Device Drivers 1 B. RAMAMURTHY 3/18/2018
Introduction 2 A device driver is computer program that allows a system to interface with hardware devices. Example driver: printer driver, bluetooth driver, pipe driver Example devices: your USB stick, sensors: accelerometer It is a translator between the operating system and applications the use the devices and the devices. A typical operating system has many device drivers built into it. A device driver converts general IO instructions into device specific operations. Device drivers operate in a privileged mode requires careful design 3/18/2018
Why Device Driver? 3 A typical computing system (lap top, computer, cell phone, PDA, Point of sale system) deals with a variety of devices. Making a hardware device work as expected is a cumbersome task. Instead adding this code every application, operating system provides a single point interface for all devices by hosting the device drivers. Adding it under the operating systems provides the protection and security needed for the device drivers from malicious use. The device drivers are essentially shared dynamically linked libraries. 3/18/2018
File abstra. Ction 4 What do you with a device? {read, write}, {read only}, {write only} Lets look at some examples: USB device, CD-ROM, LED Display, What do you do with a file? open, close, read, write, . . File is an excellent abstraction for devices. 3/18/2018
/dev partial listing 5 total 380 lrwxrwxrwx 1 root 30 Mar 7 2004 allkmem ->. . /devices/pseudo/mm@0: allkmem lrwxrwxrwx 1 root 27 Aug 15 2001 arp ->. . /devices/pseudo/arp@0: arp lrwxrwxrwx 1 root 7 Aug 15 2001 audio -> sound/0 lrwxrwxrwx 1 root 10 Aug 15 2001 audioctl -> sound/0 ctl lrwxrwxrwx 1 root 11 Oct 4 03: 06 bd. off -> /dev/term/b drwxr-xr-x 2 root 512 Aug 17 2001 cfg lrwxrwxrwx 1 root 31 Aug 15 2001 conslog ->. . /devices/pseudo/log@0 : conslog lrwxrwxrwx 1 root 30 Aug 15 2001 console ->. . /devices/pseudo/cn@0: console drwxr-xr-x 2 root 512 Aug 15 2001 cua drwxr-xr-x 2 root 2048 Aug 31 2002 dsk lrwxrwxrwx 1 root 29 Aug 15 2001 dump ->. . /devices/pseudo/dump@0: d ump lrwxrwxrwx 1 root 50 Aug 15 2001 ecpp 0 ->. . /devices/pci@1 f, 4000/eb us@1/ecpp@14, 3043 bc: ecpp 0 lrwxrwxrwx 1 root 8 Aug 15 2001 fb 0 -> fbs/ffb 0 drwxr-xr-x 2 root 512 Aug 15 2001 fbs dr-xr-xr-x 2 root 528 Nov 9 11: 51 fd lrwxrwxrwx 1 root 30 Apr 7 2002 fssnapctl ->. . /devices/pseudo/ 3/18/2018
Device SPACE 6 Typically there are multiple devices of the same type. All the devices controlled by the same device driver is given the same “major number” A “minor number” distinguishes among the devices of the same type. Example: printers have a major number since purpose is same, minor# is denote a specific printer 3/18/2018
Examples from XINU 7 Take a look at files in the include directory: device. h tty. h uart. h Also in the system directory devtable. c, initialize. c Bottom line is this, for a device xyz: 1. Include a file in include directory: xyz. h -- define the operations/functions for the device 2. Add a directory xyz -- implement all functions each in its own file 3. Add an entry in the devtable. c for the device (note that this has the “minor” device number along with other things) 3/18/2018
Lets Analyze the XINU UART Driver 8 Starting point: uart. h in include directory uart directory functions system directory devtable. c, initialize. c Usage of the devices is through device table: Ex: pdev = &devtab[i]; (pdev init)(pdev); 3/18/2018
UART Driver in EXINU 9 1. General device driver related files: device. h, devtable. c 2. Uart files: uart. h defining the physical features of the uart 3. All the files in the uart directory that implement the operations related to the uart. Control. c uart. Init. c uart. Intr. c uart. Put. Char. c uart. Write. c uart. Get. Char. c uart. Read. c 3/18/2018
Device Drivers 10 On board devices are called internal peripherals and one outside are called external peripherals UART Chip (internal) TTY (external) UART transceiver RS 232 D-9 connector laptop serial socket WRT 54 GL board and modifications 3/18/2018
Device drivers (contd. ) 11 Embedded processor interacts with a peripheral device through a set of control and status registers. Registers are part of the peripheral device. Registers within a serial controller are different from those in a timer. These devices are located in the memory space of the processor or I/O space of the processor-- two types: memory-mapped or I/O mapped respectively. 3/18/2018
Device driver (contd. ) 1. 1 2 The keyword volatile should be used when declaring pointers to device drivers. Bit patterns for testing, setting, clearing, toggling, shifting bits, bitmasks, and bitfields. Struct overlays: In embedded systems featuring memory mapped IO devices, it is common to overlay a C struct on to each peripheral’s control and status registers. This will provide the offsets for the various registers from the base address of the device. 3/18/2018
Device Driver Philosophy 13 Hide the hardware completely: hardware abstraction If the device generates any interrupts include interrupt controllers. Device driver presents a generic interface for applications at higher level to access the devices: device. h Device drivers in embedded systems are different from general purpose operating systems: See diagram in slide #14 Applications in general purpose systems accesses OS (Operating Systems) which in turn accesses device drivers. Applications in embedded systems can directly access device drivers. 3/18/2018
General Purpose OS vs. Embedded System Application process Operating System: dev/xyz Device driver Physical Device hardware 14 3/18/2018
Device Driver development steps 15 1. 2. 3. 4. 5. An interface to the control and status registers. Variables to track the current state of the physical and logical devices -- Major and minor device number, device name A routine to initialize the hardware to known state An API for users of the device driver -- Read, write, seek Interrupts service routines 3/18/2018
Example: A serial device driver 16 Read the text for explanation and general example of a timer Now lets look at the UARTdriver of the embedded xinu and WRT 54 GL. Study the tty driver that is a logical device that is layered on top of the UART driver. In our Lab 2 you will write a device driver for a framebuffer. 3/18/2018
Shift Operators 17 << left shift >> right shift Usage: unsigned int x = 70707; //x = 000000010100 0011 unsigned int y, z; y = x << 2; // y = 00000100 01010000 1100 z = x >> 2; //z = 00000000 01000101 00001100 3/18/2018
Logic Operators 18 Bitwise & (AND) Bitwise inclusive | (OR) Bitwise exclusive ^ (XOR) Bitwise negation ~ Usage: unsigned exp 1 = 1; unsigned exp 2 = 4; printf (“ %dn”, exp 1 | exp 2); printf (“ %dn”, exp 1 & exp 2); printf (“ %dn”, exp 1 ^ exp 2); printf (“ %dn”, ~exp 1); 3/18/2018
Relevance of shift and logic operators 19 Bitwise operations are necessary for much low-level programming, such as writing to device drivers, lowlevel graphics, communications protocol packet assembly and decoding. Device drivers use these operators to test the presence or absence of a bit in a serial port or a device input, for example. (checking for on or off) 3/18/2018
Summary 20 We studied the design and development of device drivers. We analyzed the code for a sample UART driver. 3/18/2018
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