eCos Reference Manual
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# SMP Support

Some eCos targets contain support for Symmetric Multi-Processing (SMP) configurations, where more than one CPU may be present. This option has a number of ramifications for the way in which device drivers must be written if they are to be SMP-compatible.

Since it is possible for the ISR, DSR and thread components of a device driver to execute on different CPUs, it is important that SMP-compatible device drivers use the driver API routines correctly.

Synchronization between threads and DSRs continues to require that the thread-side code use `cyg_drv_dsr_lock()` and `cyg_drv_dsr_unlock()` to protect access to shared data. While it is not strictly necessary for DSR code to claim the DSR lock, since DSRs are run with it claimed already, it is good practice to do so.

Synchronization between ISRs and DSRs or threads requires that access to sensitive data be protected, in all places, by calls to `cyg_drv_isr_lock()` and `cyg_drv_isr_unlock()`. Disabling or masking interrupts is not adequate, since the thread or DSR may be running on a different CPU and interrupt enable/disable only work on the current CPU.

The ISR lock, for SMP systems, not only disables local interrupts, but also acquires a spinlock to protect against concurrent access from other CPUs. This is necessary because ISRs are not run with the scheduler lock claimed. Hence they can run in parallel with the other components of the device driver.

The ISR lock provided by the driver API is just a shared spinlock that is available for use by all drivers. If a driver needs to implement a finer grain of locking, it can use private spinlocks, accessed via the `cyg_drv_spinlock_*()` functions.

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