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
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 threadarg
: Argument to pass to the function
Returns:
0 on success
< 0 on failure
Example:
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
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 structurefunc
: Function to executearg
: Argument to pass to the functionstate
: Boot state to blockseq
: Boot state sequence to block
Returns:
0 on success
< 0 on failure
Example:
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
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
). Seesrc/include/cb_err.h
for details.
Example:
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
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:
while (!condition) {
if (thread_yield() < 0) {
printk(BIOS_ERR, "Failed to yield thread\n");
break;
}
}
thread_yield_microseconds
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:
// Wait for 100 microseconds
if (thread_yield_microseconds(100) < 0) {
printk(BIOS_ERR, "Failed to yield thread\n");
}
Thread Cooperation Control
thread_coop_enable
void thread_coop_enable(void)
Enables cooperative behavior for the current thread.
Example:
thread_coop_enable(); // Allow thread to yield
thread_coop_disable
void thread_coop_disable(void)
Disables cooperative behavior for the current thread.
Example:
thread_coop_disable(); // Prevent thread from yielding
Thread Mutexes
thread_mutex_lock
void thread_mutex_lock(struct thread_mutex *mutex)
Acquires a mutex lock, waiting if necessary.
Parameters:
mutex
: Mutex to lock
Example:
struct thread_mutex mtx = THREAD_MUTEX_INITIALIZER; // Or = { .locked = false };
thread_mutex_lock(&mtx);
// Critical section
thread_mutex_unlock(&mtx);
thread_mutex_unlock
void thread_mutex_unlock(struct thread_mutex *mutex)
Releases a mutex lock.
Parameters:
mutex
: Mutex to unlock
Best Practices
Thread Safety:
Use mutexes to protect shared resources
Be careful with global variables in threaded code
Consider thread cooperation when implementing critical sections
Resource Management:
Always join threads that you create using
thread_run
to check their completion status and clean up resources. Threads started withthread_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.
Performance Considerations:
Use thread_yield_microseconds for precise timing
Minimize time spent in critical sections
Consider using thread_run_until for hardware initialization
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
(forthread_run
threads) to check the final completion status.
Common Patterns
Hardware Initialization
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
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
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
The thread system is cooperative, not preemptive.
Threads must explicitly yield to allow other threads to run.
Thread operations are typically only available after RAM initialization (in ramstage and later). Check specific environment constraints if unsure.
Thread count is limited by the
CONFIG_NUM_THREADS
Kconfig option.Thread stack size is fixed by the
CONFIG_STACK_SIZE
Kconfig option.