A device-driver for Video Memory Introduction to basic principles of the PC’s graphics display

Raster Display Technology The graphics screen is a two-dimensional array of picture elements (‘pixels’) Each pixel’s color is an individually programmable mix of red, green, and blue These pixels are redrawn sequentially, left-to-right, by rows from top to bottom

Special “dual-ported” memory VRAM RAM CPU CRT 32-MB of VRAM 1024-MB of RAM

Graphics programs What a graphics program must do is put appropriate bit-patterns into the correct locations in the VRAM, so that the CRT will show an array of colored dots which in some way is meaningful to the human eye Thus, programmer must understand what the CRT will do with the contents of VRAM

How much VRAM is needed? This depends on (1) the total number of pixels, and on (2) the number of bits-per-pixel The total number of pixels is determined by the screen’s width and height (measured in pixels) Example: when our “screen-resolution” is set to 1280-by-1024, we are seeing 1,310,720 pixels The number of bits-per-pixel (“color depth”) is a programmable parameter (varies from 1 to 32) Some types of applications also need to use extra VRAM (for multiple displays, or for “special effects” like computer game animations)

How ‘truecolor’ works R B G alpharedgreenblue pixel longword The intensity of each color-component within a pixel is an 8-bit value

Intel uses “little-endian” order BGRBGRBGR VRAM Video Screen

Recall our ‘dram.c’ driver Earlier we wrote a simple character-mode device-driver (named ‘dram.c’) that let an application program read the contents of the processor’s physical memory (1 GB) That device driver’s ‘read()’ function used the Linux kernel’s ‘kmap()’ function to map pages of physical memory to kernel space

We want a ‘vram.c’ driver We can create a similar device-driver that will let an application read, and also write, to our system’s video display memory But a few new issues will arise: –Where is physical video memory located? –How do we ‘map’ vram to virtual addresses? –How can we trasfer data to and from vram?

Physical Memory Space (4GB) VRAM DRAM 1-GigaByte 0x xFFFFFFFF 0x

Mapping a device’s memory We use a pair of special kernel functions to ‘map’ and ‘unmap’ segments of vram: void *virtaddr = ioremap( physaddr, len ); void iounmap( virtaddr ); Remember: device-memory is not tracked by the ‘struct page’ entries of ‘mem_map[]’

Virtual to Physical user space High memory Zone normal kernel space virtual address-space physical address-space video memory

Doing data-transfers Instead of using the ‘copy_to_user()’ and ‘copy_from_user()’ kernel-functions, data is transferred to and from device-memory using this pair of Linux kernel functions: void memcpy_fromio( buf, dev, length); void memcpy_toio( dev, buf, length );

Finding a device’s memory Modern peripheral devices for PCs do not employ standard fixed memory-addresses Device-memory gets physically mapped to unused areas in the CPU’s address-space during the ‘system configuration’ process The locations and sizes of device-memory are ‘remembered’ in non-volatile battery- powered RAM (‘configuration memory’)

Kernel’s PCI functions Linux provides a set of kernel functions for device-drivers to use when locating where a device’s memory was physically mapped The PCI functions (Peripheral Component Interconnect) are based on industry-wide standards based on committee consensus Devices use an identification-number pair: #define VENDOR_ID0x1039 #define DEVICE_ID0x6325

Functions we need struct pci_dev { // contains many fields }; struct pci_dev *devp = NULL; devp = pci_find_device( VID, DID, devp );

Base-address and Length For the SiS-315 Graphics Processor used in our classroom and lab workstations, the display memory is ‘resource number 0’ unsigned long base, size; base = pci_resource_start( devp, 0 ); size = pci_resource_len( devp, 0 ); NOTE: Our machines have 32MB of VRAM (although are capable of addressing 128MB)

In-class exercise We created a ‘vramdraw.cpp’ application that exercises our ‘vram.c’ device-driver’s ‘read()’, ‘write()’, and ‘llseek()’ methods – but it has a noticable programming ‘bug’. Your job is to analyze carefully the driver’s code and the program’s code, in order to find the bug – and then fix it!