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drm/i915: intel_backlight scale() math WA

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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>
This commit is contained in:
U. Artie Eoff 2014-09-29 15:49:32 -07:00 committed by Daniel Vetter
parent ebb69c9517
commit 673e7bbdb3
1 changed files with 5 additions and 3 deletions
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8
drivers/gpu/drm/i915/intel_panel.c
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@ -398,6 +398,9 @@ intel_panel_detect(struct drm_device *dev)
}
}
#define DIV_ROUND_CLOSEST_ULL(ll, d) \
({ unsigned long long _tmp = (ll)+(d)/2; do_div(_tmp, d); _tmp; })
/**
* scale - scale values from one range to another
*
@ -419,9 +422,8 @@ static uint32_t scale(uint32_t source_val,
source_val = clamp(source_val, source_min, source_max);
/* avoid overflows */
target_val = (uint64_t)(source_val - source_min) *
(target_max - target_min);
do_div(target_val, source_max - source_min);
target_val = DIV_ROUND_CLOSEST_ULL((uint64_t)(source_val - source_min) *
(target_max - target_min), source_max - source_min);
target_val += target_min;
return target_val;