Date: Mon, 12 Oct 2020 11:03:26 +0000 (UTC) From: Andriy Gapon <avg@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-stable@freebsd.org, svn-src-stable-12@freebsd.org Subject: svn commit: r366642 - stable/12/sys/arm/allwinner Message-ID: <202010121103.09CB3RsC084742@repo.freebsd.org>
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Author: avg Date: Mon Oct 12 11:03:26 2020 New Revision: 366642 URL: https://svnweb.freebsd.org/changeset/base/366642 Log: MFC r366142: aw_pwm: add a check and some comments related to long periods The hardware supports periods as long as 196 seconds[*] when using the maximal prescaling of 72000 and maximum cycle count of 2^16. But the code becomes incorrect when the period length approaches 1 second. That's because of things like NS_PER_SEC / period. [*] At the same time I must note that the KPI provides for maximum period of about 4 seconds (2^32 nanoseconds). Modified: stable/12/sys/arm/allwinner/aw_pwm.c Directory Properties: stable/12/ (props changed) Modified: stable/12/sys/arm/allwinner/aw_pwm.c ============================================================================== --- stable/12/sys/arm/allwinner/aw_pwm.c Mon Oct 12 11:01:54 2020 (r366641) +++ stable/12/sys/arm/allwinner/aw_pwm.c Mon Oct 12 11:03:26 2020 (r366642) @@ -259,6 +259,20 @@ aw_pwm_channel_config(device_t dev, u_int channel, u_i period_freq = NS_PER_SEC / period; if (period_freq > AW_PWM_MAX_FREQ) return (EINVAL); + + /* + * FIXME. The hardware is capable of sub-Hz frequencies, that is, + * periods longer than a second. But the current code cannot deal + * with those properly. + */ + if (period_freq == 0) + return (EINVAL); + + /* + * FIXME. There is a great loss of precision when the period and the + * duty are near 1 second. In some cases period_freq and duty_freq can + * be equal even if the period and the duty are significantly different. + */ duty_freq = NS_PER_SEC / duty; if (duty_freq < period_freq) { device_printf(sc->dev, "duty < period\n");
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