Martin L Roth has uploaded this change for review. ( https://review.coreboot.org/c/coreboot/+/87188?usp=email )
Change subject: Documentation: Add Threads ......................................................................
Documentation: Add Threads
Change-Id: I49a273118f19e4b332ac7f51b1f34247acf66b85 Signed-off-by: Martin Roth gaumless@gmail.com --- M Documentation/internals/index.md A Documentation/internals/threads.md 2 files changed, 311 insertions(+), 0 deletions(-)
git pull ssh://review.coreboot.org:29418/coreboot refs/changes/88/87188/1
diff --git a/Documentation/internals/index.md b/Documentation/internals/index.md index 098011c..4b4d011 100644 --- a/Documentation/internals/index.md +++ b/Documentation/internals/index.md @@ -20,5 +20,6 @@
Timestamps <timestamps.md> Timers, Stopwatch, and delays <stopwatch.md> +Threads <threads.md>
``` diff --git a/Documentation/internals/threads.md b/Documentation/internals/threads.md new file mode 100644 index 0000000..edbcb1d --- /dev/null +++ b/Documentation/internals/threads.md @@ -0,0 +1,310 @@ +# coreboot Threads + +## Thread Management + +coreboot provides a cooperative threading system that allows for +concurrent execution of tasks during the boot process. The thread API is +particularly useful for implementing asynchronous operations and +managing hardware initialization sequences. + + +### Thread Creation and Management + +#### thread_run + +```c +int thread_run(struct thread_handle *handle, enum cb_err (*func)(void *), void *arg) +``` +Creates and starts a new thread to execute the specified function. + +**Parameters:** +- `handle`: Pointer to a thread handle structure to track thread state + (Note: `struct thread_handle` is an opaque structure used by the API + to manage the thread's state.) +- `func`: Function to execute in the new thread +- `arg`: Argument to pass to the function + +**Returns:** +- 0 on success +- < 0 on failure + +**Example:** +```c +struct thread_handle th; +enum cb_err thread_func(void *arg) { + // Thread work here + return CB_SUCCESS; +} + +if (thread_run(&th, thread_func, NULL) < 0) { + printk(BIOS_ERR, "Failed to create thread\n"); +} else { + // Wait for the thread to complete and check its status + enum cb_err err = thread_join(&th); + if (err != CB_SUCCESS) { + printk(BIOS_ERR, "Thread failed with error %d\n", err); + } +} +``` + + +#### thread_run_until + +```c +int thread_run_until(struct thread_handle *handle, enum cb_err (*func)(void *), void *arg, + boot_state_t state, boot_state_sequence_t seq) +``` +Creates a thread that blocks boot state transitions until completion. + +**Parameters:** +- `handle`: Pointer to a thread handle structure +- `func`: Function to execute +- `arg`: Argument to pass to the function +- `state`: Boot state to block +- `seq`: Boot state sequence to block + +**Returns:** +- 0 on success +- < 0 on failure + +**Example:** +```c +struct thread_handle th; +enum cb_err init_func(void *arg) { + // Hardware initialization + return CB_SUCCESS; +} + +// Block BS_DEV_ENABLE until initialization completes +thread_run_until(&th, init_func, NULL, BS_DEV_ENABLE, 0); +``` + + +### Thread Synchronization + +#### thread_join + +```c +enum cb_err thread_join(struct thread_handle *handle) +``` +Waits for a thread to complete and returns its error code. + +**Parameters:** +- `handle`: Thread handle to wait for + +**Returns:** +- Thread's error code (e.g., `CB_SUCCESS`, `CB_ERR`). See + `src/include/cb_err.h` for details. + +**Example:** +```c +struct thread_handle th; +// ... create thread ... + +enum cb_err err = thread_join(&th); +if (err != CB_SUCCESS) { + printk(BIOS_ERR, "Thread failed with error %d\n", err); +} +``` + + +### Thread Yielding + +Yielding is crucial in a cooperative multitasking system like +coreboot's. Threads must explicitly yield control using `thread_yield` +or `thread_yield_microseconds` to allow other threads to run. Failure to +yield can lead to a single thread monopolizing the CPU, preventing other +tasks from executing. + +#### thread_yield + +```c +int thread_yield(void) +``` +Yields the current thread's execution to allow other threads to run. + +**Returns:** +- 0 on success +- < 0 if thread cannot yield + +**Example:** +```c +while (!condition) { + if (thread_yield() < 0) { + printk(BIOS_ERR, "Failed to yield thread\n"); + break; + } +} +``` + + +#### thread_yield_microseconds + +```c +int thread_yield_microseconds(unsigned int microsecs) +``` +Yields the current thread for a specified number of microseconds. + +**Parameters:** +- `microsecs`: Number of microseconds to yield + +**Returns:** +- 0 on success +- < 0 if thread cannot yield + +**Example:** +```c +// Wait for 100 microseconds +if (thread_yield_microseconds(100) < 0) { + printk(BIOS_ERR, "Failed to yield thread\n"); +} +``` + + +### Thread Cooperation Control + +#### thread_coop_enable + +```c +void thread_coop_enable(void) +``` +Enables cooperative behavior for the current thread. + +**Example:** +```c +thread_coop_enable(); // Allow thread to yield +``` + +#### thread_coop_disable + +```c +void thread_coop_disable(void) +``` +Disables cooperative behavior for the current thread. + +**Example:** +```c +thread_coop_disable(); // Prevent thread from yielding +``` + + +### Thread Mutexes + +#### thread_mutex_lock + +```c +void thread_mutex_lock(struct thread_mutex *mutex) +``` +Acquires a mutex lock, waiting if necessary. + +**Parameters:** +- `mutex`: Mutex to lock + +**Example:** +```c +struct thread_mutex mtx = THREAD_MUTEX_INITIALIZER; // Or = { .locked = false }; +thread_mutex_lock(&mtx); +// Critical section +thread_mutex_unlock(&mtx); +``` + + +#### thread_mutex_unlock + +```c +void thread_mutex_unlock(struct thread_mutex *mutex) +``` +Releases a mutex lock. + +**Parameters:** +- `mutex`: Mutex to unlock + +## Best Practices + +1. **Thread Safety**: + - Use mutexes to protect shared resources + - Be careful with global variables in threaded code + - Consider thread cooperation when implementing critical sections + +2. **Resource Management**: + - Always join threads that you create using `thread_run` to check + their completion status and clean up resources. Threads started + with `thread_run_until` are implicitly managed by the boot state + machine and typically do not require explicit joining. + - Consistently check return values from thread creation and operation + functions (like `thread_run`, `thread_yield`, `thread_join`) to + detect errors early. + - Clean up resources allocated or used within thread functions before + they exit. + +3. **Performance Considerations**: + - Use thread_yield_microseconds for precise timing + - Minimize time spent in critical sections + - Consider using thread_run_until for hardware initialization + +4. **Error Handling**: + - Check thread creation and operation return values (as noted in + Resource Management). + - Implement proper error handling within thread functions, returning + appropriate `cb_err` values. + - Use `thread_join` (for `thread_run` threads) to check the final + completion status. + + +## Common Patterns + +### Hardware Initialization + +```c +struct thread_handle init_th; +enum cb_err init_hardware(void *arg) { + // Initialize hardware + if (hardware_init() != 0) + return CB_ERR; + return CB_SUCCESS; +} + +// Run initialization in a thread +thread_run_until(&init_th, init_hardware, NULL, BS_DEV_ENABLE, 0); +``` + +### Asynchronous Operation + +```c +struct thread_handle async_th; +enum cb_err async_operation(void *arg) { + // Perform async operation + while (!operation_complete()) { + if (thread_yield() < 0) + return CB_ERR; + } + return CB_SUCCESS; +} + +// Start async operation +thread_run(&async_th, async_operation, NULL); +``` + + +### Critical Section Protection + +```c +struct thread_mutex resource_mtx = { .locked = false }; + +void access_shared_resource(void) { + thread_mutex_lock(&resource_mtx); + // Access shared resource + thread_mutex_unlock(&resource_mtx); +} +``` + +## Limitations + +1. The thread system is cooperative, not preemptive. +2. Threads must explicitly yield to allow other threads to run. +3. Thread operations are typically only available after RAM + initialization (in RAM stage and later). Check specific environment + constraints if unsure. +4. Thread count is limited by the `CONFIG_NUM_THREADS` Kconfig option. +5. Thread stack size is fixed by the `CONFIG_STACK_SIZE` Kconfig option. +
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Martin L Roth (Code Review)