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<html><head><title>Implementing Character and Block Device Driver Interfaces</title>
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</p><h1>10 Implementing Character and Block Device Driver Interfaces</h1>
<p>
Character and block device drivers can contain the following sections:
</p><ul>
<p></p><li>
An open and close device section, which
contains an
<tt>open</tt>
interface and a
<tt>close</tt>
interface
<p></p></li><li>
An
<tt>ioctl</tt>
section, which
contains an
<tt>ioctl</tt>
interface
<p></p></li><li>
An interrupt section, which
contains an interrupt
handler
</li></ul><p>
The following sections explain how to implement or set up each of these
interfaces.
<a name="ImplementTheOpenRoutine"></a>
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</p><h2>
10.1 Implementing the open Interface
</h2>
<p>
<a name="nx_id_511"></a>
A device driver's
<tt>open</tt>
interface performs the tasks necessary to open a device and prepare it
for I/O operations.
The kernel calls the driver's
<tt>open</tt>
interface on behalf of applications that make an
<tt>open</tt>
system call on the device special file that corresponds to the driver.
</p><p>
The code associated with an
<tt>open</tt>
interface resides in the open and close device section of the device
driver.
<a name="nx_id_512"></a>
You specify the entry point for the driver's
<tt>open</tt>
interface
in a
<tt>dsent</tt>
structure.
<a href="http://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/HTML/AA-PUBVD-TE_html/drivertut8.html#dev_swit_tbls">Section 5.4</a>
describes the
<tt>dsent</tt>
structure.
<cite>Writing Device Drivers: Reference</cite>
provides a reference page that gives additional information on the
arguments and tasks associated with
an
<tt>open</tt>
interface.
</p><p>
To implement an
<tt>open</tt>
interface you must understand the
<tt>dev_t</tt>
data type, which is discussed in
<a href="http://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/HTML/AA-PUBVD-TE_html/drivertut12.html#Thedev_tDataType">Section 8.1.1</a>.
The following list describes some typical tasks that you perform when
implementing
an
<tt>open</tt>
interface:
</p><p>
</p><ul>
<p></p><li>
Set up the
<tt>open</tt>
interface
<p></p></li><li>
Perform tasks associated with opening a device
</li></ul><p>
Your
<tt>open</tt>
interface will probably perform most of these tasks and, possibly, some
additional ones.
The following sections describe each of these tasks, using the
<tt>/dev/none</tt>
driver as an example.
<a name="SetUpOpenRoutine"></a>
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</p><h3>
10.1.1 Setting Up the open Interface
</h3>
<p>
<a name="nx_id_513"></a>
The following code shows you how to set up an
<tt>open</tt>
interface, using the
<tt>/dev/none</tt>
driver as an example:
</p><p>
</p><p>
</p><pre><p>
noneopen(dev, flag, format)
dev_t dev; <a name="co_id_80_rtn_1"></a><a href="#co_id_80_1"><strong>[1]</strong></a>
int flag; <a name="co_id_80_rtn_2"></a><a href="#co_id_80_2"><strong>[2]</strong></a>
int format; <a name="co_id_80_rtn_3"></a><a href="#co_id_80_3"><strong>[3]</strong></a>
{
</p></pre>
<p>
<br>
<tt>.</tt>
<br>
<tt>.</tt>
<br>
<tt>.</tt>
<br>
</p><p>
</p><ol>
<p></p><li>
<a name="co_id_80_1"></a>
Declares an
argument that specifies the major and minor device numbers for
a specific
<tt>NONE</tt>
device.
The minor device number is used to determine the logical unit number for
the
<tt>NONE</tt>
device that is to be opened.
<a href="#co_id_80_rtn_1">[Return to example]</a>
<p></p></li><li>
<a name="co_id_80_2"></a>
Declares an argument to contain flag bits from the
file
<tt>/usr/sys/include/sys/file.h</tt>.
These flags indicate whether the device is being opened for reading,
writing, or both.
<a href="#co_id_80_rtn_2">[Return to example]</a>
<p></p></li><li>
<a name="co_id_80_3"></a>
Declares an argument to contain a constant that
identifies whether the device is a character or a block device.
These constants are defined in
<tt>/usr/sys/include/sys/mode.h</tt>.
<a href="#co_id_80_rtn_3">[Return to example]</a>
</li></ol><p>
<a name="DetermDevOpen"></a>
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</p><h3>
10.1.2 Performing the Tasks Associated with Opening the Device
</h3>
<p>
<a name="nx_id_514"></a>
The
<tt>/dev/none</tt>
driver's
<tt>open</tt>
interface (called
<tt>noneopen</tt>)
shows some of the typical tasks that a device driver's
<tt>open</tt>
interface can perform.
Specifically, the
<tt>/dev/none</tt>
driver's
<tt>open</tt>
interface performs the following tasks: checks to ensure that the
open is unique, marks the device as open, and returns
<tt>ESUCCESS</tt>
to indicate success.
The following code implements the
<tt>noneopen</tt>
interface.
The code also shows a call to the
<tt>minor</tt>
interface.
See
<cite>Writing Device Drivers: Advanced Topics</cite>
for an example implementation of an
<tt>open</tt>
interface for disk and tape device drivers.
</p><p>
<br>
<tt>.</tt>
<br>
<tt>.</tt>
<br>
<tt>.</tt>
<br>
</p><p>
</p><pre><p>
</p><p>
register int unit = minor(dev); <a name="co_id_81_rtn_1"></a><a href="#co_id_81_1"><strong>[1]</strong></a>
struct controller *ctlr = noneinfo[unit]; <a name="co_id_81_rtn_2"></a><a href="#co_id_81_2"><strong>[2]</strong></a>
struct none_softc *sc = &none_softc[unit]; <a name="co_id_81_rtn_3"></a><a href="#co_id_81_3"><strong>[3]</strong></a>
</p><p>
</p><p>
if(unit >= num_none)
return (ENODEV); <a name="co_id_81_rtn_4"></a><a href="#co_id_81_4"><strong>[4]</strong></a>
</p><p>
</p><p>
if (sc->sc_openf == DN_OPEN)
return (EBUSY); <a name="co_id_81_rtn_5"></a><a href="#co_id_81_5"><strong>[5]</strong></a>
</p><p>
</p><p>
if ((ctlr !=0) && (ctlr->alive & ALV_ALIVE))
{
sc->sc_openf = DN_OPEN;
return(ESUCCESS); <a name="co_id_81_rtn_6"></a><a href="#co_id_81_6"><strong>[6]</strong></a>
}
</p><p>
else return(ENXIO); <a name="co_id_81_rtn_7"></a><a href="#co_id_81_7"><strong>[7]</strong></a>
}
</p></pre>
<p>
</p><p>
</p><ol>
<p></p><li>
<a name="co_id_81_1"></a>
Declares a
<tt><var>unit</var></tt>
variable and initializes it to the device minor number.
Note the use of the
<tt>minor</tt>
interface to obtain the device minor number.
<p>
The
<tt>minor</tt>
interface takes one argument: the number of the device for which an
associated device minor number will be obtained.
The minor number is encoded in the
<tt><var>dev</var></tt>
argument, which is of type
<tt>dev_t</tt>.
<a name="nx_id_515"></a>
<a href="#co_id_81_rtn_1">[Return to example]</a>
</p><p></p></li><li>
<a name="co_id_81_2"></a>
Declares a pointer to a
<tt>controller</tt>
structure and calls it
<tt>ctlr</tt>.
The driver initializes
<tt>ctlr</tt>
to the
<tt>controller</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>controller</tt>
structures to determine which
<tt>controller</tt>
structure is associated with this
<tt>NONE</tt>
device.
<a href="#co_id_81_rtn_2">[Return to example]</a>
<p></p></li><li>
<a name="co_id_81_3"></a>
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_81_rtn_3">[Return to example]</a>
<p></p></li><li>
<a name="co_id_81_4"></a>
The
<tt>NONE</tt>
device requires no real work to open; therefore, the code could simply
ignore the call and return
<tt>ESUCCESS</tt>.
To demonstrate some of the checking that a real driver might perform,
the example provides code that checks to be sure that the device exists.
<p>
If the device minor number,
<tt><var>unit</var></tt>,
is greater than or equal to the number of devices configured by the
system, returns the error code
<tt>ENODEV</tt>,
which indicates there is no such device on the system.
This error code is defined in
<tt>/usr/sys/include/sys/errno.h</tt>.
</p><p>
<a href="http://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/HTML/AA-PUBVD-TE_html/drivertut11.html#RegISIswithhandlerRtns">Section 7.1.6</a>
shows how the
<tt>noneprobe</tt>
interface increments the
<tt><var>num_none</var></tt>
variable.
<a href="#co_id_81_rtn_4">[Return to example]</a>
</p><p></p></li><li>
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