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Thu, 15 Jul 2021 09:48:35 +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> From: Peter Rosin Organization: Axentia Technologies AB Message-ID: <8417f698-eef2-3311-625a-1ceb17d3e5b2@axentia.se> Date: Thu, 15 Jul 2021 11:48:31 +0200 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 In-Reply-To: <20210715031215.1534938-6-liambeguin@gmail.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-ClientProxiedBy: HE1PR05CA0180.eurprd05.prod.outlook.com (2603:10a6:3:f8::28) 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 HE1PR05CA0180.eurprd05.prod.outlook.com (2603:10a6:3:f8::28) 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. 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... 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: >