Date: Mon, 23 May 2016 11:37:51 -0600 From: Warner Losh <imp@bsdimp.com> To: Justin Hibbits <chmeeedalf@gmail.com> Cc: Ravi Pokala <rpokala@mac.com>, src-committers <src-committers@freebsd.org>, "svn-src-head@freebsd.org" <svn-src-head@freebsd.org>, Luiz Otavio O Souza <loos@freebsd.org>, "svn-src-all@freebsd.org" <svn-src-all@freebsd.org> Subject: Re: svn commit: r300421 - in head/sys: arm/allwinner arm/broadcom/bcm2835 arm/freescale/imx arm/mv dev/acpi_support dev/acpica dev/amdtemp dev/coretemp dev/iicbus powerpc/powermac Message-ID: <CANCZdfpsEpgb_B2mbiZqFGuwOb-rcDxf5KJLaEcavw7mD%2BHj0g@mail.gmail.com> In-Reply-To: <CAHSQbTAVt6QgVNz0M_yCVpTwSc=-YFU5O0uhaqKC6XOPOmJCkQ@mail.gmail.com> References: <201605221358.u4MDwW8u097510@repo.freebsd.org> <C66E915D-A4D7-4591-AF6C-0949AF184271@panasas.com> <CAHSQbTAVt6QgVNz0M_yCVpTwSc=-YFU5O0uhaqKC6XOPOmJCkQ@mail.gmail.com>
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On Sun, May 22, 2016 at 1:12 PM, Justin Hibbits <chmeeedalf@gmail.com> wrote: > > On May 22, 2016 13:51, "Ravi Pokala" <rpokala@mac.com> wrote: >> >> -----Original Message----- >> From: <owner-src-committers@freebsd.org> on behalf of Luiz Otavio O Souza >> <loos@FreeBSD.org> >> Date: 2016-05-22, Sunday at 06:58 >> To: <src-committers@freebsd.org>, <svn-src-all@freebsd.org>, >> <svn-src-head@freebsd.org> >> Subject: svn commit: r300421 - in head/sys: arm/allwinner >> arm/broadcom/bcm2835 arm/freescale/imx arm/mv dev/acpi_support dev/acpica >> dev/amdtemp dev/coretemp dev/iicbus powerpc/powermac >> >> >Author: loos >> >Date: Sun May 22 13:58:32 2016 >> >New Revision: 300421 >> >URL: https://svnweb.freebsd.org/changeset/base/300421 >> > >> >Log: >> > Fix the deciKelvin to Celsius conversion in kernel. >> > >> > After r285994, sysctl(8) was fixed to use 273.15 instead of 273.20 as >> > 0C >> > reference and as result, the temperature read in sysctl(8) now exibits >> > a >> > +0.1C difference. >> >> Out of morbid curiosity, why do these things report in deciKelvin anyway? >> Are there sensors we support out there which report native Kelvin, or that >> report sub-degree precision? >> >> Thanks, >> >> Ravi (rpokala@) >> > > There are many i2c sensors which report in sub-degree centigrade. Though > some of them are in 1/8 (or other power of 2) degree precision, not 1/10. ACPI reports in decikelvin, and it was the first thing we supported. Some 1 wire devices support 1/16th a degree, and we support it in millikelvin since 1/16th is 0.0625. So the precision needed is actually greater than what we're reporting. However, the accuracy of the measurement isn't +/- 0.001, it's closer to +/1 0.063 or so (well, that's a relative accuracy, absolute accuracy is closer to +/- 1 degree). We report the higher level of precision so that applications that use the relative temperature can use it to inform a control loop. We have no good way to report relative precision for a measurement, or the absolute accuracy. Warner
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