From mboxrd@z Thu Jan 1 00:00:00 1970 From: Hans de Goede Subject: Re: [PATCH] drm/i915: intel_backlight scale() math WA v2 Date: Wed, 24 Sep 2014 18:06:05 +0200 Message-ID: <5422EBED.1000602@redhat.com> References: <1411574099-21977-1-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 mx1.redhat.com (mx1.redhat.com [209.132.183.28]) by gabe.freedesktop.org (Postfix) with ESMTP id A37F36E64D for ; Wed, 24 Sep 2014 09:06:13 -0700 (PDT) In-Reply-To: <1411574099-21977-1-git-send-email-joe.konno@linux.intel.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: intel-gfx-bounces@lists.freedesktop.org Sender: "Intel-gfx" To: Joe Konno , intel-gfx@lists.freedesktop.org List-Id: intel-gfx@lists.freedesktop.org Hi, On 09/24/2014 05:54 PM, Joe Konno wrote: > From: Joe Konno > > 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 > > v2: (thanks to Chris Wilson) clarify commit message, use rounded division > macro I wonder why do scaling at all, why not simply shift hw_min - hw_max range to 0 - (hw_max - hw_min) range and set max_brightness as seen by userspace to (hw_max - hw_min) ? Regards, Hans > > Signed-off-by: Joe Konno > --- > drivers/gpu/drm/i915/intel_panel.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c > index f17ada3..dcdfbb3 100644 > --- a/drivers/gpu/drm/i915/intel_panel.c > +++ b/drivers/gpu/drm/i915/intel_panel.c > @@ -421,7 +421,7 @@ 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 = DIV_ROUND_CLOSEST(target_val, source_max - source_min); > target_val += target_min; > > return target_val; >