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