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Due to the overhead of G2D for small screen-to-screen blits, CPU blits
are faster for small areas. This patch introduces are threshold below
which CPU blits are triggered. It is currently set to 1000 for 32bpp
and 2500 for 16bpp based on test results.

Some benchmarks:

1920x1080x16bppx60Hz, ShadowFB disabled:

x11perf -scroll10

Before:
1500000 trep @   0.0239 msec ( 41800.0/sec): Scroll 10x10 pixels
After:
2500000 trep @   0.0110 msec ( 90900.0/sec): Scroll 10x10 pixels

x11perf -copywinwin10

Before:
1200000 trep @   0.0247 msec ( 40500.0/sec): Copy 10x10 from window to window
After:
1800000 trep @   0.0146 msec ( 68600.0/sec): Copy 10x10 from window to window
Signed-off-by: Harm Hanemaaijer <fgenfb@yahoo.com>

The sunxi_x_g2d.c file contains the midlayer code for hooking the
G2D optimized blit into xserver. But in fact it does not strictly
need to depend on anything sunxi specific.

So now we introduce a simple "blt2d_i" interface struct which
specifically provides a pointer to the accelerated blit function.
And just use this interface struct instead of the whole "sunxi_disp_t".
This allows to easily reuse the same code for other non-G2D or even
non-sunxi blit implementations in the future.
Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>

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".