introduction to device drivers.htm

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

other utilities. After you write your device driver you need to create a single 
binary module (a file with a <TT>.mod</TT> extension) from the driver source 
file (a file with a <TT>.c</TT> extension). After you create the single binary 
module, you need to configure it into the kernel so that you can test it on a 
running system. There are two methods of device driver configuration: static 
configuration and dynamic configuration. Static configuration consists of the 
tasks and tools necessary to link a device driver (single binary module) 
directly into the kernel at kernel build time. Dynamic configuration consists of 
the tasks and tools necessary to link a device driver (single binary module) 
directly into the kernel at any point in time. <A 
href="http://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/HTML/AA-PUBVD-TE_html/drivertut19.html#ConfigModuleDynamicDrv">Chapter 
14</A> describes how to create a single binary module and then how to statically 
and dynamically configure the single binary module (the device driver) into the 
kernel. 
<P>Do not confuse device driver configuration (static configuration and dynamic 
configuration), which encompasses the tools and steps for configuring the driver 
into the kernel, with autoconfiguration and configuration. Autoconfiguration is 
a process that determines what hardware actually exists during the current 
instance of the running kernel at static configuration time. The 
autoconfiguration software (specifically, the bus's <TT>confl1</TT> interface) 
calls the driver's <TT>probe</TT>, <TT>attach</TT>, and <TT>slave</TT> 
interfaces. Thus, the driver's <TT>probe</TT>, <TT>attach</TT>, and 
<TT>slave</TT> interfaces cooperate with the bus's <TT>confl1</TT> interface to 
determine if devices exist and are functional on a given system. 
<P>Configuration is a process associated with handling user-level requests to 
the <TT>sysconfig</TT> utility to dynamically configure, unconfigure, query, and 
reconfigure devices. The <TT>cfgmgr</TT> framework calls the driver's 
<TT>configure</TT> interface as a result of these <TT>sysconfig</TT> utility 
requests. The <TT>cfgmgr</TT> framework also calls the driver's 
<TT>configure</TT> interface as a result of static configuration requests. Thus, 
the driver's <TT>configure</TT> interface cooperates with the <TT>cfgmgr</TT> 
framework to statically configure and to dynamically configure, unconfigure, 
query, and reconfigure devices. The driver's <TT>configure</TT> interface also 
cooperates with the <TT>cfgmgr</TT> framework to perform one-time initialization 
tasks such as allocating memory, initializing data structures and variables, and 
adding driver entry points to the <TT>dsent</TT> table. A driver's 
<TT>configure</TT> interface should be implemented to handle static and dynamic 
configuration. <BR><A name=placeof_devdrv></A>
<P>
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<P>
<H2>1.6&nbsp;&nbsp;&nbsp;&nbsp;Place of a Device Driver in Digital UNIX </H2>
<P><A 
href="http://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/HTML/AA-PUBVD-TE_html/drivertut3.html#placeof_drvinos">Figure 
1-2</A> shows the place of a device driver in Digital UNIX relative to the 
device: 
<UL>
  <P>
  <LI>User program or utility 
  <P>A user program, or utility, makes calls on the kernel but never directly 
  calls a device driver. 
  <P></P>
  <LI>Kernel 
  <P>The kernel runs in supervisor mode and does not communicate with a device 
  except through calls to a device driver. 
  <P></P>
  <LI>Device driver 
  <P>A device driver communicates with a device by reading and writing through a 
  bus to peripheral device registers. 
  <P></P>
  <LI>Bus 
  <P>The bus is the data path between the main processor and the device 
  controller. <A name=placeof_drvinos></A>
  <H3>Figure 1-2: Place of a Device Driver in Digital UNIX</H3>
  <P><IMG src="Introduction to Device Drivers_files/zk-0201U.gif">
  <P><A name=nx_id_16></A>
  <P></P>
  <LI>Controller 
  <P>A controller is a physical interface for controlling one or more devices. A 
  controller connects to a bus. 
  <P></P>
  <LI>Peripheral device 
  <P>A peripheral device is a device that can be connected to a controller, for 
  example, a disk or tape drive. Other devices (for example, the network) may be 
  integral to the controller. </P></LI></UL>
<P>
<P>The following sections describe these parts, with an emphasis on how a device 
driver relates to them. <A name=user_prog></A>
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<P>
<H3>1.6.1&nbsp;&nbsp;&nbsp;&nbsp;User Program or Utility </H3>
<P>User programs, or utilities, make system calls on the kernel that result in 
the kernel making requests of a device driver. <A name=nx_id_17></A>For example, 
a user program can make a <TT>read</TT> system call, which calls the driver's 
<TT>read</TT> interface. <A name=the_kernel></A>
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<P>
<H3>1.6.2&nbsp;&nbsp;&nbsp;&nbsp;Kernel </H3>
<P><A name=nx_id_18></A>The kernel makes requests to a device driver to perform 
operations on a particular device. Some of these requests result directly from 
user program requests. For example: 
<UL>
  <P>
  <LI>Block I/O (open, strategy, close) 
  <P></P>
  <LI>Character I/O (open, write, close) </LI></UL>
<P>
<P>Autoconfiguration requests, such as <TT>probe</TT> and <TT>attach</TT>, do 
not result directly from a user program, but result from activities performed by 
the kernel. At boot time, for example, the kernel (specifically, the bus code) 
calls the driver's <TT>probe</TT> interface. Configuration requests, such as 
configure, unconfigure, and query, result from a system manager's use of the 
<TT>sysconfig</TT> utility. 
<P>A device driver may call on kernel support interfaces to support such tasks 
as: 
<UL>
  <P>
  <LI>Sleeping and waking (process rescheduling) 
  <P></P>
  <LI>Scheduling events 
  <P></P>
  <LI>Managing the buffer cache 
  <P></P>
  <LI>Moving or initializing data </LI></UL>
<P><A name=dev_drvs></A>
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<P>
<H3>1.6.3&nbsp;&nbsp;&nbsp;&nbsp;Device Driver </H3>
<P><A name=nx_id_19></A>A device driver, run as part of the kernel software, 
manages each of the device controllers on the system. Often, one device driver 
manages an entire set of identical device controller interfaces. With Digital 
UNIX, you can statically configure more device drivers into the kernel than