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>SMP Support</TITLE
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>SMP Support</TITLE
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>eCos Reference Manual</TH
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>eCos Reference Manual</TH
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>Chapter 18. Device Driver Interface to the Kernel</TD
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>Chapter 18. Device Driver Interface to the Kernel</TD
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><H1
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><A
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NAME="DEVAPI-SMP-SUPPORT">SMP Support</H1
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NAME="DEVAPI-SMP-SUPPORT">SMP Support</H1
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><P
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><P
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>Some eCos targets contain support for Symmetric Multi-Processing (SMP)
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>Some eCos targets contain support for Symmetric Multi-Processing (SMP)
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configurations, where more than one CPU may be present. This option
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configurations, where more than one CPU may be present. This option
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has a number of ramifications for the way in which device drivers must
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has a number of ramifications for the way in which device drivers must
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be written if they are to be SMP-compatible. </P
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be written if they are to be SMP-compatible. </P
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><P
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><P
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>Since it is possible for the ISR, DSR and thread components of a
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>Since it is possible for the ISR, DSR and thread components of a
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device driver to execute on different CPUs, it is important that
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device driver to execute on different CPUs, it is important that
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SMP-compatible device drivers use the driver API routines correctly.</P
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SMP-compatible device drivers use the driver API routines correctly.</P
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><P
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><P
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>Synchronization between threads and DSRs continues to require that the
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>Synchronization between threads and DSRs continues to require that the
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thread-side code use <TT
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thread-side code use <TT
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CLASS="FUNCTION"
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CLASS="FUNCTION"
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>cyg_drv_dsr_lock()</TT
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>cyg_drv_dsr_lock()</TT
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> and
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> and
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<TT
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<TT
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CLASS="FUNCTION"
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CLASS="FUNCTION"
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>cyg_drv_dsr_unlock()</TT
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>cyg_drv_dsr_unlock()</TT
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> to protect access to shared
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> to protect access to shared
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data. While it is not strictly necessary for DSR code to claim the DSR
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data. While it is not strictly necessary for DSR code to claim the DSR
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lock, since DSRs are run with it claimed already, it is good practice
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lock, since DSRs are run with it claimed already, it is good practice
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to do so.</P
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to do so.</P
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><P
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><P
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>Synchronization between ISRs and DSRs or threads requires that access
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>Synchronization between ISRs and DSRs or threads requires that access
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to sensitive data be protected, in all places, by calls to
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to sensitive data be protected, in all places, by calls to
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<TT
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<TT
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CLASS="FUNCTION"
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CLASS="FUNCTION"
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>cyg_drv_isr_lock()</TT
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>cyg_drv_isr_lock()</TT
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> and
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> and
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<TT
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<TT
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CLASS="FUNCTION"
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CLASS="FUNCTION"
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>cyg_drv_isr_unlock()</TT
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>cyg_drv_isr_unlock()</TT
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>. Disabling or masking
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>. Disabling or masking
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interrupts is not adequate, since the thread or DSR may be running on
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interrupts is not adequate, since the thread or DSR may be running on
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a different CPU and interrupt enable/disable only work on the current
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a different CPU and interrupt enable/disable only work on the current
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CPU.</P
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CPU.</P
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><P
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><P
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>The ISR lock, for SMP systems, not only disables local interrupts, but
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>The ISR lock, for SMP systems, not only disables local interrupts, but
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also acquires a spinlock to protect against concurrent access from
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also acquires a spinlock to protect against concurrent access from
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other CPUs. This is necessary because ISRs are not run with the
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other CPUs. This is necessary because ISRs are not run with the
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scheduler lock claimed. Hence they can run in parallel with the other
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scheduler lock claimed. Hence they can run in parallel with the other
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components of the device driver.</P
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components of the device driver.</P
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><P
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><P
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>The ISR lock provided by the driver API is just a shared spinlock that
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>The ISR lock provided by the driver API is just a shared spinlock that
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is available for use by all drivers. If a driver needs to implement a
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is available for use by all drivers. If a driver needs to implement a
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finer grain of locking, it can use private spinlocks, accessed via the
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finer grain of locking, it can use private spinlocks, accessed via the
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<TT
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<TT
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CLASS="FUNCTION"
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CLASS="FUNCTION"
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>cyg_drv_spinlock_*()</TT
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>cyg_drv_spinlock_*()</TT
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> functions.</P
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> functions.</P
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