Question 9 which of the following actions may update your linux device drivers?

The ioctl function is useful for implementing a device driver to set the configuration on the device. e.g. a printer that has configuration options to check and set the font family, font size etc. ioctl could be used to get the current font as well as set the font to a new one. A user application uses ioctl to send a code to a printer telling it to return the current font or to set the font to a new one.

int ioctl(int fd, int request, ...)

1. fd is file descriptor, the one returned by open;
2. request is request code. e.g GETFONT will get the current font from the printer, SETFONT will set the font on the printer;
3. the third argument is void *. Depending on the second argument, the third may or may not be present, e.g. if the second argument is SETFONT, the third argument can be the font name such as "Arial";

int request is not just a macro. A user application is required to generate a request code and the device driver module to determine which configuration on device must be played with. The application sends the request code using ioctl and then uses the request code in the device driver module to determine which action to perform.

A request code has 4 main parts

    1. A Magic number - 8 bits
    2. A sequence number - 8 bits
    3. Argument type (typically 14 bits), if any.
    4. Direction of data transfer (2 bits).  

If the request code is SETFONT to set font on a printer, the direction for data transfer will be from user application to device driver module (The user application sends the font name "Arial" to the printer). If the request code is GETFONT, direction is from printer to the user application.

In order to generate a request code, Linux provides some predefined function-like macros.

1._IO(MAGIC, SEQ_NO) both are 8 bits, 0 to 255, e.g. let us say we want to pause printer. This does not require a data transfer. So we would generate the request code as below

#define PRIN_MAGIC 'P'
#define NUM 0
#define PAUSE_PRIN __IO(PRIN_MAGIC, NUM) 

and now use ioctl as

ret_val = ioctl(fd, PAUSE_PRIN);

The corresponding system call in the driver module will receive the code and pause the printer.

1. __IOW(MAGIC, SEQ_NO, TYPE) MAGIC and SEQ_NO are the same as above, and TYPE gives the type of the next argument, recall the third argument of ioctl is void *. W in __IOW indicates that the data flow is from user application to driver module. As an example, suppose we want to set the printer font to "Arial".

#define PRIN_MAGIC 'S'
#define SEQ_NO 1
#define SETFONT __IOW(PRIN_MAGIC, SEQ_NO, unsigned long)

further,

char *font = "Arial";
ret_val = ioctl(fd, SETFONT, font); 

Now font is a pointer, which means it is an address best represented as unsigned long, hence the third part of _IOW mentions type as such. Also, this address of font is passed to corresponding system call implemented in device driver module as unsigned long and we need to cast it to proper type before using it. Kernel space can access user space and hence this works. other two function-like macros are __IOR(MAGIC, SEQ_NO, TYPE) and __IORW(MAGIC, SEQ_NO, TYPE) where the data flow will be from kernel space to user space and both ways respectively.

Please let me know if this helps!

Device Driver in computing refers to a special kind of software program or a specific type of software application that controls a specific hardware device that enables different hardware devices to communicate with the computer’s Operating System. A device driver communicates with the computer hardware by computer subsystem or computer bus connected to the hardware. 

Device Drivers are essential for a computer system to work properly because without a device driver the particular hardware fails to work accordingly, which means it fails in doing the function/action it was created to do. Most use the term Driver, but some may say Hardware Driver, which also refers to the Device Driver. 

Working of Device Driver: 

Device Drivers depend upon the Operating System’s instruction to access the device and perform any particular action. After the action, they also show their reactions by delivering output or status/message from the hardware device to the Operating system. For example, a printer driver tells the printer in which format to print after getting instruction from OS, similarly, A sound card driver is there due to which 1’s and 0’s data of the MP3 file is converted to audio signals and you enjoy the music. Card reader, controller, modem, network card, sound card, printer, video card, USB devices, RAM, Speakers, etc need Device Drivers to operate. 

The following figure illustrates the interaction between the user, OS, Device driver, and the devices: 

Types of Device Driver: 

For almost every device associated with the computer system there exist a Device Driver for the particular hardware. But it can be broadly classified into two types i.e.,

1. Kernel-mode Device Driver – 
   This Kernel-mode device driver includes some generic hardware that loads with the operating system as part of the OS these are BIOS, motherboard, processor, and some other hardware that are part of kernel software. These include the minimum system requirement device drivers for each operating system.
2. User-mode Device Driver – 
   Other than the devices which are brought by the kernel for working the system the user also brings some devices for use during the using of a system that devices need device drivers to function those drivers fall under User mode device driver. For example, the user needs any plug-and-play action that comes under this.

Virtual Device Driver: 

There are also virtual device drivers(VxD), which manage the virtual device. Sometimes we use the same hardware virtually at that time virtual driver controls/manages the data flow from the different applications used by different users to the same hardware. 

It is essential for a computer to have the required device drivers for all its parts to keep the system running efficiently. Many device drivers are provided by manufacturers from the beginning and also we can later include any required device driver for our system.

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