[PATCH RESEND 4/6] rtc-mc146818-lib: reduce RTC_UIP polling period

Mateusz Jończyk
2021-10-17T19:40:02Z

A shorter period makes mc146818_get_time() more similar to
mach_get_cmos_time() in arch/x86/kernel/rtc.c, which performs the same
function, but is busy waiting for the RTC_UIP bit to clear.

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.

[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: Alessandro Zummo <a.zummo@towertech.it>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: x86@kernel.org
Cc: "H. Peter Anvin" <hpa@zytor.com>
---
drivers/rtc/rtc-mc146818-lib.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/rtc/rtc-mc146818-lib.c b/drivers/rtc/rtc-mc146818-lib.c
index 9175e11baf26..d1c42b0ef662 100644
--- a/drivers/rtc/rtc-mc146818-lib.c
+++ b/drivers/rtc/rtc-mc146818-lib.c
@@ -45,7 +45,7 @@ unsigned int mc146818_get_time(struct rtc_time *time)
#endif

again:
- if (iter_count > 10) {
+ if (iter_count > 100) {
pr_err_ratelimited("Unable to read current time from RTC\n");
memset(time, 0xff, sizeof(*time));
return 0;
@@ -57,7 +57,7 @@ unsigned int mc146818_get_time(struct rtc_time *time)
/*
* 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 an update
@@ -67,7 +67,7 @@ unsigned int mc146818_get_time(struct rtc_time *time)

if (CMOS_READ(RTC_FREQ_SELECT) & RTC_UIP) {
spin_unlock_irqrestore(&rtc_lock, flags);
- mdelay(1);
+ udelay(100);
goto again;
}

--
2.25.1