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Sat, 17 Jul 2021 08:11:25 +0000 Subject: Re: [PATCH v5 05/10] iio: afe: rescale: add INT_PLUS_{MICRO,NANO} support To: Liam Beguin , jic23@kernel.org, lars@metafoo.de, pmeerw@pmeerw.net Cc: linux-kernel@vger.kernel.org, linux-iio@vger.kernel.org, devicetree@vger.kernel.org, robh+dt@kernel.org References: <20210715031215.1534938-1-liambeguin@gmail.com> <20210715031215.1534938-6-liambeguin@gmail.com> <8417f698-eef2-3311-625a-1ceb17d3e5b2@axentia.se> From: Peter Rosin Organization: Axentia Technologies AB Message-ID: Date: Sat, 17 Jul 2021 10:11:22 +0200 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: sv-SE Content-Transfer-Encoding: 7bit X-ClientProxiedBy: HE1P189CA0002.EURP189.PROD.OUTLOOK.COM (2603:10a6:7:53::15) To DB8PR02MB5482.eurprd02.prod.outlook.com (2603:10a6:10:eb::29) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [192.168.13.3] (85.229.94.233) by HE1P189CA0002.EURP189.PROD.OUTLOOK.COM (2603:10a6:7:53::15) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4331.21 via Frontend Transport; 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>>> *val = tmp; >>> return ret; >>> + case IIO_VAL_INT_PLUS_NANO: >>> + tmp = ((s64)*val * 1000000000LL + *val2) * rescale->numerator; >>> + do_div(tmp, rescale->denominator); >>> + >>> + *val = div_s64(tmp, 1000000000LL); >>> + *val2 = tmp - *val * 1000000000LL; >>> + return ret; >> >> This is too simplistic and prone to overflow. We need something like >> this >> (untested) >> >> tmp = (s64)*val * rescale->numerator; >> rem = do_div(tmp, rescale->denominator); >> *val = tmp; >> tmp = ((s64)rem * 1000000000LL + (s64)*val2) * rescale->numerator; >> do_div(tmp, rescale->denominator); >> *val2 = tmp; >> >> Still not very safe with numerator and denominator both "large", but >> much >> better. And then we need normalizing the fraction part after the above, >> of >> course. >> > > Understood, I'll test that. I made a thinko. The remainder should not be re-multiplied with the numerator... tmp = (s64)*val * rescale->numerator; rem = do_div(tmp, rescale->denominator); *val = tmp; tmp = (s64)rem * 1000000000LL + (s64)*val2 * rescale->numerator; do_div(tmp, rescale->denominator); *val2 = tmp; And that actually reduces the risk of overflow too, which is nice! Cheers, Peter >> And, of course, I'm not sure what *val == -1 and *val2 == 500000000 >> really >> means. Is that -1.5 or -0.5? The above may very well need adjusting for >> negative values... >> > > I would've assumed the correct answer is -1 + 500000000e-9 = -0.5 > but adding a test case to iio-test-format.c seems to return -1.5... > > I believe that's a bug but we can work around if for now by moving the > integer part of *val2 to *val. > > Liam > >> Cheers, >> Peter >> >>> + case IIO_VAL_INT_PLUS_MICRO: >>> + tmp = ((s64)*val * 1000000LL + *val2) * rescale->numerator; >>> + do_div(tmp, rescale->denominator); >>> + >>> + *val = div_s64(tmp, 1000000LL); >>> + *val2 = tmp - *val * 1000000LL; >>> + return ret; >>> default: >>> + dev_err(&indio_dev->dev, "unsupported type %d\n", ret); >>> return -EOPNOTSUPP; >>> } >>> default: >>> >