Ricardo A. Baratto

1
X

• Ricardo A. Baratto
• NCL

2
Overview

• System overview
• X protocol
• X server
• Architecture
• Porting process
• XFree86 (device drivers)

3
System Overview

• History
• Created in 1984 in MIT.
• X11 released on 1987 (latest R6.6 - April 2001)
• The system
• X clients (Xlib)
• X protocol
• X server

4
System Overview (cont...)

• Features
• Event Driven
• Network Transparent
• Device Independent
• Policy Independent
• Extensible

5
X Protocol

• Communication Client Server
• Object based
• Asynchronous
• Types of Messages
• Request (Client -gt Server)
• Reply
• Event (Server -gt Client)
• Error

6
X Server

• Does
• Process Output
• Process Input
• Extension Mechanism
• Doesnt
• Define User Interface (X Toolkits)
• Window Management (WM)
• Each client responsible for its windows (Backing
  store)
• Interpret input (Input Methods Input Server)

7
X Server (cont)

• Manages a Display
• Screens (physical monitor)
• Input Devices
• Everything is a window
• Cheap, unlimited
• Tree structure
• Root window Complete screen
• Top-level windows main app window
• Parent window clips children

8
X Server Architecture

• Layered
• DIX Device Independent
• DDX Device Dependent
• Extensions
• Everything controlled through resources
• Screens, GCs, windows, pixmaps, regions,
  colormaps, fonts, cursors
• Created on demand
• Used by clients to request operations -gt Use ID

9
Dispatch Loop

• Heart of X server processing (DIX)
• Process client requests
• Send input events
• Looks like
• for()
• nready WaitForSomething() while(nready--)
  while(!isItTimeToYield) if(!ReadRequestFromCli
  ent()) break (execute request)

10
Dispatch Loop (cont)

• Requests serviced round-robin
• Execs up to 10 requests from every client
• Problems
• Busy client can monopolize server
• select() not called often enough
• Busy clients are not managed efficiently
• select() called too often

11
Dispatch Loop (cont)

• Fix XFree86 smart scheduler
• Use time metric, not request-count metric
• Dedicate time slice to each client
• Gather many clients on every iteration
• Poll, dont block
• Delay yielding
• Give clients a second chance
• Prioritize clients
• Penalize busy, praise user preferred

12
Screens

• One for each physical screen
• ScreenRec structure
• Defines functions used to handle operations on
  resources
• Main communication channel DIX-DDX

13
Drawables

• Where drawing takes place
• Windows
• Used for display output
• May have own colormap
• Pixmaps
• Off-screen own depth
• Used to speed up operations

14
Graphic Contexts

• Describe how to draw
• Colors (fg, bg), fonts, line width,
• Normally set up once, used by many
• Stored in server
• Requests specify GC, drawable and what to draw

15
Porting X Server

• Code layers
• DIX -gt Dev.Independent, dont touch
• OS -gt OS specific
• DDX -gt Graphics Device specific
• Extensions -gt Add features
• Porting Layer OS DDX
• Porting process
• Define functions required by DIX to handle
  resources

16
OS Layer

• Maintain client connections
• Schedule work to be done for clients
• Ignore/AttendClient()
• File and memory management
• Fonts stored in files
• Xalloc()/Xrealloc()/Xfree()

17
WaitForSomething()

• Input events from user or hardware
• SetInputCheck()
• Requests pending from existing clients
• ReadRequestFromClient()
• New client
• Access control -gt ClientAuthorized()
• Create it -gt NextAvailableClient()

18
DDX Layer

• Input Processing
• Mouse movement
• Key presses
• Key mappings
• Graphics Display

19
Graphics Output

• Pixel based
• Screen basic data structure
• Manage all resources
• Output done on drawables
• Pixel data format defined by DDX
• Pixmap format constant across screens
• XYPixmap -gt one bitmap for each plane
• Zpixmap -gt series of pixel values
• Pixel values interpreted at display time

20
Output Initialization

• InitOutput()
• Process arguments
• Discover screens
• AddScreen()
• Initialize pixmap formats
• Additional implementation dependent stuff

21
DDX Drawables

• Fields
• Serial number, depth, size and position
• Procedures to manipulate them
• Private DDX data
• Pixmaps
• Rectangular array of pixel values
• Reference counted
• Bitmaps are depth-1

22
DDX Drawables (cont)

• Windows
• Visible rectangle on screen
• Border drawn by DDX
• Create, move, resize
• Contents drawn by client
• Expose Events
• Server may provide backing store
• Bit gravity (resizing)
• Clipped by parent and sibbling windows

23
DDX 2D Graphics

• Procedures for drawing primitives
• Lines, arcs, arc filling, text drawing
• Clients may allow DDX optimizations
• e.g. Zero-width lines
• GC defines how to draw primitives
• May use MI functions
• Need only define basic Pixblit routines
• Read/write pixel values deal with image data

24
XFree86 Device Drivers

• Lifes easier
• Another abstraction layer
• Most previous procedures already defined
• Drivers are loadable modules
• Need not distribute/recompile whole server
• Module format X specific
• OS/Architecture independent
• Additional formats supported (dlopen())

25
Device Drivers (cont)

• Define only device specific routines
• Discovery of devices (PCI/ISA access)
• Device Initialization
• Device Drivers have access to ScreenRec
• Can redefine resource procedures
• Optimize for display hardware
• Sample VGA device driver
• Provides basic functionality usable by others
• Interface still unstable