char n block.htm

What is this ``device driver stuff anyway? Here s a very short introduction to the concept.

Declares a pointer to a
<tt>none_softc</tt>
structure and calls it
<tt>sc</tt>.
The driver initializes
<tt>sc</tt>
to the address of the
<tt>none_softc</tt>
structure associated with this
<tt>NONE</tt>
device.
The minor device number,
<tt><var>unit</var></tt>,
is used as an index into the array of
<tt>none_softc</tt>
structures to determine which
<tt>none_softc</tt>
structure is associated with this
<tt>NONE</tt>
device.
<a href="#co_id_84_rtn_7">[Return to example]</a>
<p></p></li><li>
<a name="co_id_84_8"></a>
Sets the
<tt><var>res</var></tt>
variable to point to the kernel memory allocated by the
<tt>ioctl</tt>
system call.
The
<tt>ioctl</tt>
system call copies the data to and from user address space.
<p>
Because the data types are different, this line performs a type-casting
operation that converts the
<tt><var>data</var></tt>
argument (which is of type
<tt>caddr_t</tt>)
to be of type pointer to an
<tt>int</tt>.
<a href="#co_id_84_rtn_8">[Return to example]</a>
</p><p></p></li><li>
<a name="co_id_84_9"></a>
If the
<tt>ioctl</tt>
command is equal to the
<tt>DN_GETCOUNT</tt>
macro, sets
<tt><var>res</var></tt>
to the number of bytes in the write request.
This count was previously set by the
<tt>nonewrite</tt>
interface in the
<tt>sc_count</tt>
member of the
<tt>sc</tt>
pointer.
<a href="http://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/HTML/AA-PUBVD-TE_html/drivertut8.html#DevDrvRegHeadFile">Section 5.1.2</a>
discusses the
<tt>DN_GETCOUNT</tt>
macro.
<a href="http://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/HTML/AA-PUBVD-TE_html/drivertut12.html#CopyDataWriteRoutine">Section 8.2.2</a>
discusses the
<tt>nonewrite</tt>
interface.
<a href="#co_id_84_rtn_9">[Return to example]</a>
<p></p></li><li>
<a name="co_id_84_10"></a>
If the
<tt>ioctl</tt>
command is equal to the
<tt>DN_CLRCOUNT</tt>
macro, sets the count of characters written to the device to the
value zero (0).
This line has the effect of clearing the
<tt>sc_count</tt>
member of the
<tt>softc</tt>
structure associated with this
<tt>NONE</tt>
device.
<a href="http://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/HTML/AA-PUBVD-TE_html/drivertut8.html#DevDrvRegHeadFile">Section 5.1.2</a>
discusses the
<tt>DN_CLRCOUNT</tt>
macro.
<a href="#co_id_84_rtn_10">[Return to example]</a>
<p></p></li><li>
<a name="co_id_84_11"></a>
Returns
<tt>ESUCCESS</tt>
to indicate successful completion of the
<tt>ioctl</tt>
operation.
<a href="#co_id_84_rtn_11">[Return to example]</a>
</li></ol><p>
<a name="ImplementTheInterruptRoutine"></a>
</p><p></p><hr><p align="center">
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</p><p></p><hr><p>
</p><h2>
10.4&nbsp;&nbsp;&nbsp;&nbsp;Implementing the Interrupt Handler
</h2>
<p>
<a name="nx_id_526"></a>
The interrupt section applies to both character and block device drivers
and it contains a device interrupt handler, which this book refers to as
the driver's interrupt handler.
The interrupt handler
is called as a result of a hardware interrupt.
</p><p>
Digital requires that all drivers call the
<tt>handler</tt>
interfaces to dynamically register interrupt handlers. 
Typically, a device driver registers interrupt handlers in the driver's
<tt>probe</tt>
interface.
<a href="http://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/HTML/AA-PUBVD-TE_html/drivertut11.html#RegISIswithhandlerRtns">Section 7.1.6</a>
discusses how to register a driver's interrupt handler (using the
<tt>/dev/none</tt>
driver as an example).
<a href="http://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/HTML/AA-PUBVD-TE_html/drivertut11.html#RegSharedISIswithhandlerRtns">Section 7.1.7</a>
discusses how to register a driver's shared interrupt handler (using the
<tt>/dev/xx</tt>
driver as an example).
</p><p>
When the kernel interrupt interface that does the initial handling of
interrupts receives an interrupt, it:
</p><ul>
<p></p><li>
Saves the state of the CPU (for example, the registers)
<p></p></li><li>
Sets up the argument list for the call to the driver (that is, the unit
number)
<p></p></li><li>
Transfers control to the appropriate driver, based on the interrupt
vector index provided by the bus
</li></ul><p>
Upon return from the driver's interrupt handler, the kernel
restores the state of the CPU to allow previously running processes to
run.
<a name="nx_id_527"></a>
</p><p>
The
<tt>/dev/none</tt>
driver's
interrupt
handler (called
<tt>noneintr</tt>)
is a stub because there is no physical device to generate an interrupt.
However, most devices can generate interrupts.
For example, a terminal might generate an interrupt when a character is
keyed into it.
There is an interrupt entry point for those drivers that are written for
devices that generate interrupts.
<a href="http://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/HTML/AA-PUBVD-TE_html/drivertut5.html#SpecActionsonInterrupts">Section 2.1.3.4</a>
discusses a number of important issues relating to interrupts.
Some devices generate more than one type of interrupt.
Thus, drivers controlling these devices can contain more than one
interrupt section.
For the
<tt>/dev/none</tt>
driver, these issues are not relevant because there are no interrupts,
but they will be important for most drivers.
</p><p>
To give you an idea of some tasks an interrupt handler can perform, the
following example shows the code associated with the
<tt>/dev/xx</tt>
driver's interrupt handler (called
<tt>xxintr</tt>).
The
<tt>xxintr</tt>
interrupt handler is an example of a shared interrupt.
</p><p>
</p><p>
</p><pre>int xx_intr(parameter)
   int parameter; <a name="co_id_85_rtn_1"></a><a href="#co_id_85_1"><strong>[1]</strong></a>
{
<br>.<br>.<br>.<br>
   register struct xx_softc *sc = xx_softc[unit]; <a name="co_id_85_rtn_2"></a><a href="#co_id_85_2"><strong>[2]</strong></a>
   int intr_status; <a name="co_id_85_rtn_3"></a><a href="#co_id_85_3"><strong>[3]</strong></a>
<br>
<br>.<br>.<br>.<br>
   intr_status = xx_readio(PNV_INTR_CTRL, sc); <a name="co_id_85_rtn_4"></a><a href="#co_id_85_4"><strong>[4]</strong></a>

   if (!(intr_status &amp; PNV_INTR_OCCURRED))
        return(INTR_NOT_SERVICED); <a name="co_id_85_rtn_5"></a><a href="#co_id_85_5"><strong>[5]</strong></a>

   if ((intr_status &amp; PNV_DMA_INTR) { <a name="co_id_85_rtn_6"></a><a href="#co_id_85_6"><strong>[6]</strong></a>
        xx_writeio(PNV_INTR_CTRL,
                        (intr_status &amp; ~(PNV_CRD_INTR|PNV_ABRT_INTR)), sc);
<br>
<br>.<br>.<br>.<br>
   }
   return(INTR_SERVICED); <a name="co_id_85_rtn_7"></a><a href="#co_id_85_7"><strong>[7]</strong></a>
} /* End of xx_intr */
</pre>
<p>
</p><p>
</p><ol>
<p></p><li>
<a name="co_id_85_1"></a>
Specifies
any parameter that the driver needs to control operation of the
interrupt handler.
The value that gets passed to the
<tt><var>parameter</var></tt>
argument is the value you stored in the
<tt>param</tt>
member of the
<tt>handler_intr_info</tt>
data structure.
<p>
In this example, the device driver's interrupt handler does not make
use of this argument.
<a href="#co_id_85_rtn_1">[Return to example]</a>
</p><p></p></li><li>
<a name="co_id_85_2"></a>
Specifies a pointer to a
<tt>softc</tt>
data structure and initializes it to the
<tt>softc</tt>
structure associated with this
<tt>XX</tt>
device.
<a href="#co_id_85_rtn_2">[Return to example]</a>
<p></p></li><li>
<a name="co_id_85_3"></a>
Declares a variable called
<tt><var>intr_status</var></tt>
that stores the bit associated with the interrupt controller register
after the interrupt occurred.
<a href="#co_id_85_rtn_3">[Return to example]</a>
<p></p></li><li>
<a name="co_id_85_4"></a>
Calls the device driver's
<tt>xx_readio</tt>
interface, which reads the interrupt control register
<tt>PNV_INTR_CTRL</tt>
to determine if an interrupt occurred.
(Assume that the
<tt>/dev/xx</tt>
driver implements the
<tt>xx_readio</tt>
interface to read the control register.)
<a href="#co_id_85_rtn_4">[Return to example]</a>
<p></p></li><li>
<a name="co_id_85_5"></a>
If there is no interrupt status bit set in the interrupt control
register, returns the constant
<tt>INTR_NOT_SERVICED</tt>
to the kernel's interrupt dispatch code.
This constant indicates that the driver's interrupt handler
did not service the shared
interrupt.
<a href="#co_id_85_rtn_5">[Return to example]</a>
<p></p></li><li>
<a name="co_id_85_6"></a>
If the interrupt control register is set to the
<tt>PNV_DMA_INTR</tt>
bit, then an interrupt occurred.
Calls the device driver's
<tt>xx_writeio</tt>
interface to write the bit status to the interrupt control register.
(Assume that the
<tt>/dev/xx</tt>
driver implements the
<tt>xx_writeio</tt>
interface to write the bit status.)
<a href="#co_id_85_rtn_6">[Return to example]</a>
<p></p></li><li>
<a name="co_id_85_7"></a>
Returns
<tt>INTR_SERVICED</tt>
to the kernel's interrupt dispatch code to indicate that the driver's
interrupt handler serviced the shared interrupt.
<a href="#co_id_85_rtn_7">[Return to example]</a>
</li></ol><p>
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