So , I just wanted to inform people about this problem and its solution.
I have an old Ati 6870 card. This card was causing system freeze on blender viewport on some situations.
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If the viewport draw mode is textured draw mode with GLSL enabled and any or all of the light are hidden(not visible but renderable) -> freeze. Between 10 sec - 30 sec.
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Continuous Grease pencil stroke. Freeze.
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Subdivision modeling with “View in Edit Mode” enabled from the modifier tab. Freeze.
These 3 are the ones that triggered the freeze.
SOLUTION :
Ati seems to use the DX11 tessellation feature on OpenGL by default.
If you go to catalyst control center > 3d settings > Tessellation . You can see the tessellation setting is set to “Ati Optimized” by default.
This is causing the problem. Because the driver is trying to draw tessellated mesh from OpenGL from hardware and doesn’t ask Blender. Or Because Blender uses OpenGL 2.0 - 2.1 it doesn’t understand the request.
Either way if you disable this feature the freezing stops. (Just like forcing a game to use Dx9 instead of DX11).
Hope this helps somebody.
Cheers.