kernel/Kconfig

# Kconfig - kernel configuration options

#
# Copyright (c) 2014-2015 Wind River Systems, Inc.
#
# SPDX-License-Identifier: Apache-2.0
#


menu "General Kernel Options"

module = KERNEL
module-str = kernel
source "subsys/logging/Kconfig.template.log_config"

config MULTITHREADING
	bool "Multi-threading"
	default y
	help
	  If disabled, only the main thread is available, so a main() function
	  must be provided. Interrupts are available. Kernel objects will most
	  probably not behave as expected, especially with regards to pending,
	  since the main thread cannot pend, it being the only thread in the
	  system.

	  Many drivers and subsystems will not work with this option; use only
	  when you REALLY know what you are doing.

config NUM_COOP_PRIORITIES
	int "Number of coop priorities" if MULTITHREADING
	default 1 if !MULTITHREADING
	default 16
	range 0 128
	help
	  Number of cooperative priorities configured in the system. Gives access
	  to priorities:

		K_PRIO_COOP(0) to K_PRIO_COOP(CONFIG_NUM_COOP_PRIORITIES - 1)

	  or seen another way, priorities:

		-CONFIG_NUM_COOP_PRIORITIES to -1

	  This can be set to zero to disable cooperative scheduling. Cooperative
	  threads always preempt preemptible threads.

	  Each priority requires an extra 8 bytes of RAM. Each set of 32 extra
	  total priorities require an extra 4 bytes and add one possible
	  iteration to loops that search for the next thread to run.

	  The total number of priorities is

	   NUM_COOP_PRIORITIES + NUM_PREEMPT_PRIORITIES + 1

	  The extra one is for the idle thread, which must run at the lowest
	  priority, and be the only thread at that priority.

config NUM_PREEMPT_PRIORITIES
	int "Number of preemptible priorities" if MULTITHREADING
	default 0 if !MULTITHREADING
	default 15
	range 0 128
	help
	  Number of preemptible priorities available in the system. Gives access
	  to priorities 0 to CONFIG_NUM_PREEMPT_PRIORITIES - 1.

	  This can be set to 0 to disable preemptible scheduling.

	  Each priority requires an extra 8 bytes of RAM. Each set of 32 extra
	  total priorities require an extra 4 bytes and add one possible
	  iteration to loops that search for the next thread to run.

	  The total number of priorities is

	   NUM_COOP_PRIORITIES + NUM_PREEMPT_PRIORITIES + 1

	  The extra one is for the idle thread, which must run at the lowest
	  priority, and be the only thread at that priority.

config MAIN_THREAD_PRIORITY
	int "Priority of initialization/main thread"
	default -2 if !PREEMPT_ENABLED
	default 0
	help
	  Priority at which the initialization thread runs, including the start
	  of the main() function. main() can then change its priority if desired.

config COOP_ENABLED
	def_bool (NUM_COOP_PRIORITIES != 0)

config PREEMPT_ENABLED
	def_bool (NUM_PREEMPT_PRIORITIES != 0)

config PRIORITY_CEILING
	int "Priority inheritance ceiling"
	default 0

config NUM_METAIRQ_PRIORITIES
	int "Number of very-high priority 'preemptor' threads"
	default 0
	help
	 This defines a set of priorities at the (numerically) lowest
	 end of the range which have "meta-irq" behavior.  Runnable
	 threads at these priorities will always be scheduled before
	 threads at lower priorities, EVEN IF those threads are
	 otherwise cooperative and/or have taken a scheduler lock.
	 Making such a thread runnable in any way thus has the effect
	 of "interrupting" the current task and running the meta-irq
	 thread synchronously, like an exception or system call.  The
	 intent is to use these priorities to implement "interrupt
	 bottom half" or "tasklet" behavior, allowing driver
	 subsystems to return from interrupt context but be guaranteed
	 that user code will not be executed (on the current CPU)
	 until the remaining work is finished.  As this breaks the
	 "promise" of non-preemptibility granted by the current API
	 for cooperative threads, this tool probably shouldn't be used
	 from application code.

config SCHED_DEADLINE
	bool "Enable earliest-deadline-first scheduling"
	help
	  This enables a simple "earliest deadline first" scheduling
	  mode where threads can set "deadline" deltas measured in
	  k_cycle_get_32() units.  Priority decisions within (!!) a
	  single priority will choose the next expiring deadline and
	  not simply the least recently added thread.

config SCHED_CPU_MASK
	bool "Enable CPU mask affinity/pinning API"
	depends on SCHED_DUMB
	help
	  When true, the app will have access to the
	  z_thread_*_cpu_mask() APIs which control per-CPU affinity
	  masks in SMP mode, allowing apps to pin threads to specific
	  CPUs or disallow threads from running on given CPUs.  Note
	  that as currently implemented, this involves an inherent
	  O(N) scaling in the number of idle-but-runnable threads, and
	  thus works only with the DUMB scheduler (as SCALABLE and
	  MULTIQ would see no benefit).

	  Note that this setting does not technically depend on SMP
	  and is implemented without it for testing purposes, but for
	  obvious reasons makes sense as an application API only where
	  there is more than one CPU.  With one CPU, it's just a
	  higher overhead version of k_thread_start/stop().

config MAIN_STACK_SIZE
	int "Size of stack for initialization and main thread"
	default 2048 if COVERAGE_GCOV
	default 512 if ZTEST
	default 1024
	help
	  When the initialization is complete, the thread executing it then
	  executes the main() routine, so as to reuse the stack used by the
	  initialization, which would be wasted RAM otherwise.

	  After initialization is complete, the thread runs main().

config IDLE_STACK_SIZE
	int "Size of stack for idle thread"
	default 512 if COVERAGE_GCOV
	default 1024 if XTENSA
	default 512 if RISCV32
	default 320 if ARC || (ARM && CPU_HAS_FPU)
	default 256
	help
	  Depending on the work that the idle task must do, most likely due to
	  power management but possibly to other features like system event
	  logging (e.g. logging when the system goes to sleep), the idle thread
	  may need more stack space than the default value.

config ISR_STACK_SIZE
	int "ISR and initialization stack size (in bytes)"
	default 2048
	help
	  This option specifies the size of the stack used by interrupt
	  service routines (ISRs), and during kernel initialization.

config THREAD_STACK_INFO
	bool "Thread stack info"
	help
	  This option allows each thread to store the thread stack info into
	  the k_thread data structure.

config  THREAD_CUSTOM_DATA
	bool "Thread custom data"
	help
	  This option allows each thread to store 32 bits of custom data,
	  which can be accessed using the k_thread_custom_data_xxx() APIs.

config THREAD_USERSPACE_LOCAL_DATA
	bool
	depends on USERSPACE

config THREAD_USERSPACE_LOCAL_DATA_ARCH_DEFER_SETUP
	bool
	depends on THREAD_USERSPACE_LOCAL_DATA
	default y if ARCH="arc"

config ERRNO
	bool "Enable errno support"
	default y
	select THREAD_USERSPACE_LOCAL_DATA if USERSPACE
	help
	  Enable per-thread errno in the kernel. Application and library code must
	  include errno.h provided by the C library (libc) to use the errno
	  symbol. The C library must access the per-thread errno via the
	  _get_errno() symbol.

choice SCHED_ALGORITHM
	prompt "Scheduler priority queue algorithm"
	default SCHED_DUMB
	help
	  The kernel can be built with with several choices for the
	  ready queue implementation, offering different choices between
	  code size, constant factor runtime overhead and performance
	  scaling when many threads are added.

config SCHED_DUMB
	bool "Simple linked-list ready queue"
	help
	  When selected, the scheduler ready queue will be implemented
	  as a simple unordered list, with very fast constant time
	  performance for single threads and very low code size.
	  Choose this on systems with constrained code size that will
	  never see more than a small number (3, maybe) of runnable
	  threads in the queue at any given time.  On most platforms
	  (that are not otherwise using the red/black tree) this
	  results in a savings of ~2k of code size.

config SCHED_SCALABLE
	bool "Red/black tree ready queue"
	help
	  When selected, the scheduler ready queue will be implemented
	  as a red/black tree.  This has rather slower constant-time
	  insertion and removal overhead, and on most platforms (that
	  are not otherwise using the rbtree somewhere) requires an
	  extra ~2kb of code.  But the resulting behavior will scale
	  cleanly and quickly into the many thousands of threads.  Use
	  this on platforms where you may have many threads (very
	  roughly: more than 20 or so) marked as runnable at a given
	  time.  Most applications don't want this.

config SCHED_MULTIQ
	bool "Traditional multi-queue ready queue"
	depends on !SCHED_DEADLINE
	help
	  When selected, the scheduler ready queue will be implemented
	  as the classic/textbook array of lists, one per priority
	  (max 32 priorities).  This corresponds to the scheduler
	  algorithm used in Zephyr versions prior to 1.12.  It incurs
	  only a tiny code size overhead vs. the "dumb" scheduler and
	  runs in O(1) time in almost all circumstances with very low
	  constant factor.  But it requires a fairly large RAM budget
	  to store those list heads, and the limited features make it
	  incompatible with features like deadline scheduling that
	  need to sort threads more finely, and SMP affinity which
	  need to traverse the list of threads.  Typical applications
	  with small numbers of runnable threads probably want the
	  DUMB scheduler.

endchoice # SCHED_ALGORITHM

choice WAITQ_ALGORITHM
	prompt "Wait queue priority algorithm"
	default WAITQ_DUMB
	help
	  The wait_q abstraction used in IPC primitives to pend
	  threads for later wakeup shares the same backend data
	  structure choices as the scheduler, and can use the same
	  options.

config WAITQ_SCALABLE
	bool "Use scalable wait_q implementation"
	help
	  When selected, the wait_q will be implemented with a
	  balanced tree.  Choose this if you expect to have many
	  threads waiting on individual primitives.  There is a ~2kb
	  code size increase over WAITQ_DUMB (which may be shared with
	  SCHED_SCALABLE) if the rbtree is not used elsewhere in the
	  application, and pend/unpend operations on "small" queues
	  will be somewhat slower (though this is not generally a
	  performance path).

config WAITQ_DUMB
	bool "Simple linked-list wait_q"
	help
	  When selected, the wait_q will be implemented with a
	  doubly-linked list.  Choose this if you expect to have only
	  a few threads blocked on any single IPC primitive.

endchoice # WAITQ_ALGORITHM

menu "Kernel Debugging and Metrics"

config INIT_STACKS
	bool "Initialize stack areas"
	help
	  This option instructs the kernel to initialize stack areas with a
	  known value (0xaa) before they are first used, so that the high
	  water mark can be easily determined. This applies to the stack areas
	  for threads, as well as to the interrupt stack.

config KERNEL_DEBUG
	bool "Kernel debugging"
	select INIT_STACKS
	help
	  Enable kernel debugging.

	  Note that debugging the kernel internals can be very verbose.

config BOOT_BANNER
	bool "Boot banner"
	default y
	depends on CONSOLE_HAS_DRIVER
	select PRINTK
	select EARLY_CONSOLE
	help
	  This option outputs a banner to the console device during boot up.

config BOOT_DELAY
	int "Boot delay in milliseconds"
	default 0
	help
	  This option delays bootup for the specified amount of
	  milliseconds. This is used to allow serial ports to get ready
	  before starting to print information on them during boot, as
	  some systems might boot to fast for a receiving endpoint to
	  detect the new USB serial bus, enumerate it and get ready to
	  receive before it actually gets data. A similar effect can be
	  achieved by waiting for DCD on the serial port--however, not
	  all serial ports have DCD.

config INT_LATENCY_BENCHMARK
	bool "Interrupt latency metrics [EXPERIMENTAL]"
	depends on ARCH="x86"
	help
	  This option enables the tracking of interrupt latency metrics;
	  the exact set of metrics being tracked is board-dependent.
	  Tracking begins when int_latency_init() is invoked by an application.
	  The metrics are displayed (and a new sampling interval is started)
	  each time int_latency_show() is called thereafter.

config EXECUTION_BENCHMARKING
	bool "Timing metrics"
	help
	  This option enables the tracking of various times inside the kernel
	  the exact set of metrics being tracked is board-dependent.
	  All timing measurements are enabled for X86 and ARM based architectures.
	  In other architectures only a subset are enabled.

config THREAD_MONITOR
	bool "Thread monitoring [EXPERIMENTAL]"
	help
	  This option instructs the kernel to maintain a list of all threads
	  (excluding those that have not yet started or have already
	  terminated).

config THREAD_NAME
	bool "Thread name [EXPERIMENTAL]"
	help
	  This option allows to set a name for a thread.
endmenu

menu "Work Queue Options"
config SYSTEM_WORKQUEUE_STACK_SIZE
	int "System workqueue stack size"
	default 1024

config SYSTEM_WORKQUEUE_PRIORITY
	int "System workqueue priority"
	default -2 if COOP_ENABLED && !PREEMPT_ENABLED
	default  0 if !COOP_ENABLED
	default -1
	help
	  By default, system work queue priority is the lowest cooperative
	  priority. This means that any work handler, once started, won't
	  be preempted by any other thread until finished.

config OFFLOAD_WORKQUEUE_STACK_SIZE
	int "Workqueue stack size for thread offload requests"
	default 1024

config OFFLOAD_WORKQUEUE_PRIORITY
	int "Offload requests workqueue priority"
	default -1

endmenu

menu "Atomic Operations"
config ATOMIC_OPERATIONS_BUILTIN
	bool
	help
	  Use the compiler builtin functions for atomic operations. This is
	  the preferred method. However, support for all arches in GCC is
	  incomplete.

config ATOMIC_OPERATIONS_CUSTOM
	bool
	help
	  Use when there isn't support for compiler built-ins, but you have
	  written optimized assembly code under arch/ which implements these.

config ATOMIC_OPERATIONS_C
	bool
	help
	  Use atomic operations routines that are implemented entirely
	  in C by locking interrupts. Selected by architectures which either
	  do not have support for atomic operations in their instruction
	  set, or haven't been implemented yet during bring-up, and also
	  the compiler does not have support for the atomic __sync_* builtins.
endmenu

menu "Timer API Options"

config TIMESLICING
	bool "Thread time slicing"
	default y
	depends on SYS_CLOCK_EXISTS && (NUM_PREEMPT_PRIORITIES != 0)
	help
	  This option enables time slicing between preemptible threads of
	  equal priority.

config TIMESLICE_SIZE
	int "Time slice size (in ms)"
	default 0
	range 0 2147483647
	depends on TIMESLICING
	help
	  This option specifies the maximum amount of time a thread can execute
	  before other threads of equal priority are given an opportunity to run.
	  A time slice size of zero means "no limit" (i.e. an infinitely large
	  time slice).

config TIMESLICE_PRIORITY
	int "Time slicing thread priority ceiling"
	default 0
	range 0 NUM_PREEMPT_PRIORITIES
	depends on TIMESLICING
	help
	  This option specifies the thread priority level at which time slicing
	  takes effect; threads having a higher priority than this ceiling are
	  not subject to time slicing.

config POLL
	bool "Async I/O Framework"
	help
	  Asynchronous notification framework. Enable the k_poll() and
	  k_poll_signal_raise() APIs.  The former can wait on multiple events
	  concurrently, which can be either directly triggered or triggered by
	  the availability of some kernel objects (semaphores and fifos).

endmenu

menu "Other Kernel Object Options"

config NUM_MBOX_ASYNC_MSGS
	int "Maximum number of in-flight asynchronous mailbox messages"
	default 10
	help
	  This option specifies the total number of asynchronous mailbox
	  messages that can exist simultaneously, across all mailboxes
	  in the system.

	  Setting this option to 0 disables support for asynchronous
	  mailbox messages.

config NUM_PIPE_ASYNC_MSGS
	int "Maximum number of in-flight asynchronous pipe messages"
	default 10
	help
	  This option specifies the total number of asynchronous pipe
	  messages that can exist simultaneously, across all pipes in
	  the system.

	  Setting this option to 0 disables support for asynchronous
	  pipe messages.

config HEAP_MEM_POOL_SIZE
	int "Heap memory pool size (in bytes)"
	default 0 if !POSIX_MQUEUE
	default 1024 if POSIX_MQUEUE
	help
	  This option specifies the size of the heap memory pool used when
	  dynamically allocating memory using k_malloc(). Supported values
	  are: 256, 1024, 4096, and 16384. A size of zero means that no
	  heap memory pool is defined.
endmenu

config ARCH_HAS_CUSTOM_SWAP_TO_MAIN
	bool
	# hidden
	help
	  It's possible that an architecture port cannot use _Swap() to swap to
	  the _main() thread, but instead must do something custom. It must
	  enable this option in that case.

config SWAP_NONATOMIC
	bool
	help
	  On some architectures, the _Swap() primitive cannot be made
	  atomic with respect to the irq_lock being released.  That
	  is, interrupts may be received between the entry to _Swap
	  and the completion of the context switch.  There are a
	  handful of workaround cases in the kernel that need to be
	  enabled when this is true.  Currently, this only happens on
	  ARM when the PendSV exception priority sits below that of
	  Zephyr-handled interrupts.

config ARCH_HAS_CUSTOM_BUSY_WAIT
	bool
	# hidden
	help
	  It's possible that an architecture port cannot or does not want to use
	  the provided k_busy_wait(), but instead must do something custom. It must
	  enable this option in that case.

config SYS_CLOCK_TICKS_PER_SEC
	int "System tick frequency (in ticks/second)"
	default 100
	help
	  This option specifies the frequency of the system clock in Hz.

	  Depending on the choice made, an amount of possibly expensive math must
	  occur when converting ticks to milliseconds and vice-versa. Some values
	  are optimized, and yield significantly less math.

	  The optimal values from a computational point-of-view are 1000, 500,
	  250 and 125, since in these cases there is either no computation
	  required, or it is all done via bit-shifting. These also give a
	  granularity from 1ms to 8ms.

	  Other good values are 100, 50, 25, 20 and 10. In this case, some math
	  is required but is minimized. These are also values that necessitate a
	  reduced number of clock interrupts per second, at the cost of
	  granularity (10ms to 100ms).

	  All other values require some extensive 64-bit math, and in some
	  configurations even require calls to compiler built-in functions, and
	  can require a non-trivial extra amount of stack space (e.g. around 80
	  bytes on x86).

	  Note that when available and enabled, in "tickless" mode
	  this config variable specifies the minimum available timing
	  granularity, not necessarily the number or frequency of
	  interrupts delivered to the kernel.

config SYS_CLOCK_HW_CYCLES_PER_SEC
	int "System clock's h/w timer frequency"
	help
	  This option specifies the frequency of the hardware timer used for the
	  system clock (in Hz). This option is set by the SOC's or board's Kconfig file
	  and the user should generally avoid modifying it via the menu configuration.

config SYS_CLOCK_EXISTS
	def_bool (SYS_CLOCK_TICKS_PER_SEC != 0)
	# omit prompt to signify a "hidden" option
	help
	  This option specifies that the kernel lacks timer support.

config XIP
	bool "Execute in place"
	help
	  This option allows the kernel to operate with its text and read-only
	  sections residing in ROM (or similar read-only memory). Not all boards
	  support this option so it must be used with care; you must also
	  supply a linker command file when building your image. Enabling this
	  option increases both the code and data footprint of the image.

menu "Initialization Priorities"

config KERNEL_INIT_PRIORITY_OBJECTS
	int "Kernel objects initialization priority"
	default 30
	help
	  Kernel objects use this priority for initialization. This
	  priority needs to be higher than minimal default initialization
	  priority.

config KERNEL_INIT_PRIORITY_DEFAULT
	int "Default init priority"
	default 40
	help
	  Default minimal init priority for each init level.

config KERNEL_INIT_PRIORITY_DEVICE
	int "Default init priority for device drivers"
	default 50
	help
	  Device driver, that depends on common components, such as
	  interrupt controller, but does not depend on other devices,
	  uses this init priority.

config APPLICATION_INIT_PRIORITY
	int "Default init priority for application level drivers"
	default 90
	help
	  This priority level is for end-user drivers such as sensors and display
	  which have no inward dependencies.


endmenu

menu "Security Options"

config RETPOLINE
	bool "Build with retpolines enabled"
	default y if !X86_NO_SPECTRE_V2
	# Currently only implemented for x86
	depends on X86
	help
	  This is recommended on platforms with speculative executions, to protect
	  against branch target injection (AKA Spectre-V2).  Full description of
	  how retpolines work can be found here[1].

	  [1] https://support.google.com/faqs/answer/7625886

config STACK_CANARIES
	bool "Compiler stack canaries"
	help
	  This option enables compiler stack canaries.

	  If stack canaries are supported by the compiler, it will emit
	  extra code that inserts a canary value into the stack frame when
	  a function is entered and validates this value upon exit.
	  Stack corruption (such as that caused by buffer overflow) results
	  in a fatal error condition for the running entity.
	  Enabling this option can result in a significant increase
	  in footprint and an associated decrease in performance.

	  If stack canaries are not supported by the compiler an error
	  will occur at build time.

config EXECUTE_XOR_WRITE
	bool "Enable W^X for memory partitions"
	depends on USERSPACE
	depends on ARCH_HAS_EXECUTABLE_PAGE_BIT
	default y
	help
	  When enabled, will enforce that a writable page isn't executable
	  and vice versa.  This might not be acceptable in all scenarios,
	  so this option is given for those unafraid of shooting themselves
	  in the foot.

	  If unsure, say Y.

config STACK_POINTER_RANDOM
	int "Initial stack pointer randomization bounds"
	depends on !STACK_GROWS_UP
	default 0
	help
	  This option performs a limited form of Address Space Layout
	  Randomization by offsetting some random value to a thread's
	  initial stack pointer upon creation. This hinders some types of
	  security attacks by making the location of any given stack frame
	  non-deterministic.

	  This feature can waste up to the specified size in bytes the stack
	  region, which is carved out of the total size of the stack region.
	  A reasonable minimum value would be around 100 bytes if this can
	  be spared.

	  This is currently only implemented for systems whose stack pointers
	  grow towards lower memory addresses.

endmenu

config MAX_DOMAIN_PARTITIONS
	int "Maximum number of partitions per memory domain"
	default 16
	range 0 255
	depends on USERSPACE
	help
	  Configure the maximum number of partitions per memory domain.

menu "SMP Options"

config USE_SWITCH
	bool "Use new-style _arch_switch instead of __swap"
	depends on USE_SWITCH_SUPPORTED
	help
	  The _arch_switch() API is a lower level context switching
	  primitive than the original __swap mechanism.  It is required
	  for an SMP-aware scheduler, or if the architecture does not
	  provide __swap.  In uniprocess situations where the
	  architecture provides both, _arch_switch incurs more somewhat
	  overhead and may be slower.

config USE_SWITCH_SUPPORTED
	bool
	help
	  Indicates whether _arch_switch() API is supported by the
	  currently enabled platform. This option should be selected by
	  platforms that implement it.

config SMP
	bool "Enable symmetric multithreading support"
	depends on USE_SWITCH
	help
	  When true, kernel will be built with SMP support, allowing
	  more than one CPU to schedule Zephyr tasks at a time.

config MP_NUM_CPUS
	int "Number of CPUs/cores"
	default 1
	help
	  Number of multiprocessing-capable cores available to the
	  multicpu API and SMP features.

endmenu

config TICKLESS_IDLE
	# NB: This option is deprecated, see TICKLESS_KERNEL and
	# https://github.com/zephyrproject-rtos/zephyr/pull/12234
	bool "Tickless idle"
	default y if SYS_POWER_MANAGEMENT || TICKLESS_CAPABLE
	help
	  This option suppresses periodic system clock interrupts whenever the
	  kernel becomes idle. This permits the system to remain in a power
	  saving state for extended periods without having to wake up to
	  service each tick as it occurs.

config TICKLESS_IDLE_THRESH
	int "Tickless idle threshold"
	default 3
	depends on TICKLESS_IDLE
	help
	  This option enables clock interrupt suppression when the kernel idles
	  for only a short period of time. It specifies the minimum number of
	  ticks that must occur before the next kernel timer expires in order
	  for suppression to happen.

config TICKLESS_KERNEL
	bool "Tickless kernel"
	default y if TICKLESS_CAPABLE
	help
	  This option enables a fully event driven kernel. Periodic system
	  clock interrupt generation would be stopped at all times.

source "kernel/Kconfig.power_mgmt"

endmenu