The ‘ioctl’ driver-function On implementing ‘show’ and ‘hide’ for the SiS 315 hardware cursor

‘struct file_operations’ For a Linux device-driver’s ‘init_module()’ function, there are two main actions: –Initializing the driver’s global data-structures (this includes verifying the device’s presence) –Registering the driver’s service-functions with the kernel (i.e., the ‘file_operations’ structure) The driver’s ‘cleanup_module()’ function has the duty to ‘unregister’ those services

Driver services For character-mode device-drivers (such as our ‘dram.c’, ‘mbr.c’, and ‘vramm.c’), we have implemented some (or all) of the following service-functions (i.e., ‘methods’) – read() – llseek() – write() – mmap()

The ‘file’ paradigm The UNIX approach to device-control is to create objects that represent i/o-devices, but which behave like ‘files’ do, insofar as the application programmer is concerned So ‘read()’, ‘llseek()’, ‘write()’ and ‘mmap() use the same function-call syntax – and in most respects the same semantics – for both ‘files’ and ‘devices’

An imperfect paradigm But often there are a few ways in which the file-object paradigm doesn’t quite fit with important features of an i/o device In these cases, device-drivers can provide a ‘workaround’ that allows applications to perform device-actions that deviate from customary ‘read/write/seek’ file-like actions This ‘workaround’ mechanism is ‘ioctl()’

The graphics display Our PC’s graphics display device offers an easy example of desirable behavior that is outside the traditional ‘file’ paradigm In order to display graphical images that are full-screen renderings of artistic work, we want to avoid seeing a mouse-cursor (it’s an ugly and disruptive and distraction) How can a program ‘turn off’ that cursor?

The GPU ‘resources’ The SiS 315 graphics processing units in our workstations are each equipped with 32-megabytes of video display memory (designated as PCI resource 0) These graphics adapters also implement a set of memory-mapped registers known as the 2D graphics engine (and designated as PCI resource 1)

‘pci_resource_start()’ Device-drivers can use this Linux kernel function to discover the physical address where resource 0, or resource 1, resides struct pci_dev*devp = NULL: devp = pci_find_device( VEN, DEV, devp ); vram = pci_resource_start( devp, 0 ); mmio = pci_resource_start( devp, 1 );

SiS 315 information Programming manual for the SiS 315 GPU is not usually made available to the public But some information can be derived from reading Linux device-driver source-code Examples are: –The ‘/usr/src/linux/drivers/video/sis’ directory –Also download the ‘svgalib ’ package

Graphics Cursor cursor-image source-offset x8500 cursor visibility control bit: 0=hide, 1=show cursor-image horizontal coordinate 0x850C 0x8510 cursor starting command bit: 0=no, 1=yes cursor-image vertical coordinate

Algorithm to hide cursor Map physical page containing the registers to a virtual address (with ‘ioremap()’) Read current values of these registers Clear bit #30 (to make cursor invisible) Set bit #31 (to initiate a new command) Write these adjusted register values Undo the mapping (with ‘iounmap()’)

Algorithm to show cursor Map physical page containing the registers to a virtual address (with ‘ioremap()’) Read current values of these registers Set bit #30 (to make cursor visible) Set bit #31 (to initiate a new command) Write these adjusted register values Undo the mapping (with ‘iounmap()’)

The ‘ioctl.c’ module These techniques are demonstrated in this device-driver module’s ‘ioctl()’ function Two IOCTL commands are implemented –#define CURSOR_HIDE0 –#define CURSOR_SHOW1 Applications can open the device-file, then use an ioctl-command; for example: int fd = open( “/dev/vram”, O_RDWR ); ioctl( fd, CURSOR_HIDE);

In-class exercise #1 Try adding new IOCTL commands to the ‘ioctl.c’ driver which lets applications find or move the cursor’s screen-position; #define CURSOR_FIND2 #define CURSOR_MOVE3 struct { long x, y; } location; ioctl( fd, CURSOR_FIND, &location ); location.x += 40; location.y += 40; ioctl( fd, CURSOR_MOVE, &location );

In-class exercise #2 We are not told what function is served by two of the SiS engine’s i/o locations: –0x8500 (longword) cursor-image source-offset –0x8504 (longword) –0x8508 (longword) –0x850C (longword) cursor-image horiz-coord –0x8510 (longword) cursor-image vertl-coord So can you ‘discover’ their purpose?