There are three levels of synchronization supported:

  1. Synchronization with ISRs. This normally means disabling interrupts to prevent the ISR running during a critical section. In an SMP environment, this will also require the use of a spinlock to synchronize with ISRs, DSRs or threads running on other CPUs. This is implemented by the cyg_drv_isr_lock() and cyg_drv_isr_unlock() functions. This mechanism should be used sparingly and for short periods only. For finer grained synchronization, individual spinlocks are also supplied.

  2. Synchronization with DSRs. This will be implemented in the kernel by taking the scheduler lock to prevent DSRs running during critical sections. In non-kernel configurations it will be implemented by non-kernel code. This is implemented by the cyg_drv_dsr_lock() and cyg_drv_dsr_unlock() functions. As with ISR synchronization, this mechanism should be used sparingly. Only DSRs and threads may use this synchronization mechanism, ISRs are not allowed to do this.

  3. Synchronization with threads. This is implemented with mutexes and condition variables. Only threads may lock the mutexes and wait on the condition variables, although DSRs may signal condition variables.

Any data that is accessed from more than one level must be protected against concurrent access. Data that is accessed by ISRs must be protected with the ISR lock, or a spinlock at all times, even in ISRs. Data that is shared between DSRs and threads should be protected with the DSR lock. Data that is only accessed by threads must be protected with mutexes.

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