Access to video memory

1
Access to video memory

• We create a Linux device-driver that gives
  applications access to our graphics frame-buffer

2
The role of a device-driver
A device-driver is a software module that
controls a hardware device in response to OS
kernel requests relayed, often, from an
application
hardware device
i/o memory
in
out
RAM
device-driver module
user application
ret
call
call
ret
Operating System kernel
syscall
standard runtime libraries
sysret
user space
kernel space
3
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 pixels color is an individually
programmable mix of red, green, and blue
4
Special dual-ported memory
CRT
CPU
VRAM
RAM
16-MB of VRAM
2048-MB of RAM
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
  screens width and height (measured in pixels)
• Example when our screen-resolution is set to
  1280-by-960, we are seeing 1,228,800 pixels
• The number of bits-per-pixel (color depth) is a
  programmable parameter (varies from 1 to 32)
• Certain types of applications also need to use
  extra VRAM (for multiple displays, or for
  special effects like computer game animations)

6
How truecolor works
0
8
16
24
alpha
red
green
blue
longword
R
G
B
pixel
The intensity of each color-component within a
pixel is an 8-bit value
7
x86 uses little-endian order
truecolor graphics-modes use 4-bytes per
picture-element
0 1 2 3
4 5 6 7
8 9 10
B
G
R
B
G
R
B
G
R
VRAM

Video Screen
8
Some operating system issues

• Linux is a protected-mode operating system
• I/O devices normally are not directly accessible
• Linux on x86 platforms uses virtual memory
• Privileged software must map the VRAM
• A device-driver module is needed vram.c
• We can compile it using mmake vram
• Device-node mknod /dev/vram c 98 0
• Make it writable chmod aw /dev/vram

9
Our vram.c module

• Its a character-mode Linux device-driver
• It implements four device-file methods
• read() lets a program read from video memory
• write() lets a program write to video memory
• llseek() lets a program move the files
  pointer
• mmap() lets a program map vram to user-space
• It also implements a pseudo-file that lets users
  view the RADEON X300 graphics controllers PCI
  Configuration Space parameter-values
• cat /proc/vram

10
What is PCI?

• Its an acronym for Peripheral Component
  Interconnect and refers to a collection of
  industry standards for devices used in PCs
• An Intel-sponsored initiative (from 1992-9)
  having several ambitious goals
• Reduce diversity inherent in legacy PC devices
• Improve speed and efficiency of data-transfers
• Eliminate (or reduce) platform dependencies
• Simplify adding/removing peripheral adapters
• Lower PCs total consumption of electrical power

11
PCI Configuration Space
A non-volatile parameter-storage area for each
PCI device-function
PCI Configuration Space Header (16 doublewords
fixed format)
PCI Configuration Space Body (48 doublewords
variable format)
64 doublewords
12
Example Header Type 0
16 doublewords
31
0
31
0
Dwords
Status Register
Command Register
Device ID
Vendor ID
1 - 0
BIST
Cache Line Size
Class Code Class/SubClass/ProgIF
Revision ID
Latency Timer
Header Type
3 - 2
Base Address 0
Base Address 1
5 - 4
Base Address 2
Base Address 3
7 - 6
Base Address 4
Base Address 5
9 - 8
Subsystem Device ID
Subsystem Vendor ID
CardBus CIS Pointer
11 - 10
reserved
capabilities pointer
Expansion ROM Base Address
13 - 12
Minimum Grant
Interrupt Pin
reserved
Interrupt Line
Maximum Latency
15 - 14
13
Examples of VENDOR-IDs

• 0x8086 Intel Corporation
• 0x1022 Advanced Micro Devices, Inc
• 0x1002 Advanced Technologies, Inc
• 0x10EC RealTek, Incorporated
• 0x10DE Nvidia Corporation
• 0x10B7 3Com Corporation
• 0x101C Western Digital, Inc
• 0x1014 IBM Corporation
• 0x0E11 Compaq Corporation
• 0x1057 Motorola Corporation
• 0x106B Apple Computers, Inc
• 0x5333 Silicon Integrated Systems, Inc

14
Examples of DEVICE-IDs

• 0x5347 ATI RAGE128 SG
• 0x4C58 ATI RADEON LX
• 0x5950 ATI RS480
• 0x436E ATI IXP300 SATA
• 0x438C ATI IXP600 IDE
• 0x5B60 ATI Radeon X300
• See this Linux header-file for lots more
  examples
• lt/usr/src/linux/include/linux/pci_ids.hgt

15
Defined PCI Class Codes

• 0x00 Legacy Device (i.e., built before
  class-codes were defined)
• 0x01 Mass Storage controller
• 0x02 Network controller
• 0x03 Display controller
• 0x04 Multimedia device
• 0x05 Memory Controller
• 0x06 Bridge device
• 0x07 Simple Communications controller
• 0x08 Base System peripherals
• 0x09 Input device
• 0x0A Docking stations
• 0x0B Processors
• 0x0C Serial Bus controllers
• 0x0D Wireless controllers
• 0x0E Intelligent I/O controllers
• 0x0F Encryption/Decryption controllers
• 0x10 Satellite Communications controllers
• 0x11 Data Acquisition and Signal Processing
  controllers

16
Example of Sub-Class Codes

• Class Code 0x01 Mass Storage controller
• 0x00 SCSI controller
• 0x01 IDE controller
• 0x02 Floppy Disk controller
• 0x03 IPI controller
• 0x04 RAID controller
• 0x80 Other Mass Storage controller

17
Example of Sub-Class Codes

• Class Code 0x02 Network controller
• 0x00 Ethernet controller
• 0x01 Token Ring controller
• 0x02 FDDI controller
• 0x03 ATM controller
• 0x04 ISDN controller
• 0x80 Other Network controller

18
Example of Sub-Class codes

• Class Code 0x03 Display Controller
• 0x00 VGA-compatible controller
• 0x01 XGA controller
• 0x02 3D controller
• 0x80 Other display controller

19
Hardware details may differ

• Graphics controllers use vendor-specific
  mechanisms to perform similar operations
• Theres a common core of compatibility with IBMs
  VGA (Video Graphics Array) developed in the
  mid-1980s, but since IBMs loss of market
  dominance, each manufacturer has added
  enhancements which employ incompatible
  programming interfaces you need a vendors
  manual!

20
The frame-buffer

• Todays PCI graphics systems all provide a
  dedicated amount of display memory to control the
  screen-images pixel-coloring
• But how much memory will vary with price
• And its location within the CPUs physical
  address-space cant be predicted because it
  depends upon what other PCI devices are installed
  (and mapped) during startup

21
The base address fields

• The PCI Configuration Header has several
  so-called Base Addess fields, and vendors use one
  of these to hold the frame-buffers starting
  address and to indicate how much vram the video
  controller can actually use
• The Linux kernel provides driver-writers with
  some convenient functions for getting the
  location and size of the frame-buffer

22
Radeon uses Base Address 0

• Our vram.c modules initialization routine
  employs these kernel helper-functions

include ltlinux/pci.hgt struct pci_dev devp
// for a variable that will point to a
kernel-structure // get a pointer to the PCI
devices Linux data-structure devp
pci_get_device( VENDOR_ID, DEVICE_ID, NULL ) if
( !devp ) return ENODEV // device is not
present // get starting address and length for
memory-resource 0 vram_base
pci_resource_start( devp, 0 ) vram_size
pci_resource_len( devp, 0 )
23
Reading from vram

• You can use our fileview utility to see the
  current contents of the video frame-buffer
• fileview /dev/vram
• Our vram.c drivers read() method gets
  invoked when an application-program attempts to
  read from the /dev/vram device-file
• The read-method is implemented by our driver
  using ioremap() (and iounmap()) to
  temporarily map a 4KB-page of physical vram to
  the kernels virtual address-space

24
I/O memcpy() functions

• Linux provides a platform-independent way to do
  copying from an i/o-devices memory into an
  applications buffer (or vice-versa)
• A read copies from vram to a users buffer
• memcpy_fromio( buf, vaddr, len )
• A write copies to vram from a users buffer
• memcpy_toio( vaddr, buf, len )

25
mmap()

• This is a standard UNIX system-call that lets an
  application map a file into its virtual
  address-space, where it can then treat the file
  as if it were an ordinary array
• See the man-page man mmap
• This same system-call can also work on a
  device-file if that devices driver provided
  mmap() among its file-operations

26
The user-role

• In the application-program, six arguments get
  passed to the mmap() library-function
• int mmap( (void)baseaddress,
• int memorysize,
• int accessattributes,
• int flags,
• int filehandle,
• int offset )

27
The driver-role

• In the kernel, those six arguments will get
  validated and processed, then the drivers
  mmap() callback-function will be invoked to
  supply missing information and perform further
  sanity-checks and do appropriate page-mapping
  actions
• int mmap( struct file file,
• struct vm_area_struct vma )

28
Our drivers code
int mmap( struct file file, struct
vm_area_struct vma ) // extract the paramers
we will need from the vm_area_struct unsigned
long region_length vma-gtvm_end
vma-gtvm_start unsigned long region_origin
vma-gtvm_pgoff PAGE_SIZE unsigned
long physical_addr fb_base region_origin uns
igned long user_virtaddr vma-gtvm_start //
sanity check mapped region cannot extend past
end of vram if ( region_origin region_length gt
fb_size ) return EINVAL // tell the kernel
not to try swapping out this region to the
disk vma-gtvm_flags VM_RESERVED // tell the
kernel to exclude this region from any core
dumps vma-gtvm_flags VM_IO
29
Drivers code continued
// invoke a helper-function that will set up
the page-table entries if ( remap_pfn_range(
vma, user_virtaddr, physical_addr gtgt
12, region_length, vma-gtvm_page_prot ) ) return
EAGAIN return 0 // SUCCESS
30
Demo rotation.cpp

• This application-program will demonstrate use of
  our vram.c device-drivers read(), write()
  and llseek() methods (i.e., device-file
  operations)
• It will perform a rotation of the
  color-components (R,G,B) in every displayed
  truecolor pixel R ? G G ? B B ?
  R
• After 3 times the screen will look normal again

31
Demo inherit.cpp

• This application-program will demonstrate use of
  the mmap() method in our driver, and the fact
  that memory-mappings which a parent-process
  creates will be inherited by a child-process
• You will see a rectangular purple border drawn on
  your display -- provided the program-parameters
  match your screen

32
In-class exercise

• Can you adapt the ideas in inherit.cpp to
  create a program (named backward.cpp) that will
  reverse the ordering of the pixels in each
  screen-row?