From patchwork Thu Sep 19 14:06:19 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Rasmus Villemoes X-Patchwork-Id: 11152539 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 0DB0116B1 for ; Thu, 19 Sep 2019 14:06:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E07E020882 for ; Thu, 19 Sep 2019 14:06:38 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=rasmusvillemoes.dk header.i=@rasmusvillemoes.dk header.b="MZKdSEpO" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2389501AbfISOGf (ORCPT ); Thu, 19 Sep 2019 10:06:35 -0400 Received: from mail-lj1-f194.google.com ([209.85.208.194]:38025 "EHLO mail-lj1-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2389448AbfISOGe (ORCPT ); Thu, 19 Sep 2019 10:06:34 -0400 Received: by mail-lj1-f194.google.com with SMTP id y23so3741712ljn.5 for ; Thu, 19 Sep 2019 07:06:33 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=rasmusvillemoes.dk; s=google; h=from:to:cc:subject:date:message-id:in-reply-to:references :mime-version:content-transfer-encoding; bh=Jg7JcvREMiiX5x2HEJEjAcDv3XByVb0RNmStAp9nahQ=; b=MZKdSEpOEV+5iG16oW7TXFRd1eg5H9XeUG6b1t8t2jgOvEG6oGHacyhtyg3NPIEGY/ uKiTYPItaGHpX/90K1aP/bPkpA8b+8k/Rldl4zxvx2B9det3NTU55vbdyWW1nSSp9eg3 YRFw3EAaRrmgYhPjv9gwnV6JQyEwF2erlvjsw= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:cc:subject:date:message-id:in-reply-to :references:mime-version:content-transfer-encoding; bh=Jg7JcvREMiiX5x2HEJEjAcDv3XByVb0RNmStAp9nahQ=; b=YCv6z0+/LfYMKPJEs9smrUsnCLnKwpssoSLLJT9S3wX0+oG0JcTpR1Ji7Bng3Rsi0n gB3wdOcJxcT08ffhzACliE6vcTF1eJSLZzGg5HYh5crB+Y06b+8+05i2x8rBqWiOUujH dCZIyjVObm9tyg24zw1Syx7zNV/wgwMQ9XwkAs/SOCzV0DrQpVBZ3K2/ba66tSIOBXLR ZsiPTwCKLgeU8heHx3JKCI2WsxqrLGr0+fux/sJzkBfZ+McDacWAuqtreF1CkrhOavKY 87+KHIl9C0R7UyJcfUBVxHLhYST7TPGwZ42A2M7LsenZeZGBdLOi+8uapvJfa7P4lEUB 1wqA== X-Gm-Message-State: APjAAAVJ+FkjehRptVEegbAtmyalfOPrDX4wh2xEEwDjMb3UJh4aW6oR HLvgtaaKE/UoPN2sKpNvRE/wYQ== X-Google-Smtp-Source: APXvYqwT7aVA83Vb4qPr4tBmMd1vi6mmQP81J4Kl4kTliuCxLy+GI3jRKm2Szplr4kWj55N1eonI9A== X-Received: by 2002:a05:651c:c9:: with SMTP id 9mr107103ljr.29.1568901992739; Thu, 19 Sep 2019 07:06:32 -0700 (PDT) Received: from prevas-ravi.prevas.se ([81.216.59.226]) by smtp.gmail.com with ESMTPSA id h3sm1687886ljf.12.2019.09.19.07.06.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 19 Sep 2019 07:06:32 -0700 (PDT) From: Rasmus Villemoes To: Thierry Reding , Lee Jones , Daniel Thompson , Jingoo Han , Bartlomiej Zolnierkiewicz Cc: Rasmus Villemoes , linux-pwm@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-fbdev@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH 4/5] backlight: pwm_bl: switch to power-of-2 base for fixed-point math Date: Thu, 19 Sep 2019 16:06:19 +0200 Message-Id: <20190919140620.32407-4-linux@rasmusvillemoes.dk> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20190919140620.32407-1-linux@rasmusvillemoes.dk> References: <20190919140620.32407-1-linux@rasmusvillemoes.dk> MIME-Version: 1.0 Sender: linux-fbdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fbdev@vger.kernel.org Using a power-of-2 instead of power-of-10 base makes the computations much cheaper. 2^16 is safe; retval never becomes more than 2^48 + 2^16/2. On a 32 bit platform, the very expensive 64/32 division at the end of cie1931() instead becomes essentially free (a shift by 32 is just a register rename). Signed-off-by: Rasmus Villemoes Reviewed-by: Daniel Thompson --- drivers/video/backlight/pwm_bl.c | 22 ++++++++++++---------- 1 file changed, 12 insertions(+), 10 deletions(-) diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c index aee6839e024a..102bc191310f 100644 --- a/drivers/video/backlight/pwm_bl.c +++ b/drivers/video/backlight/pwm_bl.c @@ -148,7 +148,8 @@ static const struct backlight_ops pwm_backlight_ops = { }; #ifdef CONFIG_OF -#define PWM_LUMINANCE_SCALE 10000 /* luminance scale */ +#define PWM_LUMINANCE_SHIFT 16 +#define PWM_LUMINANCE_SCALE (1 << PWM_LUMINANCE_SHIFT) /* luminance scale */ /* * CIE lightness to PWM conversion. @@ -165,23 +166,25 @@ static const struct backlight_ops pwm_backlight_ops = { * The following function does the fixed point maths needed to implement the * above formula. */ -static u64 cie1931(unsigned int lightness, unsigned int scale) +static u64 cie1931(unsigned int lightness) { u64 retval; /* * @lightness is given as a number between 0 and 1, expressed - * as a fixed-point number in scale @scale. Convert to a - * percentage, still expressed as a fixed-point number, so the - * above formulas can be applied. + * as a fixed-point number in scale + * PWM_LUMINANCE_SCALE. Convert to a percentage, still + * expressed as a fixed-point number, so the above formulas + * can be applied. */ lightness *= 100; - if (lightness <= (8 * scale)) { + if (lightness <= (8 * PWM_LUMINANCE_SCALE)) { retval = DIV_ROUND_CLOSEST(lightness * 10, 9033); } else { - retval = (lightness + (16 * scale)) / 116; + retval = (lightness + (16 * PWM_LUMINANCE_SCALE)) / 116; retval *= retval * retval; - retval = DIV_ROUND_CLOSEST_ULL(retval, (scale * scale)); + retval += PWM_LUMINANCE_SCALE/2; + retval >>= 2*PWM_LUMINANCE_SHIFT; } return retval; @@ -215,8 +218,7 @@ int pwm_backlight_brightness_default(struct device *dev, /* Fill the table using the cie1931 algorithm */ for (i = 0; i < data->max_brightness; i++) { retval = cie1931((i * PWM_LUMINANCE_SCALE) / - data->max_brightness, PWM_LUMINANCE_SCALE) * - period; + data->max_brightness) * period; retval = DIV_ROUND_CLOSEST_ULL(retval, PWM_LUMINANCE_SCALE); if (retval > UINT_MAX) return -EINVAL;