I so totally suck.
This can cause a black screen if (for whatever reason) the bios
hasn't set this bit itself.
This regression has been introduced in
commit
7cf4160148136deb31ee5f2802857dd935a38529
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date: Tue Jun 5 10:07:09 2012 +0200
drm/i915: clear up backlight #define confusion on gen4+
Tested-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
/* PCH CTL1 is totally different, all but the below bits are reserved. CTL2 is
* like the normal CTL from gen4 and earlier. Hooray for confusing naming. */
#define BLC_PWM_PCH_CTL1 0xc8250
-#define BLM_PCH_PWM_ENABLE (1 << 30)
+#define BLM_PCH_PWM_ENABLE (1 << 31)
#define BLM_PCH_OVERRIDE_ENABLE (1 << 30)
#define BLM_PCH_POLARITY (1 << 29)
#define BLC_PWM_PCH_CTL2 0xc8254