From 63732c2f37093d63102d53e70866cf87bf0c0479 Mon Sep 17 00:00:00 2001 From: Matt Mackall Date: Tue, 28 Mar 2006 01:55:58 -0800 Subject: [PATCH] [PATCH] RTC: Remove RTC UIP synchronization on x86 Reading the CMOS clock on x86 and some other arches currently takes up to one second because it synchronizes with the CMOS second tick-over. This delay shows up at boot time as well a resume time. This is the currently the most substantial boot time delay for machines that are working towards instant-on capability. Also, a quick back of the envelope calculation (.5sec * 2M users * 1 boot a day * 10 years) suggests it has cost Linux users in the neighborhood of a million man-hours. An earlier thread on this topic is here: http://groups.google.com/group/linux.kernel/browse_frm/thread/8a24255215ff6151/2aa97e66a977653d?hl=en&lr=&ie=UTF-8&rnum=1&prev=/groups%3Fhl%3Den%26lr%3D%26ie%3DUTF-8%26selm%3D1To2R-2S7-11%40gated-at.bofh.it#2aa97e66a977653d ..from which the consensus seems to be that it's no longer desirable. In my view, there are basically four cases to consider: 1) networked, need precise walltime: use NTP 2) networked, don't need precise walltime: use NTP anyway 3) not networked, don't need sub-second precision walltime: don't care 4) not networked, need sub-second precision walltime: get a network or a radio time source because RTC isn't good enough anyway So this patch series simply removes the synchronization in favor of a simple seqlock-like approach using the seconds value. Note that for purposes of timer accuracy on wakeup, this patch will cause us to fire timers up to one second late. But as the current timer resume code will already sync once (or more!), it's no worse for short timers. Signed-off-by: Matt Mackall Cc: Andi Kleen Cc: "David S. Miller" Cc: Benjamin Herrenschmidt Cc: Paul Mackerras Cc: Russell King Cc: Ralf Baechle Cc: Paul Mundt Cc: Kazumoto Kojima Cc: Alessandro Zummo Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- include/asm-i386/mach-default/mach_time.h | 16 ++-------------- 1 file changed, 2 insertions(+), 14 deletions(-) diff --git a/include/asm-i386/mach-default/mach_time.h b/include/asm-i386/mach-default/mach_time.h index b749aa44a8..ff03cf0fdc 100644 --- a/include/asm-i386/mach-default/mach_time.h +++ b/include/asm-i386/mach-default/mach_time.h @@ -82,21 +82,8 @@ static inline int mach_set_rtc_mmss(unsigned long nowtime) static inline unsigned long mach_get_cmos_time(void) { unsigned int year, mon, day, hour, min, sec; - int i; - /* The Linux interpretation of the CMOS clock register contents: - * When the Update-In-Progress (UIP) flag goes from 1 to 0, the - * RTC registers show the second which has precisely just started. - * Let's hope other operating systems interpret the RTC the same way. - */ - /* read RTC exactly on falling edge of update flag */ - for (i = 0 ; i < 1000000 ; i++) /* may take up to 1 second... */ - if (CMOS_READ(RTC_FREQ_SELECT) & RTC_UIP) - break; - for (i = 0 ; i < 1000000 ; i++) /* must try at least 2.228 ms */ - if (!(CMOS_READ(RTC_FREQ_SELECT) & RTC_UIP)) - break; - do { /* Isn't this overkill ? UIP above should guarantee consistency */ + do { sec = CMOS_READ(RTC_SECONDS); min = CMOS_READ(RTC_MINUTES); hour = CMOS_READ(RTC_HOURS); @@ -104,6 +91,7 @@ static inline unsigned long mach_get_cmos_time(void) mon = CMOS_READ(RTC_MONTH); year = CMOS_READ(RTC_YEAR); } while (sec != CMOS_READ(RTC_SECONDS)); + if (!(CMOS_READ(RTC_CONTROL) & RTC_DM_BINARY) || RTC_ALWAYS_BCD) { BCD_TO_BIN(sec); -- 2.39.5