| From | Greg Kroah-Hartman <> | Subject | [PATCH 3.17 167/319] drm/i915: intel_backlight scale() math WA | Date | Wed, 12 Nov 2014 10:15:05 +0900 |
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3.17-stable review patch. If anyone has any objections, please let me know.
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From: "U. Artie Eoff" <ullysses.a.eoff@intel.com>
commit 673e7bbdb3920b62cfc6c710bea626b0a9b0f43a upstream.
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> Reviewed-By: Joe Konno <joe.konno@intel.com> [danvet: Add backporting note.] Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
--- drivers/gpu/drm/i915/intel_panel.c | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-)
--- a/drivers/gpu/drm/i915/intel_panel.c +++ b/drivers/gpu/drm/i915/intel_panel.c @@ -398,6 +398,9 @@ intel_panel_detect(struct drm_device *de } } +#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_va 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;
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