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Operation without Threads

Thread support is not necessary in some applications:

• Bootloaders

• Simple event-driven applications

• Examples intended to demonstrate core functionality

Thread support can be disabled by setting CONFIG_MULTITHREADING to n. Since this configuration has a significant impact on Zephyr’s functionality and testing of it has been limited, there are conditions on what can be expected to work in this configuration.

What Can be Expected to Work

These core capabilities shall function correctly when CONFIG_MULTITHREADING is disabled:

• The build system

• The ability to boot the application to main()

• Interrupt management

• The system clock including k_uptime_get()

• Timers, i.e. k_timer()

• Non-sleeping delays e.g. k_busy_wait().

• Sleeping k_cpu_idle().

• Pre main() drivers and subsystems initialization e.g. SYS_INIT.

• Memory Management

• Specifically identified drivers in certain subsystems, listed below.

The expectations above affect selection of other features; for example CONFIG_SYS_CLOCK_EXISTS cannot be set to n.

What Cannot be Expected to Work

Functionality that will not work with CONFIG_MULTITHREADING includes majority of the kernel API:

• Threads

• Scheduling

• Workqueue Threads

• Polling API

• Semaphores

• Mutexes

• Condition Variables

• Data Passing

Subsystem Behavior Without Thread Support

The sections below list driver and functional subsystems that are expected to work to some degree when CONFIG_MULTITHREADING is disabled. Subsystems that are not listed here should not be expected to work.

Some existing drivers within the listed subsystems do not work when threading is disabled, but are within scope based on their subsystem, or may be sufficiently isolated that supporting them on a particular platform is low-impact. Enhancements to add support to existing capabilities that were not originally implemented to work with threads disabled will be considered.

Flash

The Flash is expected to work for all SoC flash peripheral drivers. Bus-accessed devices like serial memories may not be supported.

List/table of supported drivers to go here

GPIO

The GPIO is expected to work for all SoC GPIO peripheral drivers. Bus-accessed devices like GPIO extenders may not be supported.

List/table of supported drivers to go here

UART

A subset of the UART is expected to work for all SoC UART peripheral drivers.

• Applications that select CONFIG_UART_INTERRUPT_DRIVEN may work, depending on driver implementation.

• Applications that select CONFIG_UART_ASYNC_API may work, depending on driver implementation.

• Applications that do not select either CONFIG_UART_ASYNC_API or CONFIG_UART_INTERRUPT_DRIVEN are expected to work.

List/table of supported drivers to go here, including which API options are supported