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A device-driver for Video Memory

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’). These pixels are redrawn sequentially, left-to-right, by rows from top to bottom.

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A device-driver for Video Memory

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

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

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

  4. 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

  5. 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)

  6. How ‘truecolor’ works 24 16 8 0 longword alpha red green blue R G B pixel The intensity of each color-component within a pixel is an 8-bit value

  7. Intel uses “little-endian” order 0 1 2 3 4 5 6 7 8 9 10 VRAM B G R B G R B G R Video Screen

  8. 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

  9. 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?

  10. Physical Memory Space (4GB) 0xFFFFFFFF VRAM 0x40000000 DRAM 1-GigaByte 0x00000000

  11. 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[]’

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

  13. 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 );

  14. 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’)

  15. 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_ID 0x1039 #define DEVICE_ID 0x6325

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

  17. 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)

  18. 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!

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