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

drivers/rtc/rtc-mc146818-lib.c

@@ -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;
 		}