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>Chapter 13. Device Driver Interface to the Kernel</H1
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>Interrupt Model</A
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>Synchronization</A
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>Device Driver Models</A
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><P
>This chapter describes the API that device drivers may use
to interact with the kernel and HAL. It is primarily concerned with
the control and management of interrupts and the synchronization of
ISRs, DSRs and threads.</P
><P
>The same API will be present in configurations where the kernel
is not present. In this case the functions will be supplied by code
acting directly on the HAL.</P
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>Interrupt Model</A
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> presents a three level interrupt model to
device drivers. This consists of Interrupt Service Routines (ISRs) that are invoked
in response to a hardware interrupt; Deferred
Service Routines (DSRs) that are invoked in response to a request by
an ISR; and threads that are the clients of the driver. </P
><P
>Hardware interrupts are delivered with minimal intervention to an
ISR. The HAL decodes the hardware source of the interrupt and calls
the ISR of the attached interrupt object. This ISR may manipulate the
hardware but is only allowed to make a restricted set of calls on the
driver API. When it returns, an ISR may request that its DSR should be
scheduled to run.</P
><P
>A DSR will be run when it is safe to do so without interfering with
the scheduler. Most of the time the DSR will run immediately after the
ISR, but if the current thread is in the scheduler, it will be delayed
until the thread is finished. A DSR is allowed to make a larger set of
driver API calls, including, in particular, being able to call
<CODE
CLASS="FUNCTION"
>cyg_drv_cond_signal()</CODE
> to wake up waiting
threads.</P
><P
>Finally, threads are able to make all API calls and in particular are
allowed to wait on mutexes and condition variables. </P
><P
>For a device driver to receive interrupts it must first define ISR and
DSR routines as shown below, and then call
<CODE
CLASS="FUNCTION"
>cyg_drv_interrupt_create()</CODE
>. Using the handle
returned, the driver must then call
<CODE
CLASS="FUNCTION"
>cyg_drv_interrupt_attach()</CODE
> to actually attach the
interrupt to the hardware vector.</P
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