[Bug 61457] [Intel G45] Screen turns blank when terminating a low res X server on a different low res KMS mode.

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=61457

--- Comment #10 from Egbert Eich <eich at pdx.freedesktop.org> ---
I've played around a bit more - my original workaround seems to work without
the not-understood mdelay(9) if I do a:

u32 fw_blc_self = I915_READ(FW_BLC_SELF);
I915_WRITE(FW_BLC_SELF, fw_blc_self & ~FW_BLC_SELF_EN);

before I enable the cursor plane and restore the previous value after I disable
it again.
If the cursor plane is off already or (fw_blc_self & FW_BLC_SELF_EN) == 0 I
don't have to do anything.

Of course this is only to the extent I've tested using the described test case
and
and a limited number of test cycles - since this involves visual inspection if
the screen lights up it's hard to automate.

I was a bit surprised that it wasn't sufficient to unset the FW_BLC_SELF_EN bit
before enabling the display plane and restore the old value after it's enabled
but that also the cursor had to be enabled when enabling the display plane.

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