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authorMatt Mackall <mpm@selenic.com>2006-03-28 04:55:58 -0500
committerLinus Torvalds <torvalds@g5.osdl.org>2006-03-28 12:16:00 -0500
commit63732c2f37093d63102d53e70866cf87bf0c0479 (patch)
treebf364131da50132b6e27cfb028eee725fb4fe3e5 /include/asm-i386
parent329b10bb0feacb7fb9a41389313ff0a51ae56f2a (diff)
[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 <mpm@selenic.com> Cc: Andi Kleen <ak@muc.de> Cc: "David S. Miller" <davem@davemloft.net> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> Cc: Paul Mackerras <paulus@samba.org> Cc: Russell King <rmk@arm.linux.org.uk> Cc: Ralf Baechle <ralf@linux-mips.org> Cc: Paul Mundt <lethal@linux-sh.org> Cc: Kazumoto Kojima <kkojima@rr.iij4u.or.jp> Cc: Alessandro Zummo <a.zummo@towertech.it> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Diffstat (limited to 'include/asm-i386')
-rw-r--r--include/asm-i386/mach-default/mach_time.h16
1 files 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 b749aa44a86f..ff03cf0fdc3c 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)
82static inline unsigned long mach_get_cmos_time(void) 82static inline unsigned long mach_get_cmos_time(void)
83{ 83{
84 unsigned int year, mon, day, hour, min, sec; 84 unsigned int year, mon, day, hour, min, sec;
85 int i;
86 85
87 /* The Linux interpretation of the CMOS clock register contents: 86 do {
88 * When the Update-In-Progress (UIP) flag goes from 1 to 0, the
89 * RTC registers show the second which has precisely just started.
90 * Let's hope other operating systems interpret the RTC the same way.
91 */
92 /* read RTC exactly on falling edge of update flag */
93 for (i = 0 ; i < 1000000 ; i++) /* may take up to 1 second... */
94 if (CMOS_READ(RTC_FREQ_SELECT) & RTC_UIP)
95 break;
96 for (i = 0 ; i < 1000000 ; i++) /* must try at least 2.228 ms */
97 if (!(CMOS_READ(RTC_FREQ_SELECT) & RTC_UIP))
98 break;
99 do { /* Isn't this overkill ? UIP above should guarantee consistency */
100 sec = CMOS_READ(RTC_SECONDS); 87 sec = CMOS_READ(RTC_SECONDS);
101 min = CMOS_READ(RTC_MINUTES); 88 min = CMOS_READ(RTC_MINUTES);
102 hour = CMOS_READ(RTC_HOURS); 89 hour = CMOS_READ(RTC_HOURS);
@@ -104,6 +91,7 @@ static inline unsigned long mach_get_cmos_time(void)
104 mon = CMOS_READ(RTC_MONTH); 91 mon = CMOS_READ(RTC_MONTH);
105 year = CMOS_READ(RTC_YEAR); 92 year = CMOS_READ(RTC_YEAR);
106 } while (sec != CMOS_READ(RTC_SECONDS)); 93 } while (sec != CMOS_READ(RTC_SECONDS));
94
107 if (!(CMOS_READ(RTC_CONTROL) & RTC_DM_BINARY) || RTC_ALWAYS_BCD) 95 if (!(CMOS_READ(RTC_CONTROL) & RTC_DM_BINARY) || RTC_ALWAYS_BCD)
108 { 96 {
109 BCD_TO_BIN(sec); 97 BCD_TO_BIN(sec);