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This is a quick little graphics hack I put together to experiment with libdrm
and this "dumb buffer" thingy David Airlie added to the kernel back in 2011.
If you're like me and miss the pre-KMS days of functional SVGAlib on linux
where we could write purely software-rendered graphics toys like demos that
were still beautiful and synchronized to vertical retrace without any tearing
or flickering, there is cause to rejoice.
Using libdrm and these two ioctls:
DRM_IOCTL_MODE_CREATE_DUMB
DRM_IOCTL_MODE_MAP_DUMB
We can then mmap into our address space a 32bpp buffer that can be drawn to
while off-screen, and submitted to the gpu for displaying in a page-flipping
fashion, synchronized to the vertical retrace. It's revisiting the 90s, it's
VESA 2.0 linear frame buffers but actually supporting all our crazy native
resolutions and abundant memory for 32bpp with page flipping.
In my testing so far, this seems to work without even requiring root.
The test machine is a 1.6Ghz Core2 duo w/i915 and 1400x1050 (x61s w/SXGA+), and
rototiller64 only uses 30-40% of one core, rendering a flawless 50HZ.
As for the files, they're identical, except rototiller64 writes 64-bit words at
a time to the graphics buffer, rototiller32 writes 32-bit words at a time. On
the test machine 64-bit is good for a few % CPU savings. I included the 32-bit
version since it's simpler to read and maybe you have a 32-bit box.
Before you try to run these things, realize this is direct libdrm graphics,
it's going to compete with your X/wayland server. Switch to a plain virtual
console to run the program. You don't need to quit X, just switch away from the
X vt so it's not visible.
To quit it's as simple as Ctrl-C, rototiller will otherwise run forever.
Do not try switching back to X while rototiller is running, Ctrl-C it first, or
X will get angry and exit when it tries to do drm things and can't.
Your display may be left in an inconsistent state after exiting rototiller.
Don't panic! Just switch virtual consoles or go back to X, graphics will be
restored. This seems like a bug in drm to me. It's genuine SVGAlib dejavu,
corrupt displays and all. BUCKETS OF NOSTALGIA
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