Machine is one of those hybrids with both an  Intel(R) HD Graphics 4600
(according to Xorg.log) and a Venus PRO [Radeon HD 8850M / R9 M265X] (rev ff)
according to lspci.

Can't install fglrx as the ATI card is not yet supported by the proprietary AMD
driver, thus it does not seem to be related to the raeon driver.

The tearing and flickering start as soon as the kernel loads the i915 driver
during the boot sequence, before starting X.

It only affects the first third of the screen (height) an the first two thirds
of that section (width) And only happens with the black background, be it in
text or graphics mode. Setting the desktop background to white, only a small
set of pixels in the top line of the screen seem to flicker.

I've tried disabling the radeon module and the problem persists. Since the
problem seems to be data dependent (only with black background and different
degrees of tearing according to the amount of black on screen, and only in part
of the screen) I suspect it is related to frame buffer compression, but not
sure how to test it.

I've tried with a kernel from the latest drm-intel nightly, and in that one
both the i915 and radeon drivers work without issues.