linux-kernel - [PATCH v2 3/4] backlight: pwm_bl: drop use of int

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Date:   Tue,  8 Oct 2019 14:03:26 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Thierry Reding <thierry.reding@...il.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>, Lee Jones <lee.jones@...aro.org>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Jingoo Han <jingoohan1@...il.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-pwm@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-fbdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH v2 3/4] backlight: pwm_bl: drop use of int_pow()

For a fixed small exponent of 3, it is more efficient to simply use
two explicit multiplications rather than calling the int_pow() library
function: Aside from the function call overhead, its implementation
using repeated squaring means it ends up doing four 64x64
multiplications.

Reviewed-by: Daniel Thompson <daniel.thompson@...aro.org>
Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
---
 drivers/video/backlight/pwm_bl.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
index 672c5e7cfcd1..273d3fb628a0 100644
--- a/drivers/video/backlight/pwm_bl.c
+++ b/drivers/video/backlight/pwm_bl.c
@@ -179,7 +179,8 @@ static u64 cie1931(unsigned int lightness, unsigned int scale)
 	if (lightness <= (8 * scale)) {
 		retval = DIV_ROUND_CLOSEST(lightness * 10, 9033);
 	} else {
-		retval = int_pow((lightness + (16 * scale)) / 116, 3);
+		retval = (lightness + (16 * scale)) / 116;
+		retval *= retval * retval;
 		retval = DIV_ROUND_CLOSEST_ULL(retval, (scale * scale));
 	}
 
-- 
2.20.1