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rtc: mc146818-lib: reduce RTC_UIP polling period 

Waiting 1ms every time is not necessary, for example on some AMD boxes
the RTC_UIP bit is documented as being high for around 270 microseconds
in some cases [1], which agreed with experiments on an SB710
southbridge. So 100us seems optimal.

This in preparation for mach_get_cmos_time() refactoring.
The functions mc146818_get_time() and mach_get_cmos_time() in
arch/x86/kernel/rtc.c perform the same function and the code is
duplicated. mach_get_cmos_time() is busy waiting for the RTC_UIP
bit to clear, so make mc146818_get_time() more similar to it by reducing
the polling period.

[1] AMD SB700/710/750 Register Reference Guide, page 307,
https://developer.amd.com/wordpress/media/2012/10/43009_sb7xx_rrg_pub_1.00.pdf

        "SB700 A12: The UIP high pulse is 270 μS Typical when SS on SRC
        clock is OFF and 100μ min when SRC SS is ON." [sic]

Signed-off-by: Mateusz Jończyk <mat.jonczyk@o2.pl>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: x86@kernel.org
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220225215011.861477-2-mat.jonczyk@o2.pl
This commit is contained in:
Mateusz Jończyk 2022-02-25 22:50:09 +01:00 committed by Alexandre Belloni
parent f2906aa863
commit d2a632a8a1
1 changed files with 4 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

8
drivers/rtc/rtc-mc146818-lib.c
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@ -21,13 +21,13 @@ bool mc146818_avoid_UIP(void (*callback)(unsigned char seconds, void *param),
unsigned long flags;
unsigned char seconds;
for (i = 0; i < 10; i++) {
for (i = 0; i < 100; i++) {
spin_lock_irqsave(&rtc_lock, flags);
/*
* Check whether there is an update in progress during which the
* readout is unspecified. The maximum update time is ~2ms. Poll
* every msec for completion.
* every 100 usec for completion.
*
* Store the second value before checking UIP so a long lasting
* NMI which happens to hit after the UIP check cannot make
@ -37,7 +37,7 @@ bool mc146818_avoid_UIP(void (*callback)(unsigned char seconds, void *param),
if (CMOS_READ(RTC_FREQ_SELECT) & RTC_UIP) {
spin_unlock_irqrestore(&rtc_lock, flags);
mdelay(1);
udelay(100);
continue;
}
@ -56,7 +56,7 @@ bool mc146818_avoid_UIP(void (*callback)(unsigned char seconds, void *param),
*/
if (CMOS_READ(RTC_FREQ_SELECT) & RTC_UIP) {
spin_unlock_irqrestore(&rtc_lock, flags);
mdelay(1);
udelay(100);
continue;
}