introduction to device drivers.htm

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

there are physical devices in the hardware system. At boot time, the 
autoconfiguration software determines which of the physical devices are 
accessible and functional and can produce a correct run-time configuration for 
that instance of the running kernel. Similarly, when a driver is dynamically 
configured, the kernel performs the configuration sequence for each instance of 
the physical device. 
<P>As stated previously, the kernel makes requests of a driver by calling the 
driver's standard entry points (such as the <TT>probe</TT>, <TT>attach</TT>, 
<TT>open</TT>, <TT>read</TT>, <TT>write</TT>, <TT>close</TT> entry points). In 
the case of I/O requests such as <TT>read</TT> and <TT>write</TT>, it is typical 
that the device causes an interrupt upon completion of each I/O operation. Thus, 
a <TT>write</TT> system call from a user program may result in several calls on 
the <TT>interrupt</TT> entry point in addition to the original call on the 
<TT>write</TT> entry point. This is the case when the write request is segmented 
into several partial transfers at the driver level. 
<P>Device drivers, in turn, make calls upon kernel support interfaces to perform 
the tasks mentioned earlier. 
<P>The device register offset definitions giving the layout of the control 
registers for a device are part of the source for a device driver. Device 
drivers, unlike the rest of the kernel, can access and modify these registers. 
Digital UNIX provides generic CSR I/O access kernel interfaces that allow device 
drivers to read from and write to these registers. <A name=buses></A>
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<P>
<H3>1.6.4&nbsp;&nbsp;&nbsp;&nbsp;Bus </H3>
<P><A name=nx_id_20></A>When a device driver reads or writes to the hardware 
registers of a controller, the data travels across a bus. 
<P>A bus is a physical communication path and an access protocol between a 
processor and its peripherals. A bus standard, with a predefined set of logic 
signals, timings, and connectors, provides a means by which many types of device 
interfaces (controllers) can be built and easily combined within a computer 
system. <A name=nx_id_21></A>The term <EM>OPENbus</EM> refers to those buses 
whose architectures and interfaces are publicly documented, allowing a vendor to 
easily plug in hardware and software components. The TURBOchannel bus, the EISA 
bus, the PCI bus, and the VMEbus, for example, can be classified as having 
OPENbus architectures. <A name=nx_id_22></A><A name=nx_id_23></A><A 
name=nx_id_24></A>
<P>Device driver writers must understand the bus that the device is connected 
to. This book covers topics that all driver writers need to know regardless of 
the bus. <A name=dev_contrler></A>
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<P>
<H3>1.6.5&nbsp;&nbsp;&nbsp;&nbsp;Device Controller </H3>
<P><A name=nx_id_25></A>A device controller is the hardware interface between 
the computer and a peripheral device. Sometimes a controller handles several 
devices. In other cases, a controller is integral to the device. <A 
name=nx_id_26></A><A name=periph_devs></A>
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<P>
<H3>1.6.6&nbsp;&nbsp;&nbsp;&nbsp;Peripheral Device </H3>
<P><A name=nx_id_27></A>A peripheral device is hardware, such as a disk 
controller, that connects to a computer system. It can be controlled by commands 
from the computer and can send data to the computer and receive data from it. <A 
name=nx_id_28></A>Examples of peripheral devices include: 
<UL>
  <P>
  <LI>A data acquisition device, like a digitizer 
  <P></P>
  <LI>A line printer 
  <P></P>
  <LI>A disk or tape drive </LI></UL>
<P><A name=exofread_char></A>
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<P>
<H2>1.7&nbsp;&nbsp;&nbsp;&nbsp;Example of Reading a Character </H2>
<P><A name=nx_id_29></A>This section provides an example of how Digital UNIX 
processes a read request of a single character in raw mode from a terminal. (Raw 
mode returns single characters.) Although the example takes a simplified view of 
character processing, it does show how control can pass from a user program to 
the kernel to the device driver. It also shows that interrupt processing occurs 
asynchronously from other device driver activity. <A name=nx_id_30></A>
<P><A 
href="http://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/HTML/AA-PUBVD-TE_html/drivertut3.html#chardrv_interex">Figure 
1-3</A> summarizes the flow of control between a user program, the kernel, the 
device driver, and the hardware. The figure shows the following sequence of 
events: 
<UL>
  <P>
  <LI>A read request is made to the device driver (C-1 to C-3). 
  <P></P>
  <LI>The character is captured by the hardware (I-4 and I-5). 
  <P></P>
  <LI>The interrupt is generated (I-6). 
  <P></P>
  <LI>The interrupt handler services the interrupt (I-7 to I-9). 
  <P></P>
  <LI>The character is returned (C-10 to C-13). </LI></UL>
<P><A 
href="http://h30097.www3.hp.com/docs/base_doc/DOCUMENTATION/HTML/AA-PUBVD-TE_html/drivertut3.html#chardrv_interex">Figure 
1-3</A> provides a snapshot of the processing that occurs in the reading of a 
single character. The following sections elaborate on this sequence. <A 
name=chardrv_interex></A>
<H3>Figure 1-3: Simple Character Driver Interrupt Example</H3>
<P><IMG src="Introduction to Device Drivers_files/zk-0479U.gif">
<P><A name=nx_id_31></A><A name=areadreq_ismade></A>
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