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Now source and destination pixmaps don't need to be the same for
using G2D acceleration (as long as both of them are allocated in
the framebuffer). This allows using G2D to copy pixels from DRI2
buffers to the framebuffer on the fallback path (when the window
of an OpenGL ES application is partially overlapped by some other
windows). Though it only works when composite extension is
disabled, for example by adding the following to xorg.conf:

    Section "Extensions"
        Option "Composite" "Disable"
    EndSection

If composite extension is enabled, windows have backing pixmaps, and
we have a longer chain of copies:

   DRI2 buffer -> backing pixmap -> framebuffer

Because backing pixmap is not allocated in a physically contiguous
memory, it can't be copied using G2D yet.
Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>

Avoid creating a new mapping because that's a waste of the virtual address
space. Also we are going to use this xserver framebuffer mapping address
for testing whether window backing pixmaps are allocated in the framebuffer
and can be accelerated by G2D.
Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>

The existing kernel driver from Allwinner for G2D accelerator
is quite bad because ioctls are synchronous and blocking the
caller thread, compromise security (basically it is a backdoor
for copying data in memory between any arbitrary physical
addresses) and have high overhead (each individual fill or
blit operation needs an ioctl). But we need to start with
something, so use this stuff as a placeholder.

The g2d_driver.h header file is taken from linux-sunxi-3.4
Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>

Actually they are converted to 32x32 with 256 color palette. In the
case if we have more than 256 unique colors, the color components
of the pixels are reduced from 8-bit to 7-bit, then to 6-bit if
necessary and so on (until we reduce the number of unique colors
so that they can fit the palette). In the worst case we may
theoretically end up with just 2 bits per A, R, G and B channels,
but in practice 7 or 6 bits seem to be enough.
Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>

Note: the header file "sunxi_disp_ioctl.h" is GPL licensed. So until
it is gets a MIT/X11 replacement, the DDX driver is GPL licensed as
a whole. The individual source files still have their own license.
Also in order to avoid any possible confusion, the MIT/X11 license
header from COPYING has been added to "fbdev.c" and "fbdev_priv.h".