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Mon, 19 Jul 2021 08:31:05 +0000 Subject: Re: [PATCH v5 05/10] iio: afe: rescale: add INT_PLUS_{MICRO,NANO} support To: Liam Beguin , Jonathan Cameron Cc: lars@metafoo.de, pmeerw@pmeerw.net, 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> <20210717175551.20265ac4@jic23-huawei> From: Peter Rosin Organization: Axentia Technologies AB Message-ID: Date: Mon, 19 Jul 2021 10:31:03 +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: HE1PR0901CA0044.eurprd09.prod.outlook.com (2603:10a6:3:45::12) 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 HE1PR0901CA0044.eurprd09.prod.outlook.com (2603:10a6:3:45::12) 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. >>> >>>> 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... >> > > Hi Jonathan, > >> No. -1.5 is as intended, though the IIO_VAL_PLUS_MICRO is rather >> confusing >> naming :( We should perhaps add more documentation for that. Signs were >> always a bit of a pain with this two integer scheme for fixed point. >> >> The intent is to have moderately readable look up tables with the >> problem that >> we don't have a signed 0 available. Meh, maybe this decision a long time >> back wasn't a the right one, but it may be a pain to change now as too >> many >> drivers to check! >> >> 1, 0000000 == 1 >> 0, 5000000 == 0.5 >> 0, 0000000 == 0 >> 0, -5000000 == -0.5 >> -1, 5000000 == -1.5 >> > > Understood, thanks for clearing that out. I just realized that do_div assumes unsigned operands... :-( Cheers, Peter