From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Eoff, Ullysses A" Subject: Re: [PATCH] drm/i915: intel_backlight scale() math WA Date: Tue, 23 Sep 2014 23:40:37 +0000 Message-ID: <1411515637.5214.37.camel@localhost> References: <1411512625-10657-1-git-send-email-joe.konno@linux.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Received: from mga11.intel.com (mga11.intel.com [192.55.52.93]) by gabe.freedesktop.org (Postfix) with ESMTP id C94C66E01C for ; Tue, 23 Sep 2014 16:40:38 -0700 (PDT) In-Reply-To: <1411512625-10657-1-git-send-email-joe.konno@linux.intel.com> Content-Language: en-US Content-ID: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: intel-gfx-bounces@lists.freedesktop.org Sender: "Intel-gfx" To: "joe.konno@linux.intel.com" Cc: "intel-gfx@lists.freedesktop.org" List-Id: intel-gfx@lists.freedesktop.org On Tue, 2014-09-23 at 15:50 -0700, Joe Konno wrote: > From: Joe Konno > > Improper integer division-- truncated rather than rounded-- 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 > Reviewed-by: U. Artie Eoff > Signed-off-by: Joe Konno > --- > drivers/gpu/drm/i915/intel_panel.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c > index f17ada3..c40f837 100644 > --- a/drivers/gpu/drm/i915/intel_panel.c > +++ b/drivers/gpu/drm/i915/intel_panel.c > @@ -421,7 +421,8 @@ static uint32_t scale(uint32_t source_val, > /* avoid overflows */ > target_val = (uint64_t)(source_val - source_min) * > (target_max - target_min); > - do_div(target_val, source_max - source_min); > + target_val = (target_val + ((source_max - source_min) / 2)) / > + (source_max - source_min); > target_val += target_min; > > return target_val;