linux-kernel - [PATCH RESEND 4/6] rtc-mc146818-lib: reduce RTC

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Date:   Sun, 17 Oct 2021 21:39:25 +0200
From:   Mateusz Jończyk <mat.jonczyk@...pl>
To:     linux-kernel@...r.kernel.org, linux-rtc@...r.kernel.org
Cc:     Mateusz Jończyk <mat.jonczyk@...pl>,
        Alessandro Zummo <a.zummo@...ertech.it>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH RESEND 4/6] rtc-mc146818-lib: reduce RTC_UIP polling period

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@...pl>
Cc: Alessandro Zummo <a.zummo@...ertech.it>
Cc: Alexandre Belloni <alexandre.belloni@...tlin.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: x86@...nel.org
Cc: "H. Peter Anvin" <hpa@...or.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