developing a device driver.htm

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

<P>
<H3>2.1.4.1&nbsp;&nbsp;&nbsp;&nbsp;Listing the Device Documentation </H3>
<P><A name=nx_id_58></A>For a real device, you should have on hand the manual 
for the device that is supplied by the manufacturer. The <TT>NONE</TT> device is 
a fictitious device; therefore, this section of the worksheet shown in <A 
href="http://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/HTML/AA-PUBVD-TE_html/drivertut29.html#devuse_wkshtfordevnone">Figure 
C-7</A> is left blank. <A name=SpecNumofDeviceTypes></A>
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<H3>2.1.4.2&nbsp;&nbsp;&nbsp;&nbsp;Specifying the Number of Device Types to 
Reside on the System </H3>
<P><A name=nx_id_59></A>The number of devices that can be supported has a direct 
effect on the design of the driver. If only one will be supported, the driver 
need not worry about determining which device is being accessed. If a small 
number (for example, 2 - 5) is supported, the driver can use simple data 
structures and indexing to keep track of device access. If a greater number is 
to be supported, the driver must use more sophisticated methods to keep track of 
which device is being accessed. 
<P>The <TT>/dev/none</TT> driver can accommodate more than one instance. The 
worksheet shown in <A 
href="http://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/HTML/AA-PUBVD-TE_html/drivertut29.html#devuse_wkshtfordevnone">Figure 
C-7</A> indicates that the <TT>/dev/none</TT> device driver can support a 
maximum of eight instances of the <TT>NONE</TT> device. However, the driver 
writers at EasyDriver Incorporated ship the <TT>/dev/none</TT> driver product to 
support a maximum of one instance. Customers of EasyDriver Incorporated can 
change this number by editing the <TT>none:</TT> entry in the 
<TT>/etc/sysconfigtab</TT> database. <A name=DescribePurposeofDevice></A>
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<P>
<H3>2.1.4.3&nbsp;&nbsp;&nbsp;&nbsp;Describing the Purpose of the Device </H3>
<P><A name=nx_id_60></A>For this device usage item, enter a short description of 
the purpose of the device. For example, the purpose of most disk devices is to 
provide storage for user data and files. The purpose of most terminal devices is 
to provide a means for interacting with users of the system. 
<P>The worksheet shown in <A 
href="http://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/HTML/AA-PUBVD-TE_html/drivertut29.html#devuse_wkshtfordevnone">Figure 
C-7</A> indicates that this item is not applicable to the <TT>NONE</TT> device 
because it is a fictitious device. <A name=provdescof_devreg></A>
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<P>
<H3>2.1.5&nbsp;&nbsp;&nbsp;&nbsp;Providing a Description of the Device Registers 
</H3>
<P><A 
href="http://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/HTML/AA-PUBVD-TE_html/drivertut29.html#devreg_wkshtfordevnone">Figure 
C-8</A> and <A 
href="http://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/HTML/AA-PUBVD-TE_html/drivertut29.html#devreg2_wkshtfordevnone">Figure 
C-9</A> show the device register information for the <TT>/dev/none</TT> driver. 
<A name=nx_id_61></A><A name=nx_id_62></A><A name=nx_id_63></A>As the worksheets 
show, you gather the following information about the device registers: 
<UL>
  <P>
  <LI>A description or sketch of the layout of the device registers 
  <P>The manual supplied with the device would most likely have the following: 
  <UL>
    <P>
    <LI>The layout of the registers and their offset 
    <P></P>
    <LI>How the registers are used </LI></UL>
  <P>
  <P>The worksheet shown in <A 
  href="http://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/HTML/AA-PUBVD-TE_html/drivertut29.html#devreg_wkshtfordevnone">Figure 
  C-8</A> indicates a device register offset definition of the device register 
  for the <TT>NONE</TT> device along with a comment as to its function. 
  <P></P>
  <LI>A mapping of the device register with the memory address </LI></UL>
<P><A name=identsup_wrtdrv></A>
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<H3>2.1.6&nbsp;&nbsp;&nbsp;&nbsp;Identifying Support in Writing the Driver </H3>
<P><A 
href="http://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/HTML/AA-PUBVD-TE_html/drivertut29.html#devsupp_wkshtfordevnone">Figure 
C-10</A> shows the support associated with writing the <TT>/dev/none</TT> 
driver. <A name=nx_id_64></A>As the worksheet shows, you gather the following 
information about device driver support: 
<UL>
  <P>
  <LI>Whether you are writing the driver from scratch 
  <P><A name=nx_id_65></A>Answering this question can help determine the amount 
  of time you need to spend writing the driver. Most device drivers are written 
  by modifying an existing device driver that contains similar functionality. 
  However, if source code for an existing driver is not available, it may take 
  you more time to develop the driver. The <TT>/dev/none</TT> driver will be 
  developed from scratch; therefore, the Yes box on the worksheet shown in <A 
  href="http://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/HTML/AA-PUBVD-TE_html/drivertut29.html#devsupp_wkshtfordevnone">Figure 
  C-10</A> is marked. 
  <P></P>
  <LI>Whether the driver for the device was written previously for an ULTRIX 
  system 
  <P><A name=nx_id_66></A>If the source code is available, you can update the 
  device driver to reflect the Digital UNIX operating system. <A 
  href="http://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/HTML/AA-PUBVD-TE_html/drivertut7.html#PortingDevDrvs">Section 
  4.1</A> provides information to help you understand the tasks involved in 
  porting from an ULTRIX system to Digital UNIX. In this case, no source code is 
  available so the No box on the worksheet shown in <A 
  href="http://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/HTML/AA-PUBVD-TE_html/drivertut29.html#devsupp_wkshtfordevnone">Figure 
  C-10</A> is marked. 
  <P></P>
  <LI>Whether the driver for the device was written previously for a UNIX system 

  <P><A name=nx_id_67></A>If the source code is available, you can begin 
  updating the device driver by identifying areas that are different from 
  Digital UNIX. Other versions of the UNIX operating system would probably have 
  different data structures and some of the kernel interfaces would probably 
  behave differently and expect different arguments. In this case, no source 
  code is available so the No box on the worksheet shown in <A 
  href="http://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/HTML/AA-PUBVD-TE_html/drivertut29.html#devsupp_wkshtfordevnone">Figure 
  C-10</A> is marked. 
  <P></P>
  <LI>Whether the driver for the device was written previously for another 
  operating system 
  <P><A name=nx_id_68></A>If the source code is available, you must study the 
  differences between writing a device driver on that operating system and on 
  the Digital UNIX operating system. This would probably be more difficult than 
  the pre