SiS 315 Graphics Engine
Introduction to some capabilities
of graphics accelerator hardware
SVGA incompatibilities
• SVGA manufacturers have different ways
of implementing their accelerator features
• SiS provides 2D and 3D graphics engines
• Access is via memory-mapped i/o ports
• This requires a new Linux device-driver, to
allow mapping the io-ports into user-space
• A suitable driver is our ‘engine2d.c’
• It only works with SiS graphics hardware
SiS policy
• SiS officials say it is not company policy to
provide individuals with programming info
• But some programming info is available in
‘unofficial’ sources (e.g., in-line comments
by programmers who wrote ‘open source’
device-drivers for Linux XFree86 systems)
• Not everything is fully explained, though
• So a lot of ‘trial-and-error’ is necessary!
Where to look for info
• The source-code for drivers distributed
with the Linux kernel can be found in:
/usr/src/local/linux/drivers/video/sis
• Recent versions of the SVGALIB package
have some SiS-specific code you can view
• There is also a website maintained by the
author of the SiS driver for Linux (Thomas
Winischhofer):
http://www.winischhofer.net
Linux kernel modules
• Linux permits installing new kernel code at
runtime (i.e., without recompiling kernel)
• A system administrator can install/remove
kernel modules, and may grant users this
same privilege (by adjusting permissions
on the ‘insmod’ and ‘rmmod’ commands)
• Modules are written in the C language (not
C++) and include special header-files that
are distributed with the kernel source-code
Module requirements
• Must define __KERNEL__ and MODULE
before any #include statements
• Must have: #include <linux/module.h>
• Maybe others: e.g., #include <linux/pci.h>
• Must have these two public functions:
int init_module( void );
void cleanup_module( void );
• Usually device-specific function(s), too
Driver-Module Structure
// filename and module abstract
#include --------#define ----------typedef ------------static data objects
read()
write()
lseek()
mmap()
This is the device-driver core
struct file_operations
init_module()
These are for module mgmt
cleanup_module()
MODULE_LICENSE
Our ‘engine2d.c’ module
• Our module only needs one extra function:
int my_mmap( );
• Also needs a ‘struct file_operations’ object:
struct file_operations my_fops;
• The ‘init_module()’ function will install that
structure-object in kernel-space, together
with executable code which it references
• The ‘cleanup_module()’ function removes
that code and data after we’re finished
How it works
user-space
kernel-space
int $0x80
runtime
library
iret
syscall
handler
ret
mmap
application
program
call
ret
device-driver
module
Pentium’s Page-Tables
• Our driver’s ‘mmap’ method calls a kernel
procedure that knows how to setup some
new entries in the CPU’s page-directory
and page-table data-structures which give
the effect of mapping the GPU’s i/o-ports
into an application’s virtual address-space
• Then the program can read or write these
i/o-ports as if they were memory-locations
The PCI Interface
• The graphics hardware connects with the
CPU using the AGP bus, conforming to a
standard PCI-bus programming interface
• Linux kernel functions can be called from
our ‘init_module()’ to query the GPU chip
– Identify the chip’s make and model
– Get physical address for its i/o-memory
– Determine the length of the i/o-memory
Linux device-nodes
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Linux treats devices as if they were files
We must create a device-file for our GPU
Device-files normally go in ‘/dev’ directory
We invent a filename for our device-file
We pick an unused device id-number
A system administrator creates the file:
root# mknod /dev/sismmio c 101 0
root# chmod a+rw /dev/sismmio
Our ‘sisaccel.cpp’ demo
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We have written a short demo-program
It uses the SiS 315’s 2D graphics engine
It fills some rectangles with a solid color
It also shows how to draw a line-segment
These operations could be done, as we
know, with software algorithms – but it’s
faster to let the hardware do it instead
• You are invited to experiment further!
#include “sisaccel.h”
• This header defines symbolic names for
some of the 2D engine’s i/o addresses
• Accelerator commands involve writing the
values for various parameters to these i/o
port-addresses, concluding with a value
that encodes a desired engine ‘command’
• Some Extended Sequencer registers must
be initialized beforehand, to enable engine
Truecolor Graphics
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We used VESA graphics mode 0x413B
Screen-resolution is 800x600
Pixels are 32-bits in size (‘Truecolor’)
Recall the Truecolor pixel-format:
byte3
Alpha channel
byte2
byte1
byte0
Makefile
• In order to compile the ‘engine2d.c’ driver,
we recommend using the ‘Makefile’ on our
class website (copy it to your directory):
$ make engine2d.o
• Be sure you compile it BEFORE you try to
run the ‘gpuaccel.cpp’ demo-program
• Don’t forget that your IOPL needs to be 3
e.g., run the ‘iopl3’ program first