <linux/cpu.h>
       6                 :            : #include <linux/err.h>
       7                 :            : #include <linux/hrtimer.h>
       8                 :            : #include <linux/interrupt.h>
       9                 :            : #include <linux/percpu.h>
      10                 :            : #include <linux/profile.h>
      11                 :            : #include <linux/clockchips.h>
      12                 :            : #include <linux/sched.h>
      13                 :            : #include <linux/smp.h>
      14                 :            : #include <linux/module.h>
      15                 :            : 
      16                 :            : #include "tick-internal.h"
      17                 :            : 
      18                 :            : static struct hrtimer bctimer;
      19                 :            : 
      20                 :          0 : static int bc_shutdown(struct clock_event_device *evt)
      21                 :            : {
      22                 :            :         /*
      23                 :            :          * Note, we cannot cancel the timer here as we might
      24                 :            :          * run into the following live lock scenario:
      25                 :            :          *
      26                 :            :          * cpu 0                cpu1
      27                 :            :          * lock(broadcast_lock);
      28                 :            :          *                      hrtimer_interrupt()
      29                 :            :          *                      bc_handler()
      30                 :            :          *                         tick_handle_oneshot_broadcast();
      31                 :            :          *                          lock(broadcast_lock);
      32                 :            :          * hrtimer_cancel()
      33                 :            :          *  wait_for_callback()
      34                 :            :          */
      35                 :          0 :         hrtimer_try_to_cancel(&bctimer);
      36                 :          0 :         return 0;
      37                 :            : }
      38                 :            : 
      39                 :            : /*
      40                 :            :  * This is called from the guts of the broadcast code when the cpu
      41                 :            :  * which is about to enter idle has the earliest broadcast timer event.
      42                 :            :  */
      43                 :          0 : static int bc_set_next(ktime_t expires, struct clock_event_device *bc)
      44                 :            : {
      45                 :            :         /*
      46                 :            :          * This is called either from enter/exit idle code or from the
      47                 :            :          * broadcast handler. In all cases tick_broadcast_lock is held.
      48                 :            :          *
      49                 :            :          * hrtimer_cancel() cannot be called here neither from the
      50                 :            :          * broadcast handler nor from the enter/exit idle code. The idle
      51                 :            :          * code can run into the problem described in bc_shutdown() and the
      52                 :            :          * broadcast handler cannot wait for itself to complete for obvious
      53                 :            :          * reasons.
      54                 :            :          *
      55                 :            :          * Each caller tries to arm the hrtimer on its own CPU, but if the
      56                 :            :          * hrtimer callbback function is currently running, then
      57                 :            :          * hrtimer_start() cannot move it and the timer stays on the CPU on
      58                 :            :          * which it is assigned at the moment.
      59                 :            :          *
      60                 :            :          * As this can be called from idle code, the hrtimer_start()
      61                 :            :          * invocation has to be wrapped with RCU_NONIDLE() as
      62                 :            :          * hrtimer_start() can call into tracing.
      63                 :            :          */
      64                 :          0 :         RCU_NONIDLE( {
      65                 :            :                 hrtimer_start(&bctimer, expires, HRTIMER_MODE_ABS_PINNED_HARD);
      66                 :            :                 /*
      67                 :            :                  * The core tick broadcast mode expects bc->bound_on to be set
      68                 :            :                  * correctly to prevent a CPU which has the broadcast hrtimer
      69                 :            :                  * armed from going deep idle.
      70                 :            :                  *
      71                 :            :                  * As tick_broadcast_lock is held, nothing can change the cpu
      72                 :            :                  * base which was just established in hrtimer_start() above. So
      73                 :            :                  * the below access is safe even without holding the hrtimer
      74                 :            :                  * base lock.
      75                 :            :                  */
      76                 :            :                 bc->bound_on = bctimer.base->cpu_base->cpu;
      77                 :            :         } );
      78                 :          0 :         return 0;
      79                 :            : }
      80                 :            : 
      81                 :            : static struct clock_event_device ce_broadcast_hrtimer = {
      82                 :            :         .name                   = "bc_hrtimer",
      83                 :            :         .set_state_shutdown     = bc_shutdown,
      84                 :            :         .set_next_ktime         = bc_set_next,
      85                 :            :         .features               = CLOCK_EVT_FEAT_ONESHOT |
      86                 :            :                                   CLOCK_EVT_FEAT_KTIME |
      87                 :            :                                   CLOCK_EVT_FEAT_HRTIMER,
      88                 :            :         .rating                 = 0,
      89                 :            :         .bound_on               = -1,
      90                 :            :         .min_delta_ns           = 1,
      91                 :            :         .max_delta_ns           = KTIME_MAX,
      92                 :            :         .min_delta_ticks        = 1,
      93                 :            :         .max_delta_ticks        = ULONG_MAX,
      94                 :            :         .mult                   = 1,
      95                 :            :         .shift                  = 0,
      96                 :            :         .cpumask                = cpu_possible_mask,
      97                 :            : };
      98                 :            : 
      99                 :          0 : static enum hrtimer_restart bc_handler(struct hrtimer *t)
     100                 :            : {
     101                 :          0 :         ce_broadcast_hrtimer.event_handler(&ce_broadcast_hrtimer);
     102                 :            : 
     103                 :          0 :         return HRTIMER_NORESTART;
     104                 :            : }
     105                 :            : 
     106                 :          0 : void tick_setup_hrtimer_broadcast(void)
     107                 :            : {
     108                 :          0 :         hrtimer_init(&bctimer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS_HARD);
     109                 :          0 :         bctimer.function = bc_handler;
     110                 :          0 :         clockevents_register_device(&ce_broadcast_hrtimer);
     111                 :          0 : }