From 673e7bbdb3920b62cfc6c710bea626b0a9b0f43a Mon Sep 17 00:00:00 2001 From: "U. Artie Eoff" Date: Mon, 29 Sep 2014 15:49:32 -0700 Subject: [PATCH] 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 Cc: stable@vger.kernel.org Reviewed-By: Joe Konno [danvet: Add backporting note.] Signed-off-by: Daniel Vetter --- Reading git-format-patch failed