Study case: ASPEED Linux version 5.15.41
In the kernel, two clock objects are essential for building the timer frameworks:
- Clock source: Provides the kernel with the ability to read the current time and update the internal wall time accordingly.
- Clock event device: Allows the kernel to configure hardware and issue interrupts at specific times.
The configuration CONFIG_HZ determines the number of ticks per second, impacting the interrupt frequency of the clock event device.
This device can operate in either 'periodic' or 'oneshot' mode, with the latter configuring each interrupt based on the previous one.
Achieving higher resolution in this framework involves handling additional ticks, which can burden the kernel with managing timer interrupts. To address this, the high-resolution timer framework uses the 'oneshot' mode for precision while simulating routine timer events.
For example, if the kernel ticks 100 times per second, the granularity is 10ms. See the picture below for a visual comparison.
In the design of a regular-timer base, multiple granularity levels exist, each containing 64 buckets. When a regular timer is added, the kernel identifies the current CPU's base and selects an appropriate bucket based on the timer's expiration.
During a timer interrupt, a routine task involves updating the timer base and executing functions within each expired timer, following detachment. The previous 'cascade' action, which refilled lower levels from higher ones, has been replaced. Instead, the kernel now waits until all timers of the same bucket expire before executing them collectively. Consequently, this change leads to increased latency in higher granularity scenarios.
In contrast to the regular timer base's bucket array, the high-resolution timer utilizes a simpler and more recognizable approach: the red-black tree.
When transitioning to the high-resolution timer mechanism, the hrtimer determines the next tick and configures the hardware timer. Each timer can then autonomously decide whether to re-queue itself.
It's important to note that regardless of the switch to the high-resolution timer mechanism, timers in both bases are always managed. The distinction lies in the event of a switch failure, wherein the high-resolution timer mechanism remains in the low granularity mode similar to the regular timer mechanism.
(skip)
In our case study, time initiates upon registering the first clock source, fttmr010, as part of the driver's probe process.
[ 0.000000] i2c controller registered, irq 17
[ 0.000000] clocksource: FTTMR010-TIMER2: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 77222644334 ns
[ 0.000142] sched_clock: 32 bits at 24MHz, resolution 40ns, wraps every 86767015915ns
[ 0.001900] Switching to timer-based delay loop, resolution 40ns
The driver also establishes an interrupt service routine for the hardware timer, enabling it to regularly execute the following actions (tick_handle_periodic):
- Update jiffies and wall time.
- Process expired high-resolution timers.
- Validate availability of a suitable hardware timer for the high-resolution framework.
- Process expired regular timers.
- Adjust the scheduler tick.
- Opportunity for context switching.
- Handle expired posix timers.
Subsequently, in the function clocksource_done_booting, fttmr010 is stored in the time keeper.
[ 0.238478] clocksource: Switched to clocksource FTTMR010-TIMER2
In the subsequent timer routine, upon recognizing the qualified hardware timer, the system transitions to the high-resolution mechanism by undertaking these steps:
- Prepare a hrtimer and add it to the hrtimer base. Within this function:
- Perform actions from the prior routine, excluding hrtimer execution (managed externally).
- Forward the hrtimer for future use.
- Replace
tick_handle_periodicwithtick_sched_timer, which manages expired hrtimers and schedules the next tick.
More Details
kernel/time/posix-timers.c
+----------------------------+
| sys_clock_nanosleep_time32 | : set up hrtimer sleeper, sleep till condition met
+-|--------------------------+
| +-------------------+
|--> | clockid_to_kclock | given clock id, get k_clock
| +-------------------+
|
+--> call ->nsleep(), e.g.,
+---------------+
| common_nsleep | set up hrtimer sleeper, sleep till condition met
+---------------+
kernel/time/posix-timers.c
+---------------+
| common_nsleep | : set up hrtimer sleeper, sleep till condition met
+-|-------------+
| +---------------------+
|--> | timespec64_to_ktime | convert timespec to ktime
| +---------------------+
| +-------------------+
+--> | hrtimer_nanosleep | set up hrtimer sleeper, sleep till condition met
+-------------------+
kernel/time/hrtimer.c
+-------------------+
| hrtimer_nanosleep | : set up hrtimer sleeper, sleep till condition met
+-|-----------------+
| +-------------------------------+
|--> | hrtimer_init_sleeper_on_stack | set up hrtimer sleeper for 'current'
| +-------------------------------+
| +------------------------------+
|--> | hrtimer_set_expires_range_ns | set timer expires
| +------------------------------+
| +--------------+
|--> | do_nanosleep | task sleeps till condition met
| +--------------+
| +--------------------------+
+--> | destroy_hrtimer_on_stack | do nothing bc of disabled config
+--------------------------+
kernel/time/hrtimer.c
+-------------------------------+
| hrtimer_init_sleeper_on_stack | : set up hrtimer sleeper for 'current'
+----------------------+--------+
| hrtimer_init_sleeper | : set up hrtimer sleeper for 'current'
+------------------------+
| __hrtimer_init_sleeper | : set up hrtimer sleeper for 'current'
+-|----------------------+
| +----------------+
|--> | __hrtimer_init | save base in hrtimer
| +----------------+
|
|--> install function
| +----------------+
| | hrtimer_wakeup | wake up task, return no-restart
| +----------------+
|
+--> save 'current' in sleeper
kernel/time/hrtimer.c
+----------------+
| __hrtimer_init | : save base in hrtimer
+-|--------------+
|
|--> get percpu hrtimer bases
|
+--> determine base and save in hrtimer
kernel/time/hrtimer.c
+----------------+
| hrtimer_wakeup | : wake up task, return no-restart
+-|--------------+
|
|--> if task
| |
| | +-----------------+
| +--> | wake_up_process |
| +-----------------+
|
+--> return no-restart
kernel/time/hrtimer.c
+--------------+
| do_nanosleep | : task sleeps till condition met
+|-------------+
|
|--> while saved task isn't cleared && no signal pending
| |
| | +-------------------------------+
| |--> | hrtimer_sleeper_start_expires | start a hrtimer sleeper
| | +-------------------------------+
| | +--------------------+
| |--> | freezable_schedule | schedule
| | +--------------------+
| | +----------------+
| +--> | hrtimer_cancel | cancel a hrtimer
| +----------------+
|
+--> if saved task info is cleared, return (normal path)
kernel/time/tick-common.c
+----------------------+
| tick_handle_periodic | : update jiffies/wall_time, run expired hrtimers/timers/posix_timers, update rq clock
+-|--------------------+
| +---------------+
|--> | tick_periodic | update jiffies/wall_time, run expired hrtimers/timers/posix_timers, update rq clock
| +---------------+
|
|--> if event handler is overwritten (not tick_handle_periodic anymore), return
|
+--> if state isn't oneshot, return
|
| +---------------------------+
+--> | clockevents_program_event | program next event
+---------------------------+
kernel/time/tick-common.c
+---------------+
| tick_periodic | : update jiffies/wall_time, run expired hrtimers/timers/posix_timers, update rq clock
+-|-------------+
|
|--> if current cpu is owner of the action
| |
| |--> update 'tick_next_period' (next event)
| |
| | +----------+
| |--> | do_timer | update 'jiffies_64' and global loads
| | +----------+
| | +------------------+
| +--> | update_wall_time | update wall time
| +------------------+
| +----------------------+
|--> | update_process_times | run hrtimers/timers, update rq clock, run posix timers
| +----------------------+
| +--------------+
+--> | profile_tick | do nothing bc of disabled config
+--------------+
kernel/time/timer.c
+----------------------+
| update_process_times | : run hrtimers/timers, update rq clock, run posix timers
+-|--------------------+
| +------------------+
|--> | run_local_timers | run expired hrtimers and timers
| +------------------+
|
|--> if it's in irq
| |
| | +---------------+
| +--> | irq_work_tick | (skip)
| +---------------+
|
| +----------------+
|--> | scheduler_tick | update rq clock, call ->task_tick(), balance runqueues if necessary
| +----------------+
|
+--> if CONFIG_POSIX_TIMERS
|
| +----------------------+
+--> | run_posix_cpu_timers | collect expired timers from task/group, trigger them
+----------------------+
kernel/time/timer.c
+------------------+
| run_local_timers | : switch to hrtimer mechanism, or run expired hrtimers
+-|----------------+
|
|--> get percpu timer base (std)
|
| +--------------------+
|--> | hrtimer_run_queues | switch to hrtimer mechanism, or run expired hrtimers
| +--------------------+
|
+--> if 'jiffies' exceeds expire time
|
| +---------------+
+--> | raise_softirq | timer
+---------------+ +-------------------+
| run_timer_softirq | run each expired timer of timer base(s)
+-------------------+
kernel/time/hrtimer.c
+--------------------+
| hrtimer_run_queues | : switch to hrtimer mechanism, or run expired hrtimers
+-|------------------+
|
|--> get percpu hrtimer base
|
|--> if hrtimer mechanism is active already, return
|
|--> if we are to to switch to hrtimer mechanism
| |
| | +------------------------+
| |--> | hrtimer_switch_to_hres | overwrite handler for hrtimer, switch to oneshot mode,
| | +------------------------+ lable 'hres_active', setup sched hrtimer
| |
| +--> return
|
|--> if at lease one timer expires
| |
| | +----------------------++---------------------+
| +--> | raise_softirq_irqoff || hrtimer_run_softirq |
| +----------------------++---------------------+
| run expired hrtimers of percpu hrtimer bases
|
| +----------------------+
+--> | __hrtimer_run_queues | for each base, run expired hrtimers
+----------------------+
kernel/time/hrtimer.c
+------------------------+
| hrtimer_switch_to_hres | : overwrite handler for hrtimer, switch to oneshot mode, lable 'hres_active', setup sched hrtimer
+-|----------------------+
| +-------------------+
|--> | tick_init_highres | overwrite evtdev handler for hrtimer, switch to state/mode to oneshot
| +-------------------+
|
|--> label hres_active in base
|
| +------------------------+
|--> | tick_setup_sched_timer | setup a sched hrtimer
| +------------------------+
| +----------------------+
+--> | retrigger_next_event | retrigger the interrupt to get things going?
+----------------------+
kernel/time/tick-oneshot.c
+-------------------+
| tick_init_highres | : overwrite evtdev handler for hrtimer, switch to state/mode to oneshot
+------------------------+
| tick_switch_to_oneshot | : overwrite evtdev handler for hrtimer, switch to state/mode to oneshot
+-|----------------------+
|
|--> if evtdev doesn't support one shot, return error
|
|--> tick_dev mode = oneshot
|
|--> install arg handler, e.g.,
| +-------------------+
| | hrtimer_interrupt | for each base: run expired hrtimers, program next event
| +-------------------+
|
| +--------------------------+
|--> | clockevents_switch_state | oneshot
| +--------------------------+
| +----------------------------------+
+--> | tick_broadcast_switch_to_oneshot | (skip)
+----------------------------------+
kernel/time/hrtimer.c
+-------------------+
| hrtimer_interrupt | : for each base: run expired hrtimers, program next event
+-|-----------------+
|
|--> get percpu hrtimer base
|
| +---------------------+
|--> | hrtimer_update_base | update boot/real/tai offset of arg base
| +---------------------+
|
|--> if softriq expires
| |
| | +----------------------+
| +--> | raise_softirq_irqoff | raise the corresponding flag (hrtimer) in softirq bitmap
| +----------------------+
| +----------------------+
|--> | __hrtimer_run_queues | for each base, run expired hrtimers
| +----------------------+
| +---------------------------+
|--> | hrtimer_update_next_event | calculate next expire time
| +---------------------------+
| +--------------------+
+--> | tick_program_event | given next expire time, program evtdev event
+--------------------+
kernel/time/tick-sched.c
+------------------------+
| tick_setup_sched_timer | : setup a sched hrtimer
+-|----------------------+
| +--------------+
|--> | hrtimer_init | init hrtimer
| +--------------+
|
|--> install function
| +------------------+
| | tick_sched_timer | update jiffies/wall_time, run hrtimers/timers/posix_timers, update rq clock, forward hrtimer
| +------------------+
|
| +---------------------+
|--> | hrtimer_set_expires | set next expiry
| +---------------------+
|
|--> if 'sched_skew_tick' is set (probably not our case), blabla
|
| +-----------------+
|--> | hrtimer_forward | forward hrtimer
| +-----------------+
| +-----------------------+
|--> | hrtimer_start_expires | dequeue hrtimer, set next expiry, switch base and enqueue, re-program next event is necessary
| +-----------------------+
| +--------------------+
+--> | tick_nohz_activate | (skip)
+--------------------+
kernel/time/tick-sched.c
+------------------+
| tick_sched_timer | : update jiffies/wall_time, run hrtimers/timers/posix_timers, update rq clock, forward hrtimer
+-|----------------+
| +---------------------+
|--> | tick_sched_do_timer | update jiffies/global_load/wall_time
| +---------------------+
|
|--> if we're in irq context
| |
| | +-------------------+
| +--> | tick_sched_handle | run hrtimers/timers, update rq clock, run posix timers
| +-------------------+
| +-----------------+
+--> | hrtimer_forward | updatre next expiry of hrtimer
+-----------------+
kernel/time/tick-sched.c
+---------------------+
| tick_sched_do_timer | : update jiffies/global_load/wall_time
+-|-------------------+
|
+--> if current cpu is the one
|
| +--------------------------+
+--> | tick_do_update_jiffies64 | update jiffies/global_load/wall_time
+--------------------------+
kernel/time/tick-sched.c
+--------------------------+
| tick_do_update_jiffies64 | : update jiffies/global_load/wall_time
+-|------------------------+
|
|--> get 'tick_next_period'
|
|--> if 'now' doesn't reach next expire, return
|
|--> update 'jiffies_64'
|
|--> update 'tick_next_period'
|
| +------------------+
|--> | calc_global_load |
| +------------------+
| +------------------+
+--> | update_wall_time |
+------------------+
kernel/time/tick-sched.c
+-------------------+
| tick_sched_handle | : run hrtimers/timers, update rq clock, run posix timers
+-|-----------------+
| +----------------------+
|--> | update_process_times | run hrtimers/timers, update rq clock, run posix timers
| +----------------------+
| +--------------+
+--> | profile_tick | do nothing bc of disabled config
+--------------+
kernel/time/tick-sched.c
+-----------------------+
| hrtimer_start_expires | : dequeue hrtimer, set next expiry, switch base and enqueue, re-program next event is necessary
+------------------------+
| hrtimer_start_range_ns | : dequeue hrtimer, set next expiry, switch base and enqueue, re-program next event is necessary
+-|----------------------+
| +--------------------------+
|--> | __hrtimer_start_range_ns | dequeue hrtimer, set next expiry, switch base and enqueue, re-program next event is necessary
| +--------------------------+
|
+--> if need reprogram
|
| +-------------------+
+--> | hrtimer_reprogram | re-program next event
+-------------------+
kernel/time/hrtimer.c
+--------------------------+
| __hrtimer_start_range_ns | : dequeue hrtimer, set next expiry, switch base and enqueue, re-program next event is necessary
+-|------------------------+
| +----------------+
|--> | remove_hrtimer | remove hrtimer, might reprogram next event accordingly
| +----------------+
| +------------------------------+
|--> | hrtimer_set_expires_range_ns | set next expiry
| +------------------------------+
|
|--> if no force_local
| |
| | +---------------------+
| +--> | switch_hrtimer_base | save target base in hrtimer
| +---------------------+
| +-----------------+
|--> | enqueue_hrtimer | add hrtimer to target base
| +-----------------+
|
|--> if no force_local, return
|
| +-------------------------+
+--> | hrtimer_force_reprogram | program next event
+-------------------------+
kernel/time/posix-cpu-timers.c
+----------------------+
| run_posix_cpu_timers | : collect expired timers from task/group, trigger them
+-|--------------------+
| +----------------------+
|--> | fastpath_timer_check | check if there's expired timer
| +----------------------+
|
|--> if no, return
|
| +------------------------+
+--> | __run_posix_cpu_timers | collect expired timers from task/group, trigger them
+------------------------+
kernel/time/posix-cpu-timers.c
+------------------------+
| __run_posix_cpu_timers | : collect expired timers from task/group, trigger them
+-------------------------+
| handle_posix_cpu_timers | : collect expired timers from task/group, trigger them
+-|-----------------------+
| +---------------------+
|--> | check_thread_timers | move expired timers of given task to arg list
| +---------------------+
| +----------------------+
|--> | check_process_timers | move expired timers of given task's signal to arg list
| +----------------------+
|
+--> for each expired timer in list
|
|--> remove it from list
|
| +----------------+
+--> | cpu_timer_fire | determine pid type and send signal to target pid
+----------------+
kernel/time/posix-cpu-timers.c
+---------------------+
| check_thread_timers | : move expired timers of given task to arg list
+-|-------------------+
|
+--> get cpu timers from task
|
| +---------------------+
|--> | task_sample_cputime | take utime/stime from task and save in samples
| +---------------------+
| +-------------------------+
+--> | collect_posix_cputimers | move expired timers of all bases to arg list
+-------------------------+
kernel/time/posix-cpu-timers.c
+-------------------------+
| collect_posix_cputimers | : move expired timers of all bases to arg list
+-|-----------------------+
|
+--> for each clock base
|
| +--------------------+
+--> | collect_timerqueue | move expired timers to arg list
+--------------------+
kernel/time/posix-cpu-timers.c
+--------------------+
| collect_timerqueue | : move expired timers to arg list
+-|------------------+
|
+--> while time queue has next entry
|
|--> if the timer hans't expired, return
|
| +-------------------+
|--> | cpu_timer_dequeue | remove timer from where it is
| +-------------------+
| +---------------+
+--> | list_add_tail | add to arg list
+---------------+
kernel/time/posix-cpu-timers.c
+----------------------+
| check_process_timers | : move expired timers of given task's signal to arg list
+-|--------------------+
|
|--> get timer base(s) from signal
|
| +-------------------------+
+--> | collect_posix_cputimers | move expired timers of all bases to arg list
+-------------------------+
kernel/time/posix-cpu-timers.c
+----------------+
| cpu_timer_fire | : determine pid type and send signal to target pid
+-|--------------+
|
|--> if it's a oneshot timer
| |
| | +-------------------+
| |--> | posix_timer_event | determine pid type and send signal to target pid
| | +-------------------+
| |
| +--> clear expiry
|
+--> else
|
| +-------------------+
+--> | posix_timer_event | determine pid type and send signal to target pid
+-------------------+
start_kernel
|
|--> time_init
| -
| +--> timer_probe
| -
| +--> aspeed_timer_init
| fttmr010_common_init
| |
| |--> clocksource_mmio_init
| | -
| | +--> clocksource_register_hz [ 0.000000] clocksource: FTTMR010-TIMER2: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 77222644334 ns
| |
| |--> sched_clock_register [ 0.000124] sched_clock: 32 bits at 24MHz, resolution 40ns, wraps every 86767015915ns
| |
| +--> register_current_timer_delay [ 0.001753] Switching to timer-based delay loop, resolution 40ns
|
+--> calibrate_delay [ 0.005060] Calibrating delay loop (skipped), value calculated using timer frequency.. 49.50 BogoMIPS (lpj=247500)
kernel_init
-
+--> kernel_init_freeable
-
+--> do_basic_setup
|
|--> init_jiffies_clocksource
| -
| +--> __clocksource_register [ 0.081094] clocksource: jiffies: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 19112604462750000 ns
|
|--> clocksource_done_booting
| -
| +--> clocksource_select [ 0.212355] clocksource: Switched to clocksource FTTMR010-TIMER2
|
+--> aspeed_i2c_bus_driver_init
-
+--> aspeed_i2c_probe_bus
-
+--> i2c_new_client_device
-
+--> rv8803_probe [ 1.722342] rtc-rv8803 11-0032: registered as rtc0
[ 1.723329] rtc-rv8803 11-0032: setting system clock to 2023-07-05T03:02:00 UTC (1688526120)
init_timers: init percpu timer bases, register softirq action
hrtimers_init: init hrtimer bases, register softirq action
timekeeping_init: init timekeeping mechanism
time_init: init clock, probe timer, setup broadcast
dummy_timer_starting_cpu: register dummy evtdev
init_jiffies_clocksource: register 'jiffies' clock source
clocksource_done_booting: update time keeper's clock source with the best one (fttmr010 in our case)
init_timer_list_procfs: create '/proc/timer_list' with 'timer_list_sops' installed
alarmtimer_init: (skip, it matches no device)
init_posix_timers: create kmem cache for 'k_itimer'
timeriomem_rng_driver_init: (skip, random# related)
timer_led_trigger_init: (skip)
kernel/time/timer.c
+-------------+
| init_timers | : init percpu timer bases, register softirq action
+-|-----------+
| +-----------------+
|--> | init_timer_cpus | init percpu timer_bases
| +-----------------+
| +---------------------------+
|--> | posix_cputimers_init_work | do nothing bc of disabled config
| +---------------------------+
| +--------------+
+--> | open_softirq | register softirq action
+--------------+ +-------------------+
| run_timer_softirq | run each expired timer of timer base(s)
+-------------------+
kernel/time/timer.c
+-------------------+
| run_timer_softirq | : run each expired timer of timer base(s)
+-|-----------------+
|
|--> get percpu timer base (std)
|
| +--------------+
|--> | __run_timers | run each expired timer
| +--------------+
|
+--> if deferrable timer base exists
|
| +--------------+
+--> | __run_timers | run each expired timer
+--------------+
kernel/time/timer.c
+--------------+
| __run_timers | : run each expired timer
+-|------------+
|
|--> if nothing expires yet, return
|
+--> while there's at least one expired timer
|
| +------------------------+
|--> | collect_expired_timers | collect expired timers from arg base
| +------------------------+
|
|--> base->clk++
|
| +------------------------+
|--> | __next_timer_interrupt | look into timer base to get next expiration
| +------------------------+
|
+--> while level--
|
| +---------------+
+--> | expire_timers | given list, detach each timer and call its function
+---------------+
kernel/time/timer.c
+------------------------+
| collect_expired_timers | : collect expired timers from arg base
+-|----------------------+
|
+--> for each level_depth
|
|--> determine idx
|
|--> if it's set in bitmap (clear it)
| |
| |--> get vector from base (base contains all vectors in all wheels)
| |
| |--> add vector to head list
| |
| +--> heads++, level++
|
|--> if no need to check next level, break
|
+--> adjust level to fit next granularity
kernel/time/timer.c
+------------------------+
| __next_timer_interrupt | : look into timer base to get next expiration
+-|----------------------+
|
+--> for each level depth
|
| +---------------------+
|--> | next_pending_bucket | get pos of next pending bucket
| +---------------------+
|
|--> if it expires early than next, update next with it
|
|--> if no timer can expire earlier, break
|
+--> adjust clk for next round
kernel/time/timer.c
+---------------+
| expire_timers | : given list, detach each timer and call its function
+-|-------------+
|
+--> while list isn't empty
|
|--> get first timer from list
|
| +--------------+
|--> | detach_timer |
| +--------------+
| +---------------+
+--> | call_timer_fn |
+---------------+
kernel/time/hrtimer.c
+---------------+
| hrtimers_init | : init hrtimer bases, register softirq action
+-|-------------+
| +----------------------+
|--> | hrtimers_prepare_cpu | init hrtimer bases for the current cpu
| +----------------------+
| +--------------+
+--> | open_softirq | register softirq action
+--------------+ +---------------------+
| hrtimer_run_softirq | run expired hrtimers of percpu hrtimer bases
+---------------------+
kernel/time/hrtimer.c
+---------------------+
| hrtimer_run_softirq | : run expired hrtimers of percpu hrtimer bases
+-|-------------------+
|
|--> get percpu hrtimer base
|
| +---------------------+
|--> | hrtimer_update_base | given hrtimer abse, update its realtime/boottime/tai offset
| +---------------------+
| +----------------------+
+--> | __hrtimer_run_queues | for each base, run expired hrtimers
+----------------------+
kernel/time/hrtimer.c
+----------------------+
| __hrtimer_run_queues | : for each base, run expired hrtimers
+-|--------------------+
|
+--> for each base in hrtimer_bases
-
+--> while we can still get active hrtimer from base
|
|--> if it's not expired, break
|
| +---------------+
+--> | __run_hrtimer | detach hrtimer, call function, attach back if needed
+---------------+
kernel/time/hrtimer.c
+---------------+
| __run_hrtimer | : detach hrtimer, call function, attach back if needed
+-|-------------+
| +------------------+
|--> | __remove_hrtimer | detach hrtimer from queue, if queue becomes empty, clear bit@bitmap
| +------------------+
|
|--> call timer's function
|
+--> if need restart
|
| +-----------------+
+--> | enqueue_hrtimer | add hrtimer back to base->active
+-----------------+
kernel/time/timekeeping.c
+------------------+
| timekeeping_init | : init timekeeping mechanism
+-|----------------+
| +--------------------------------------+
|--> | read_persistent_wall_and_boot_offset |
| +--------------------------------------+
| +----------+
|--> | ntp_init | (skip)
| +----------+
| +---------------------------+
|--> | clocksource_default_clock | get 'clocksource_jiffies'
| +---------------------------+
|
|--> if it has ->enable(), call it (not our case)
|
| +--------------------+
|--> | tk_setup_internals | set up internals to use clocksource clock
| +--------------------+
|
|--> set walltime to 'tk_core'
|
| +--------------------+
+--> | timekeeping_update |
+--------------------+
arch/arm/kernel/time.c
+-----------+
| time_init | : init clock, probe timer, setup broadcast
+-|---------+
|
|--> if machine desc has ->init_time()
| -
| +--> (not our case)
|
+--> else
|
| +-------------+
|--> | of_clk_init | for each clk provider, call its callback and remove from list
| +-------------+
| +-------------+
|--> | timer_probe | probe timer (register clocksource/schedclock_read()/timer_isr/clockevent_dev)
| +-------------+
| +------------------------------+
+--> | tick_setup_hrtimer_broadcast | (skip)
+------------------------------+
drivers/clk/clk.c
+-------------+
| of_clk_init | : for each clk provider, call its callback and remove from list
+-|-----------+
|
|--> if arg 'matches' isn't provided
| -
| +--> use '__clk_of_table' by default
|
|--> for each dt node of entry in table
| |
| |--> alloc 'parent' for the match
| | (the matched one is probably 'aspeed,ast2500-scu')
| |
| +--> add 'parent' to 'clk_provider_list'
|
+--> while 'clk_provider_list' isn't empty
-
+--> for each provider in list
|
|--> call ->clk_init_cb(), e.g.,
| +----------------+
| | aspeed_cc_init | read 'strap' reg, register fixed factors, register clk_provider
| +----------------+
|
| +---------------------+
|--> | of_clk_set_defaults | (seems do nothing)
| +---------------------+
|
+--> remove clk_provider from list and free it
drivers/clk/clk-aspeed.c
+----------------+
| aspeed_cc_init | : read 'strap' reg, register fixed factors, register clk_provider
+-|--------------+
| +----------+
|--> | of_iomap | map scu base
| +----------+
|
|--> alloc 'aspeed_clk_data'
|
| +-----------------------+
|--> | syscon_node_to_regmap | get syscon regmap (what's the difference from scu base？）
| +-----------------------+
| +-------------+
|--> | regmap_read | read 'aspeed_strap'
| +-------------+
|
|--> if the node is compatible with 'aspeed,ast2500-scu'
| |
| | +-------------------+
| +--> | aspeed_ast2500_cc | read 'strap' register, register fixed factors for 'clkin', 'ahb', 'apb'
| +-------------------+
| +------------------------+
+--> | of_clk_add_hw_provider | alloc 'clk_provider' and register to 'of_clk_providers'
+------------------------+
drivers/clocksource/timer-probe.c
+-------------+
| timer_probe | : probe timer (register clocksource/schedclock_read()/timer_isr/clockevent_dev)
+-|-----------+
|
+--> for each matched dt_node of entry in '__timer_of_table'
|
|--> (the matched node is probably compatible with 'aspeed,ast2400-timer')
|
+--> call init_func_ret(), e.g.,
+-------------------+
| aspeed_timer_init | register clocksource, register read() for sched, request timer irq, register clockevent_dev
+-------------------+
drivers/clocksource/timer-probe.c
+-------------------+
| aspeed_timer_init | : register clocksource, register read() for sched, request timer irq, register clockevent_dev
+----------------------+
| fttmr010_common_init | : register clocksource, register read() for sched, request timer irq, register clockevent_dev
+-|--------------------+
|
|--> alloc and setup 'fttmr010'
|
| +----------+
|--> | of_iomap |
| +----------+
| +----------------------+
|--> | irq_of_parse_and_map | get timer irq#
| +----------------------+
|
+--> write hw reg to get timer start and disable its interrupt
|
| +-----------------------+
|--> | clocksource_mmio_init | prepare clocksource and register it, update 'curr_clocksource' is condition met
| +-----------------------+
| +----------------------+
|--> | sched_clock_register | register read() function for sched clock
| +----------------------+
|
|--> write hw reg to set up clockevent timer (interrupt-driven)
|
| +-------------+
|--> | request_irq | register isr for timer interrupt
| +-------------+ +--------------------------+
| | fttmr010_timer_interrupt | call clockevent_dev->event_handler(), e.g.,
| +--------------------------+ +----------------------+
| | tick_handle_periodic | or
| +-------------------+--+
| | hrtimer_interrupt |
| +-------------------+
|
|--> set up clockevent_dev (install ops)
|
| +---------------------------------+
|--> | clockevents_config_and_register | register evtdev, install to tick_dev if preferred
| +---------------------------------+
| +------------------------------+
+--> | register_current_timer_delay | install delay timer
+------------------------------+
drivers/clocksource/timer-fttmr010.c
+-----------------------+
| clocksource_mmio_init | : prepare clocksource and register it, update 'curr_clocksource' is condition met
+-|---------------------+
|
|--> alloc and setup 'cs' (clocksource), e.g., label 'CLOCK_SOURCE_IS_CONTINUOUS'
|
| +-------------------------+
+--> | clocksource_register_hz | register clocksource, switch 'curr_clocksource' to it if it's better
+-------------------------+
drivers/clocksource/mmio.c
+-------------------------+
| clocksource_register_hz | : register clocksource, switch 'curr_clocksource' to it if it's better
+------------------------------+
| __clocksource_register_scale | : register clocksource, switch 'curr_clocksource' to it if it's better
+-|----------------------------+
| +-----------------------+
|--> | clocksource_arch_init | (do nothing on arm)
| +-----------------------+
| +---------------------------------+
|--> | __clocksource_update_freq_scale | update mult and shift of cs
| +---------------------------------+
| +---------------------+
|--> | clocksource_enqueue | add to 'clocksource_list' ('rating' ascending list)
| +---------------------+
| +------------------------------+
|--> | clocksource_enqueue_watchdog | label 'CLOCK_SOURCE_VALID_FOR_HRES' if condition met
| +------------------------------+
| +--------------------+
|--> | clocksource_select | select the best clocksource and switch to it
| +--------------------+
| +-----------------------------+
|--> | clocksource_select_watchdog | (do nothing bc of disabled config)
| +-----------------------------+
| +------------------------------+
+--> | __clocksource_suspend_select | determine 'suspend_clocksource'
+------------------------------+
kernel/time/clocksource.c
+---------------------------------+
| __clocksource_update_freq_scale | : update mult and shift of cs
+-|-------------------------------+
|
|--> if arg freq is provided
| |
| | +------------------------+
| +--> | clocks_calc_mult_shift | update mult and shift of cs
| +------------------------+
|
+--> print "%s: mask: 0x%llx max_cycles: 0x%llx, max_idle_ns: %lld ns\n"
kernel/time/clocksource.c
+--------------------+
| clocksource_select | : select the best clocksource and switch to it
+----------------------+
| __clocksource_select | : select the best clocksource and switch to it
+-|--------------------+
| +-----------------------+
|--> | clocksource_find_best | get the best clocksource
| +-----------------------+
|
|--> if no 'override_name' (our case)
| -
| +--> go to 'found'
|
|found:
|
+--> if 'curr_clocksource' isn't the best clocksource yet
|
|--> print "Switched to clocksource %s\n"
|
+--> 'curr_clocksource' = best
kernel/time/sched_clock.c
+----------------------+
| sched_clock_register | : register read() function for sched clock
+-|--------------------+
|
|--> set up 'cd' (clock data)
|
|--> if sched_clock_timer is ready
| |
| | +---------------+
| +--> | hrtimer_start | sched_clock_timer
| +---------------+
|
+--> print "sched_clock: %u bits at %lu%cHz, resolution %lluns, wraps every %lluns\n"
drivers/clocksource/timer-fttmr010.c
+---------------------------------+
| clockevents_config_and_register | : register evtdev, install to tick_dev if preferred
+-|-------------------------------+
| +--------------------+
|--> | clockevents_config | colnfig min_delta_ns/max_delta_ns of clockevent_dev
| +--------------------+
| +-----------------------------+
+--> | clockevents_register_device | register evtdev, install to tick_dev if preferred
+-----------------------------+
kernel/time/clockevents.c
+-----------------------------+
| clockevents_register_device | : register evtdev, install to tick_dev if preferred
+-|---------------------------+
|
|--> register clockevent_dev to 'clockevent_devices'
|
| +-----------------------+
|--> | tick_check_new_device | if arg evtdev is preferred, install it to tick_dev
| +-----------------------+
| +-----------------------------+
+--> | clockevents_notify_released | try to install those released evtdev to tick_dev (should fail)
+-----------------------------+
kernel/time/tick-common.c
+-----------------------+
| tick_check_new_device | : if arg evtdev is preferred, install it to tick_dev
+-|---------------------+
|
|--> get percpu tick_dev, and further get current clockevent_dev from it
|
| +------------------------+
|--> | tick_check_replacement | check if new clockevent_dev is preferred
| +------------------------+
|
|--> if not, go to 'out'
|
| +-----------------------------+
|--> | clockevents_exchange_device | remove old one and shutdown new one
| +-----------------------------+
| +-------------------+
+--> | tick_setup_device | install arg evtdev to tick_dev, ensure next event happens
| +-------------------+
|out
| +-------------------------------+
+--> | tick_install_broadcast_device | (skip)
+-------------------------------+
kernel/time/tick-common.c
+-------------------+
| tick_setup_device | : install arg evtdev to tick_dev, ensure next event happens
+-|-----------------+
|
|--> if tick_dev has no clockevent_dev yet
| |
| |--> if no cpu responsible for do_timer
| | |
| | |--> assign that job to current cpu
| | |
| | +--> tick_next_period = now
| |
| +--> td->mode = periodic
|
|--> else
| |
| |--> get handler from td->evtdev
| |
| +--> reset td->evtdev's handler
|
|--> td->evtdev = arg dev
|
|--> if td->mode == periodic
| |
| | +---------------------+
| +--> | tick_setup_periodic | do nothing if it's periodic, otherwise (oneshot), program timer for next event
| +---------------------+
|
+--> else
|
| +--------------------+
+--> | tick_setup_oneshot | install handler, program next event
+--------------------+
kernel/time/tick-common.c
+---------------------+
| tick_setup_periodic | : do nothing if it's periodic, otherwise (oneshot), program timer for next event
+-|-------------------+
| +---------------------------+
|--> | tick_set_periodic_handler | install evtdev handler
| +---------------------------+ +----------------------+
| | tick_handle_periodic |
| +----------------------+
|
|--> if it's a dummy dev, return
|
|--> if state == periodic
| |
| | +--------------------------+
| +--> | clockevents_switch_state | periodic
| +--------------------------+
|
+--> else
|
| +--------------------------+
|--> | clockevents_switch_state | oneshot
| +--------------------------+
| +---------------------------+
+--> | clockevents_program_event | program timer for next event
+---------------------------+
arch/arm/lib/delay.c
+------------------------------+
| register_current_timer_delay | : install delay timer
+-|----------------------------+
|
|--> install delay timer
|
+--> print "Switching to timer-based delay loop, resolution %lluns\n"
kernel/time/timer_list.c
+------------------------+
| init_timer_list_procfs | ： create '/proc/timer_list' with 'timer_list_sops' installed
+-|----------------------+
|
+--> create '/proc/timer_list' with 'timer_list_sops' installed
static const struct seq_operations timer_list_sops = {
.start = timer_list_start,
.next = timer_list_next,
.stop = timer_list_stop,
.show = timer_list_show,
};