Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1752838AbdLULmq (ORCPT <rfc822;w@1wt.eu>);
        Thu, 21 Dec 2017 06:42:46 -0500
Received: from smtp-out6.electric.net ([192.162.217.185]:50644 "EHLO
        smtp-out6.electric.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1751867AbdLULmn (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Thu, 21 Dec 2017 06:42:43 -0500
From: David Laight <David.Laight@ACULAB.COM>
To: "'Crt Mori'" <cmo@melexis.com>
CC: Peter Zijlstra <peterz@infradead.org>,
        Jonathan Cameron <jic23@kernel.org>, Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        Rusty Russell <rusty@rustcorp.com.au>, Ian Abbott <abbotti@mev.co.uk>,
        Larry Finger <Larry.Finger@lwfinger.net>,
        Niklas Soderlund <niklas.soderlund+renesas@ragnatech.se>,
        Thomas Gleixner <tglx@linutronix.de>,
        Krzysztof Kozlowski <krzk@kernel.org>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-iio@vger.kernel.org" <linux-iio@vger.kernel.org>,
        Joe Perches <joe@perches.com>
Subject: RE: [PATCH v10 1/3] lib: Add strongly typed 64bit int_sqrt
Thread-Topic: [PATCH v10 1/3] lib: Add strongly typed 64bit int_sqrt
Thread-Index: AQHTeZ24vj4uJGdJVEmg6EfyY0tMJ6NMSiFAgAAfCS+AAAbEMIAADUYAgAEwoOA=
Date: Thu, 21 Dec 2017 11:43:00 +0000
Message-ID: <95b9b2b52554410a85a9f10c7f5e8b13@AcuMS.aculab.com>
References: <20171220142001.18161-1-cmo@melexis.com>
 <1c1d0ffa8ee140bf9adbc78f1559b1e8@AcuMS.aculab.com>
 <20171220160001.manjff26gfbjccsw@hirez.programming.kicks-ass.net>
 <CAKv63uuL9+xzF7KruhYwSY68-M0=aJSvJOr5Y0vVBiX8ebqfeg@mail.gmail.com>
 <c3462afd27d14c8684ee33ef6623a31a@AcuMS.aculab.com>
 <CAKv63utKNW2--ZnkoJ0z++68764cX_S9xS9Gst_vj7eDsFAZrg@mail.gmail.com>
In-Reply-To: <CAKv63utKNW2--ZnkoJ0z++68764cX_S9xS9Gst_vj7eDsFAZrg@mail.gmail.com>
Accept-Language: en-GB, en-US
Content-Language: en-US
X-MS-Has-Attach: 
X-MS-TNEF-Correlator: 
x-ms-exchange-transport-fromentityheader: Hosted
x-originating-ip: [10.202.205.33]
Content-Type: text/plain; charset="utf-8"
MIME-Version: 1.0
X-Outbound-IP: 156.67.243.126
X-Env-From: David.Laight@ACULAB.COM
X-Proto: esmtps
X-Revdns: 
X-HELO: AcuMS.aculab.com
X-TLS: TLSv1.2:ECDHE-RSA-AES256-SHA384:256
X-Authenticated_ID: 
X-PolicySMART: 3396946, 3397078
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Transfer-Encoding: 8bit
X-MIME-Autoconverted: from base64 to 8bit by mail.home.local id vBLBgoVO020983
Content-Length: 1266
Lines: 59

From: Crt Mori
> Sent: 20 December 2017 17:30
> I did a quick run through unit tests for the sensor and the results
> are way off
> ...

Try this version instead:
unsigned int sqrt64(unsigned long long x_in)
{
        unsigned int x = x_in >> 32;

        unsigned int b = 0;
        unsigned int y = 0;
        unsigned int i;

        i = 31;
        if (!x) {
                x = x_in;
                i = 15;
        }
        if (!(x & 0xffff0000)) {
                x <<= 16;
                i -= 8;
        }
        if (!(x & 0xff000000)) {
                x <<= 8;
                i -= 4;
        }
        if (!(x & 0xf0000000)) {
                x <<= 4;
                i -= 2;
        }

        do {
                b <<= 2;
                b |= x >> 30;
                x <<= 2;
                if (i == 16)
                        x = x_in;
                y <<= 1;
                if (b > y) {
                        b -= ++y;
                        y++;
                }
        } while (--i);

        /* 'b' becomes 33 bits if the input is greater than 2^62 */
        b <<= 1;
        b |= x >> 31;
        if (b > y || (b == y && x & (1u << 30)))
                y |= 1;

        return y;
}

I've tested that one with more values.

	David