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<HTML
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><HEAD
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><TITLE
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>Device Driver Models</TITLE
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"><LINK
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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 Levels"
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>eCos Reference Manual</TH
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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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VALIGN="bottom"
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><A
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NAME="DEVAPI-DEVICE-DRIVER-MODELS">Device Driver Models</H1
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><P
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>There are several ways in which device drivers
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may be built. The exact model chosen will depend on the properties of
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the device and the behavior desired. There are three basic models that
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may be adopted.</P
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><P
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>The first model is to do all device processing in the ISR. When it is
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invoked the ISR programs the device hardware directly and accesses
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data to be transferred directly in memory. The ISR should also call
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<TT
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CLASS="FUNCTION"
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>cyg_drv_interrupt_acknowledge()</TT
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>. When it is
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finished it may optionally request that its DSR be invoked. The DSR
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does nothing but call <TT
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CLASS="FUNCTION"
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>cyg_drv_cond_signal()</TT
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> to
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cause a thread to be woken up. Thread level code must call
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<TT
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CLASS="FUNCTION"
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>cyg_drv_isr_lock()</TT
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>, or
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<TT
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CLASS="FUNCTION"
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>cyg_drv_interrupt_mask()</TT
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> to prevent ISRs running
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while it manipulates shared memory.</P
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><P
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>The second model is to defer device processing to the DSR. The ISR
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simply prevents further delivery of interrupts by either programming
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the device, or by calling
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<TT
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CLASS="FUNCTION"
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>cyg_drv_interrupt_mask()</TT
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>. It must then call
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<TT
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CLASS="FUNCTION"
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>cyg_drv_interrupt_acknowledge()</TT
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> to allow other
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interrupts to be delivered and then request that its DSR be
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called. When the DSR runs it does the majority of the device handling,
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optionally signals a condition variable to wake a thread, and finishes
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by calling <TT
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CLASS="FUNCTION"
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>cyg_drv_interrupt_unmask()</TT
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> to re-allow
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device interrupts. Thread level code uses
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<TT
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CLASS="FUNCTION"
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>cyg_drv_dsr_lock()</TT
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> to prevent DSRs running while
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it manipulates shared memory. The eCos serial device drivers use this
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approach.</P
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><P
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>The third model is to defer device processing even further to a
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thread. The ISR behaves exactly as in the previous model and simply
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blocks and acknowledges the interrupt before request that the DSR
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run. The DSR itself only calls
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<TT
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CLASS="FUNCTION"
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>cyg_drv_cond_signal()</TT
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> to wake the thread. When
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the thread awakens it performs all device processing, and has full
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access to all kernel facilities while it does so. It should finish by
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calling <TT
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CLASS="FUNCTION"
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>cyg_drv_interrupt_unmask()</TT
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> to re-allow
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device interrupts. The eCos ethernet device drivers are written to
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this model.</P
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><P
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>The first model is good for devices that need immediate processing and
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interact infrequently with thread level. The second model trades a
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little latency in dealing with the device for a less intrusive
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synchronization mechanism. The last model allows device processing to
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be scheduled with other threads and permits more complex device
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handling.</P
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HREF="devapi-smp-support.html"
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>Prev</A
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>Home</A
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>Next</A
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WIDTH="33%"
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>SMP Support</TD
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ALIGN="center"
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HREF="devapi-device-driver-interface-to-the-kernel.html"
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ALIGN="right"
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>Synchronization Levels</TD
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