Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754943AbaFCMAg (ORCPT ); Tue, 3 Jun 2014 08:00:36 -0400 Received: from youngberry.canonical.com ([91.189.89.112]:43625 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932795AbaFCLly (ORCPT ); Tue, 3 Jun 2014 07:41:54 -0400 From: Luis Henriques To: linux-kernel@vger.kernel.org, stable@vger.kernel.org, kernel-team@lists.ubuntu.com Cc: Aaron Lu , Jani Nikula , Luis Henriques Subject: [PATCH 3.11 082/138] drm/i915: restore backlight precision when converting from ACPI Date: Tue, 3 Jun 2014 12:38:48 +0100 Message-Id: <1401795584-22664-83-git-send-email-luis.henriques@canonical.com> X-Mailer: git-send-email 1.9.1 In-Reply-To: <1401795584-22664-1-git-send-email-luis.henriques@canonical.com> References: <1401795584-22664-1-git-send-email-luis.henriques@canonical.com> X-Extended-Stable: 3.11 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 3.11.10.11 -stable review patch. If anyone has any objections, please let me know. ------------------ From: Aaron Lu commit 721e82c08c1afd6b47367b0e0c4a62140b0667f3 upstream. When we set backlight on behalf of ACPI opregion, we will convert the backlight value in the 0-255 range defined in opregion to the actual hardware level. Commit 22505b82a2 (drm/i915: avoid brightness overflow when doing scale) is meant to fix the overflow problem when doing the conversion, but it also caused a problem that the converted hardware level doesn't quite represent the intended value: say user wants maximum backlight level(255 in opregion's range), then we will calculate the actual hardware level to be: level = freq / max * level, where freq is the hardware's max backlight level(937 on an user's box), and max and level are all 255. The converted value should be 937 but the above calculation will yield 765. To fix this issue, just use 64 bits to do the calculation to keep the precision and avoid overflow at the same time. Buglink: https://bugzilla.kernel.org/show_bug.cgi?id=72491 Reported-by: Nico Schottelius Reviewed-by: Chris Wilson Signed-off-by: Aaron Lu Signed-off-by: Jani Nikula [ luis: backported to 3.11: adjusted context ] Signed-off-by: Luis Henriques --- drivers/gpu/drm/i915/intel_panel.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c index 5950888ae1d0..7a66423c93ab 100644 --- a/drivers/gpu/drm/i915/intel_panel.c +++ b/drivers/gpu/drm/i915/intel_panel.c @@ -488,6 +488,7 @@ void intel_panel_set_backlight(struct drm_device *dev, u32 level, u32 max) struct drm_i915_private *dev_priv = dev->dev_private; u32 freq; unsigned long flags; + u64 n; spin_lock_irqsave(&dev_priv->backlight.lock, flags); @@ -498,10 +499,9 @@ void intel_panel_set_backlight(struct drm_device *dev, u32 level, u32 max) } /* scale to hardware, but be careful to not overflow */ - if (freq < max) - level = level * freq / max; - else - level = freq / max * level; + n = (u64)level * freq; + do_div(n, max); + level = n; dev_priv->backlight.level = level; if (dev_priv->backlight.device) -- 1.9.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/