Android Framebuffer emulation technical notes:
==============================================

This document tries to explain how framebuffer emulation works in the
Android emulator.

1 - Hardware emulation (hw/goldfish_fb.c):
------------------------------------------

  The emulated hardware provides a bunch of i/o registers to allow
  the following:

  - let the kernel query the framebuffer's dimensions (both in pixels
    and millimeters) (see goldfish_fb_read)

  - let the kernel control the physical base address for the framebuffer
    and the internal pixel rotation to apply before display (see
    goldfish_fb_write).

  The pixel buffer is itself a set of physical pages allocated by the
  kernel driver in the emulated system. These pages are contiguous in
  the emulated system, but not in the emulator's process space which
  places them randomly in the heap.

  Also, a function called goldfish_fb_update_display() is in charge of
  checking the dirty bits of the framebuffer physical pages, in order to
  compute the bounding rectangle of pixel updates since the last call, and
  send them to the UI through qframebuffer_update(). More on this later.


2 - Framebuffer abstract interface (framebuffer.h):
---------------------------------------------------

  The Android-specific header 'framebuffer.h' is used to provide a generic
  interface between framebuffer 'producers' and 'clients'. Essentially, each
  QFrameBuffer object:

    - holds a contiguous pixel buffer allocated by the emulator.
    - can have one producer in charge of drawing into the pixel buffer
    - can have zero or more clients, in charge of displaying the pixel
      buffer to the final UI window (or remote VNC connection, whatever).

  The emulator will periodically call 'qframebuffer_check_updates()' which
  does the following:

      foreach fb in framebuffers:
          if fb.producer:
            fb.producer.check_updates()
              => in producer
               foreach up in updates:
                 qframebuffer_update(fb, up.x, up.y, up.w, up.h)
                   =>
                     foreach cl in fb.clients:
                        cl.update(cl.opaque, up.x, up.y. up.w, up.h)

  hw/goldfish_fb.c implements a producer
  the QEmulator type in android/main.c implements a client.


3 - DisplayState (console.h):
-----------------------------

  The upstream QEMU sources use a DisplayState object to model the state
  of each "display". This is not conceptually exactly the same thing than
  a QFrameBuffer object as it can also be used to emulated text-based
  displays instead of pixel framebuffers, and incorporates input state
  as well.

  See sdl_display_init() in android/main.c which is called from vl-android.c
  on startup to initialize the display. This really registers a function,
  sdl_refresh() (in android/main.c), that will get called every 1/60th of
  a second to handle pending inputs.

  sdl_refresh() also calls qframebuffer_check_updates(), ensuring that
  any animation in the emulated framebuffer will be displayed at the same
  frequency.

  The refresh frequency is defined by the GUI_REFRESH_INTERVAL defined
  at the top of console.h


4 - QEmulator object:
---------------------

  Currently defined and implemented in android/main.c (we may want to move
  it to a different location). The QEmulator object bridges provides a
  framebuffer client that uses the "generic" skin code under android/skin
  to display the main UI window.