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: 6dda730e 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: NU. Artie Eoff <ullysses.a.eoff@intel.com> Cc: stable@vger.kernel.org Reviewed-By: NJoe Konno <joe.konno@intel.com> [danvet: Add backporting note.] Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
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