Remember the rule of three Test them Formats for documenting your messaging.
Quick Reference The goal of this chapter is to write a complete char device driver. We'll develop a character driver because this class is suitable for most simple hardware devices. Char drivers are also easier to understand than, for example, block drivers or network drivers.
Our ultimate aim is to write a modularized char driver, but we won't talk about modularization issues in this chapter.
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Throughout the chapter, we'll present code fragments extracted from a real device driver: A side effect of this behavior is that, as far as scull is concerned, the word device can be used interchangeably with "the memory area used by scull.
Anyone can compile and run scull, and scull is portable across the computer architectures on which Linux runs. On the other hand, the device doesn't do anything "useful" other than demonstrating the interface between the kernel and char drivers and allowing the user to run some tests.
The Design of scull The first step of driver writing is defining the capabilities the mechanism the driver will offer to user programs. Since our "device" is part of the computer's memory, we're free to do what we want with it.
It can be a sequential or random-access device, one device or many, and so on. To make scull be useful as a template for writing real drivers for real devices, we'll show you how to implement several device abstractions on top of the computer memory, each with a different personality.
The scull source implements the following devices. Each kind of device implemented by the module is referred to as a type: Global means that if the device is opened multiple times, the data contained within the device is shared by all the file descriptors that opened it.
Persistent means that if the device is closed and reopened, data isn't lost. One process reads what another process writes. If multiple processes read the same device, they contend for data.
The internals of scullpipe will show how blocking and nonblocking read and writecan be implemented without having to resort to interrupts.
Although real drivers synchronize with their devices using hardware interrupts, the topic of blocking and nonblocking operations is an important one and is separate from interrupt handling covered in Chapter 9, "Interrupt Handling".
These variations of scull add more "policy" than "mechanism;" this kind of behavior is interesting to look at anyway, because some devices require types of management like the ones shown in these scull variations as part of their mechanism. Each of the scull devices demonstrates different features of a driver and presents different difficulties.
This chapter covers the internals of scull0 to skull3; the more advanced devices are covered in Chapter 5, "Enhanced Char Driver Operations": Major and Minor Numbers Char devices are accessed through names in the filesystem.
Special files for char drivers are identified by a "c" in the first column of the output of ls -l. If you issue the ls -l command, you'll see two numbers separated by a comma in the device file entries before the date of last modification, where the file length normally appears.
These numbers are the major device number and minor device number for the particular device. The following listing shows a few devices as they appear on a typical system.
Their major numbers are 1, 4, 7, and 10, while the minors are 1, 3, 5, 64, 65, and The kernel uses the major number at open time to dispatch execution to the appropriate driver.A persuasive message is written or spoken communication that compels the intended audience to respond or act in a certain way.
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syn: meaning, sense, significance, purport denote that which is expressed or indicated by language or action.
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sense may also be used of meaning that is intelligible or reasonable. 1) A short message is a brief text message sent to or from a mobile phone subscriber through the Short Message Service.
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