Date: Mon, 11 Oct 2021 09:53:31 +0200 From: Sebastian Huber <sebastian.huber@embedded-brains.de> To: FreeBSD Hackers <freebsd-hackers@freebsd.org> Subject: Large timecounter delta handling Message-ID: <5318327d-d247-bb73-81d9-967c4ae18d32@embedded-brains.de>
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Hello, I synchronize currently the port of the FreeBSD timecounters to RTEMS. I have to write test cases for all code we use in RTEMS. In 2020 some code was added to fix integer overflow issues with large time deltas while getting the time from a timehand. https://github.com/freebsd/freebsd-src/commit/6cf2362e2c7e9061611f93a48ec654a5b7451d6b#diff-8b8e2f8e41e6a847f14ab08c7d50454c20a4a135f2c2241d91687c0832c1d99e If a time delta obtained by tc_delta(th) is greater than or equal to th->th_large_delta, then some extra calculation is carried out. The th->th_large_delta is computed like this scale = (uint64_t)1 << 63; scale += (th->th_adjustment / 1024) * 2199; scale /= th->th_counter->tc_frequency; th->th_scale = scale * 2; th->th_large_delta = MIN(((uint64_t)1 << 63) / scale, UINT_MAX); If we ignore the th->th_adjustment (== 0), then we have ideally scale = 2**64 / f th->th_large_delta = MIN( f / 2, UINT_MAX ) Does this mean that we only need the large delta processing if a timehand was not updated for about 0.5s? -- embedded brains GmbH Herr Sebastian HUBER Dornierstr. 4 82178 Puchheim Germany email: sebastian.huber@embedded-brains.de phone: +49-89-18 94 741 - 16 fax: +49-89-18 94 741 - 08 Registergericht: Amtsgericht München Registernummer: HRB 157899 Vertretungsberechtigte Geschäftsführer: Peter Rasmussen, Thomas Dörfler Unsere Datenschutzerklärung finden Sie hier: https://embedded-brains.de/datenschutzerklaerung/
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