1Android Framebuffer emulation technical notes:
2==============================================
3
4This document tries to explain how framebuffer emulation works in the
5Android emulator.
6
71 - Hardware emulation (hw/android/goldfish/fb.c):
8--------------------------------------------------
9
10  The emulated hardware provides a bunch of i/o registers to allow
11  the following:
12
13  - let the kernel query the framebuffer's dimensions (both in pixels
14    and millimeters) (see goldfish_fb_read)
15
16  - let the kernel control the physical base address for the framebuffer
17    and the internal pixel rotation to apply before display (see
18    goldfish_fb_write).
19
20  Read docs/GOLDFISH-VIRTUAL-HARDWARE.TXT for low-level details.
21
22  The pixel buffer is itself a set of physical pages allocated by the
23  kernel driver in the emulated system. These pages are contiguous in
24  the emulated system, but not in the emulator's process space which
25  places them randomly in the heap.
26
27  Also, a function called goldfish_fb_update_display() is in charge of
28  checking the dirty bits of the framebuffer physical pages, in order to
29  compute the bounding rectangle of pixel updates since the last call, and
30  send them to the UI through qframebuffer_update(). More on this later.
31
32
332 - Framebuffer abstract interface (framebuffer.h):
34---------------------------------------------------
35
36  The Android-specific header 'framebuffer.h' is used to provide a generic
37  interface between framebuffer 'producers' and 'clients'. Essentially, each
38  QFrameBuffer object:
39
40    - holds a contiguous pixel buffer allocated by the emulator.
41    - can have one producer in charge of drawing into the pixel buffer
42    - can have zero or more clients, in charge of displaying the pixel
43      buffer to the final UI window (or remote VNC connection, whatever).
44
45  The emulator will periodically call 'qframebuffer_check_updates()' which
46  does the following:
47
48      foreach fb in framebuffers:
49          if fb.producer:
50            fb.producer.check_updates()
51              => in producer
52               foreach up in updates:
53                 qframebuffer_update(fb, up.x, up.y, up.w, up.h)
54                   =>
55                     foreach cl in fb.clients:
56                        cl.update(cl.opaque, up.x, up.y. up.w, up.h)
57
58  hw/android/goldfish/fb.c implements a producer
59  the QEmulator type in android/main.c implements a client.
60
61
623 - DisplayState (console.h):
63-----------------------------
64
65  The upstream QEMU sources use a DisplayState object to model the state
66  of each "display". This is not conceptually exactly the same thing than
67  a QFrameBuffer object as it can also be used to emulated text-based
68  displays instead of pixel framebuffers, and incorporates input state
69  as well.
70
71  See sdl_display_init() in android/main.c which is called from vl-android.c
72  on startup to initialize the display. This really registers a function,
73  sdl_refresh() (in android/main.c), that will get called every 1/60th of
74  a second to handle pending inputs.
75
76  sdl_refresh() also calls qframebuffer_check_updates(), ensuring that
77  any animation in the emulated framebuffer will be displayed at the same
78  frequency.
79
80  The refresh frequency is defined by the GUI_REFRESH_INTERVAL defined
81  at the top of console.h
82
83
844 - QEmulator object:
85---------------------
86
87  Currently defined and implemented in android/main.c (we may want to move
88  it to a different location). The QEmulator object bridges provides a
89  framebuffer client that uses the "generic" skin code under android/skin
90  to display the main UI window.
91