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<!-- Copyright (C) 2003 Red Hat, Inc.                                -->
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<HTML
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><HEAD
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><TITLE
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>SMP Support</TITLE
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><meta name="MSSmartTagsPreventParsing" content="TRUE">
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<META
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"><LINK
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REL="HOME"
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TITLE="eCos Reference Manual"
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HREF="ecos-ref.html"><LINK
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REL="UP"
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TITLE="Device Driver Interface to the Kernel"
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HREF="devapi-device-driver-interface-to-the-kernel.html"><LINK
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REL="PREVIOUS"
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TITLE="Synchronization"
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TITLE="Device Driver Models"
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><TR
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><TH
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>eCos Reference Manual</TH
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></TR
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><TR
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><TD
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WIDTH="10%"
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ALIGN="left"
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VALIGN="bottom"
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><A
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HREF="devapi-synchronization.html"
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ACCESSKEY="P"
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>Prev</A
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></TD
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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>Chapter 18. Device Driver Interface to the Kernel</TD
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><TD
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WIDTH="10%"
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HREF="devapi-device-driver-models.html"
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>Next</A
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><DIV
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CLASS="SECTION"
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><H1
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CLASS="SECTION"
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><A
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NAME="DEVAPI-SMP-SUPPORT">SMP Support</H1
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><P
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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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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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><P
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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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SMP-compatible device drivers use the driver API routines correctly.</P
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><P
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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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CLASS="FUNCTION"
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>cyg_drv_dsr_lock()</TT
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> and
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<TT
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CLASS="FUNCTION"
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>cyg_drv_dsr_unlock()</TT
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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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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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><P
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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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<TT
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CLASS="FUNCTION"
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>cyg_drv_isr_lock()</TT
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> and
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<TT
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CLASS="FUNCTION"
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>cyg_drv_isr_unlock()</TT
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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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a different CPU and interrupt enable/disable only work on the current
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CPU.</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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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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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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><P
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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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finer grain of locking, it can use private spinlocks, accessed via the
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<TT
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CLASS="FUNCTION"
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>cyg_drv_spinlock_*()</TT
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> functions.</P
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HREF="devapi-synchronization.html"
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>Prev</A
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>Home</A
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>Next</A
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></TD
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><TR
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><TD
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ALIGN="left"
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>Synchronization</TD
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><TD
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ALIGN="center"
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VALIGN="top"
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><A
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HREF="devapi-device-driver-interface-to-the-kernel.html"
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ACCESSKEY="U"
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>Up</A
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></TD
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><TD
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
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>Device Driver Models</TD
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></TR
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></TABLE
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></DIV
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></HTML
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>

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