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Android Framebuffer emulation technical notes:
This document tries to explain how framebuffer emulation works in the
Android emulator.
1 - Hardware emulation (hw/android/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
Read docs/GOLDFISH-VIRTUAL-HARDWARE.TXT for low-level details.
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:
=> 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/android/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
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.