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

diff --git a/drivers/rtc/rtc-mc146818-lib.c b/drivers/rtc/rtc-mc146818-lib.c
index 522449b25921efd0ac23f6d3c6bac0173654d3ba..f1c09f1db044c8481fca7ddb8801c952954e246e 100644 (file)
--- a/drivers/rtc/rtc-mc146818-lib.c
+++ b/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;
                }