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+><TITLE
+>SMP Support</TITLE
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+TITLE="Device Driver Interface to the Kernel"
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+TITLE="Synchronization"
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+>eCos Reference Manual</TH
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+>Prev</A
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+><TD
+WIDTH="80%"
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+>Chapter 18. Device Driver Interface to the Kernel</TD
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+><DIV
+CLASS="SECTION"
+><H1
+CLASS="SECTION"
+><A
+NAME="DEVAPI-SMP-SUPPORT">SMP Support</H1
+><P
+>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. </P
+><P
+>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.</P
+><P
+>Synchronization between threads and DSRs continues to require that the
+thread-side code use <TT
+CLASS="FUNCTION"
+>cyg_drv_dsr_lock()</TT
+> and
+<TT
+CLASS="FUNCTION"
+>cyg_drv_dsr_unlock()</TT
+> 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.</P
+><P
+>Synchronization between ISRs and DSRs or threads requires that access
+to sensitive data be protected, in all places, by calls to
+<TT
+CLASS="FUNCTION"
+>cyg_drv_isr_lock()</TT
+> and
+<TT
+CLASS="FUNCTION"
+>cyg_drv_isr_unlock()</TT
+>. 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.</P
+><P
+>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.</P
+><P
+>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
+<TT
+CLASS="FUNCTION"
+>cyg_drv_spinlock_*()</TT
+> functions.</P
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+>
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