Commits
U. Artie Eoff committed 673e7bbdb39
drm/i915: intel_backlight scale() math WA
Improper truncated integer division in the scale() function causes
actual_brightness != brightness. This (partial) work-around should be
sufficient for a majority of use-cases, but it is by no means a complete
solution.
TODO: Determine how best to scale "user" values to "hw" values, and
vice-versa, when the ranges are of different sizes. That would be a
buggy scenario even with this work-around.
The issue was introduced in the following (v3.17-rc1) commit:
6dda730 drm/i915: respect the VBT minimum backlight brightness
Note that for easier backporting this commit adds a duplicated macro.
A follow-up cleanup patch rectifies this for 3.18+
v2: (thanks to Chris Wilson) clarify commit message, use rounded division
macro
v3: -DIV_ROUND_CLOSEST() fails to build with CONFIG_X86_32=y. (Jani)
-Use DIV_ROUND_CLOSEST_ULL() instead. (Damien)
-v1 and v2 originally authored by Joe Konno.
Signed-off-by: U. Artie Eoff <ullysses.a.eoff@intel.com>
Cc: stable@vger.kernel.org
Reviewed-By: Joe Konno <joe.konno@intel.com>
[danvet: Add backporting note.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>