[PATCH 0/4] mips: CMOS RTC refactoring

[PATCH 0/4] mips: CMOS RTC refactoring

[Mateusz Jończyk]

Hello,

A while ago I wrote some patches that refactor a bit the RTC CMOS support on the
MIPS architecture. These were in response to a bug report about locking
problems during suspend / resume:

Link: https://lore.kernel.org/all/CAP-bSRZ0CWyZZsMtx046YV8L28LhY0fson2g4EqcwRAVN1Jk+Q@mail.gmail.com/

Back then, I was trying to convert the rtc_lock to a raw spinlock. This proved
to be unnecessary and harmful, but in the process I removed some dead and
duplicated code that touches the MC146818 (the chip of the CMOS RTC) for
the MIPS architecture.

These patches were tested only in an emulator, as I don't have the hardware.
They are roughly in the order of increasing risk.

Greetings,
Mateusz

Mateusz Jończyk (4):
  mips: remove unused function mc146818_set_rtc_mmss
  mips/mach-rm: remove custom mc146818rtc.h file
  mips: remove redundant macro mc146818_decode_year
  mips/malta,loongson2ef: use generic mc146818_get_time function

 arch/mips/Kconfig                             |   2 +
 .../include/asm/mach-generic/mc146818rtc.h    |   4 -
 arch/mips/include/asm/mach-jazz/mc146818rtc.h |   2 -
 .../mips/include/asm/mach-malta/mc146818rtc.h |   2 -
 arch/mips/include/asm/mach-rm/mc146818rtc.h   |  21 ----
 arch/mips/include/asm/mc146818-time.h         | 105 ++----------------
 6 files changed, 9 insertions(+), 127 deletions(-)
 delete mode 100644 arch/mips/include/asm/mach-rm/mc146818rtc.h


base-commit: d7b8f8e20813f0179d8ef519541a3527e7661d3a
-- 
2.25.1

[PATCH 1/4] mips: remove unused function mc146818_set_rtc_mmss

[Mateusz Jończyk]

I have checked carefully: this function is unused, so remove it.

The last caller appears to have been removed in 2007 in
commit 4b550488f894 ("[MIPS] Deforest the function pointer jungle in the time code.")

mc146818-time.h is included only in three files:
- arch/mips/mti-malta/malta-time.c
- arch/mips/loongson64/numa.c
- arch/mips/loongson2ef/common/time.c

Also, remove unused macros USEC_AFTER/USEC_BEFORE.

Signed-off-by: Mateusz Jończyk <mat.jonczyk@o2.pl>
---
 arch/mips/include/asm/mc146818-time.h | 74 ---------------------------
 1 file changed, 74 deletions(-)

diff --git a/arch/mips/include/asm/mc146818-time.h b/arch/mips/include/asm/mc146818-time.h
index cbf5cec345f1..07bf30bee792 100644
--- a/arch/mips/include/asm/mc146818-time.h
+++ b/arch/mips/include/asm/mc146818-time.h
@@ -12,80 +12,6 @@
 #include <linux/mc146818rtc.h>
 #include <linux/time.h>
 
-/*
- * For check timing call set_rtc_mmss() 500ms; used in timer interrupt.
- */
-#define USEC_AFTER	500000
-#define USEC_BEFORE	500000
-
-/*
- * In order to set the CMOS clock precisely, set_rtc_mmss has to be
- * called 500 ms after the second nowtime has started, because when
- * nowtime is written into the registers of the CMOS clock, it will
- * jump to the next second precisely 500 ms later. Check the Motorola
- * MC146818A or Dallas DS12887 data sheet for details.
- *
- * BUG: This routine does not handle hour overflow properly; it just
- *	sets the minutes. Usually you'll only notice that after reboot!
- */
-static inline int mc146818_set_rtc_mmss(unsigned long nowtime)
-{
-	int real_seconds, real_minutes, cmos_minutes;
-	unsigned char save_control, save_freq_select;
-	int retval = 0;
-	unsigned long flags;
-
-	spin_lock_irqsave(&rtc_lock, flags);
-	save_control = CMOS_READ(RTC_CONTROL); /* tell the clock it's being set */
-	CMOS_WRITE((save_control|RTC_SET), RTC_CONTROL);
-
-	save_freq_select = CMOS_READ(RTC_FREQ_SELECT); /* stop and reset prescaler */
-	CMOS_WRITE((save_freq_select|RTC_DIV_RESET2), RTC_FREQ_SELECT);
-
-	cmos_minutes = CMOS_READ(RTC_MINUTES);
-	if (!(save_control & RTC_DM_BINARY) || RTC_ALWAYS_BCD)
-		cmos_minutes = bcd2bin(cmos_minutes);
-
-	/*
-	 * since we're only adjusting minutes and seconds,
-	 * don't interfere with hour overflow. This avoids
-	 * messing with unknown time zones but requires your
-	 * RTC not to be off by more than 15 minutes
-	 */
-	real_seconds = nowtime % 60;
-	real_minutes = nowtime / 60;
-	if (((abs(real_minutes - cmos_minutes) + 15)/30) & 1)
-		real_minutes += 30;		/* correct for half hour time zone */
-	real_minutes %= 60;
-
-	if (abs(real_minutes - cmos_minutes) < 30) {
-		if (!(save_control & RTC_DM_BINARY) || RTC_ALWAYS_BCD) {
-			real_seconds = bin2bcd(real_seconds);
-			real_minutes = bin2bcd(real_minutes);
-		}
-		CMOS_WRITE(real_seconds, RTC_SECONDS);
-		CMOS_WRITE(real_minutes, RTC_MINUTES);
-	} else {
-		printk_once(KERN_NOTICE
-		       "set_rtc_mmss: can't update from %d to %d\n",
-		       cmos_minutes, real_minutes);
-		retval = -1;
-	}
-
-	/* The following flags have to be released exactly in this order,
-	 * otherwise the DS12887 (popular MC146818A clone with integrated
-	 * battery and quartz) will not reset the oscillator and will not
-	 * update precisely 500 ms later. You won't find this mentioned in
-	 * the Dallas Semiconductor data sheets, but who believes data
-	 * sheets anyway ...			       -- Markus Kuhn
-	 */
-	CMOS_WRITE(save_control, RTC_CONTROL);
-	CMOS_WRITE(save_freq_select, RTC_FREQ_SELECT);
-	spin_unlock_irqrestore(&rtc_lock, flags);
-
-	return retval;
-}
-
 static inline time64_t mc146818_get_cmos_time(void)
 {
 	unsigned int year, mon, day, hour, min, sec;
-- 
2.25.1

[PATCH 2/4] mips/mach-rm: remove custom mc146818rtc.h file

[Mateusz Jończyk]

The mc146818_decode_year macro is used only in mc146818_get_cmos_time,
which in turn is called only in
	arch/mips/loongson2ef/common/time.c
and
	arch/mips/mti-malta/malta-time.c

So no SNI or mach-rm content. SNI did not use mc146818_get_cmos_time
apparently since
commit 06cf5583fd9a ("[MIPS] SNI RM updates")
in 2007.

Signed-off-by: Mateusz Jończyk <mat.jonczyk@o2.pl>
---
 arch/mips/include/asm/mach-rm/mc146818rtc.h | 21 ---------------------
 1 file changed, 21 deletions(-)
 delete mode 100644 arch/mips/include/asm/mach-rm/mc146818rtc.h

diff --git a/arch/mips/include/asm/mach-rm/mc146818rtc.h b/arch/mips/include/asm/mach-rm/mc146818rtc.h
deleted file mode 100644
index a074f4f84f75..000000000000
--- a/arch/mips/include/asm/mach-rm/mc146818rtc.h
+++ /dev/null
@@ -1,21 +0,0 @@
-/*
- * This file is subject to the terms and conditions of the GNU General Public
- * License.  See the file "COPYING" in the main directory of this archive
- * for more details.
- *
- * Copyright (C) 2004 by Ralf Baechle
- *
- * RTC routines for PC style attached Dallas chip with ARC epoch.
- */
-#ifndef __ASM_MACH_RM_MC146818RTC_H
-#define __ASM_MACH_RM_MC146818RTC_H
-
-#ifdef CONFIG_CPU_BIG_ENDIAN
-#define mc146818_decode_year(year) ((year) < 70 ? (year) + 2000 : (year) + 1900)
-#else
-#define mc146818_decode_year(year) ((year) + 1980)
-#endif
-
-#include <asm/mach-generic/mc146818rtc.h>
-
-#endif /* __ASM_MACH_RM_MC146818RTC_H */
-- 
2.25.1

[PATCH 3/4] mips: remove redundant macro mc146818_decode_year

[Mateusz Jończyk]

The mc146818_decode_year macro is used only in mc146818_get_cmos_time(),
which in turn is called only in
	arch/mips/loongson2ef/common/time.c
and
	arch/mips/mti-malta/malta-time.c
so on mach-jazz it is unused and can be removed. On other platforms it
is defined in the same way, so it can be safely folded into
mc146818_get_cmos_time().

Signed-off-by: Mateusz Jończyk <mat.jonczyk@o2.pl>
---
 arch/mips/include/asm/mach-generic/mc146818rtc.h | 4 ----
 arch/mips/include/asm/mach-jazz/mc146818rtc.h    | 2 --
 arch/mips/include/asm/mach-malta/mc146818rtc.h   | 2 --
 arch/mips/include/asm/mc146818-time.h            | 5 ++++-
 4 files changed, 4 insertions(+), 9 deletions(-)

diff --git a/arch/mips/include/asm/mach-generic/mc146818rtc.h b/arch/mips/include/asm/mach-generic/mc146818rtc.h
index 9c72e540ff56..249279b0494d 100644
--- a/arch/mips/include/asm/mach-generic/mc146818rtc.h
+++ b/arch/mips/include/asm/mach-generic/mc146818rtc.h
@@ -29,8 +29,4 @@ static inline void CMOS_WRITE(unsigned char data, unsigned long addr)
 
 #define RTC_ALWAYS_BCD	0
 
-#ifndef mc146818_decode_year
-#define mc146818_decode_year(year) ((year) < 70 ? (year) + 2000 : (year) + 1900)
-#endif
-
 #endif /* __ASM_MACH_GENERIC_MC146818RTC_H */
diff --git a/arch/mips/include/asm/mach-jazz/mc146818rtc.h b/arch/mips/include/asm/mach-jazz/mc146818rtc.h
index 987f727afe25..639bff8ebca3 100644
--- a/arch/mips/include/asm/mach-jazz/mc146818rtc.h
+++ b/arch/mips/include/asm/mach-jazz/mc146818rtc.h
@@ -33,6 +33,4 @@ static inline void CMOS_WRITE(unsigned char data, unsigned long addr)
 
 #define RTC_ALWAYS_BCD	0
 
-#define mc146818_decode_year(year) ((year) + 1980)
-
 #endif /* __ASM_MACH_JAZZ_MC146818RTC_H */
diff --git a/arch/mips/include/asm/mach-malta/mc146818rtc.h b/arch/mips/include/asm/mach-malta/mc146818rtc.h
index e8cc7fdf7415..7da2c0ea55da 100644
--- a/arch/mips/include/asm/mach-malta/mc146818rtc.h
+++ b/arch/mips/include/asm/mach-malta/mc146818rtc.h
@@ -31,6 +31,4 @@ static inline void CMOS_WRITE(unsigned char data, unsigned long addr)
 
 #define RTC_ALWAYS_BCD	0
 
-#define mc146818_decode_year(year) ((year) < 70 ? (year) + 2000 : (year) + 1900)
-
 #endif /* __ASM_MACH_MALTA_MC146818RTC_H */
diff --git a/arch/mips/include/asm/mc146818-time.h b/arch/mips/include/asm/mc146818-time.h
index 07bf30bee792..4e07914e94e6 100644
--- a/arch/mips/include/asm/mc146818-time.h
+++ b/arch/mips/include/asm/mc146818-time.h
@@ -37,7 +37,10 @@ static inline time64_t mc146818_get_cmos_time(void)
 		year = bcd2bin(year);
 	}
 	spin_unlock_irqrestore(&rtc_lock, flags);
-	year = mc146818_decode_year(year);
+	if (year < 70)
+		year += 2000;
+	else
+		year += 1900;
 
 	return mktime64(year, mon, day, hour, min, sec);
 }
-- 
2.25.1

[PATCH 4/4] mips/malta,loongson2ef: use generic mc146818_get_time function

[Mateusz Jończyk]

mc146818_get_cmos_time() is now mostly equivalent to mc146818_get_time()
from drivers/rtc/rtc-mc146818-lib.c, with the latter using a more
advanced algorithm (which checks the UIP bit in the CMOS). The Malta
and Loongson2ef platforms, the only users of mc146818_get_cmos_time()
have RTC devices that should be MC146818 compatible.

So, rewrite mc146818_get_cmos_time() in a way that uses
mc146818_get_time() and add CONFIG_RTC_MC146818_LIB as a dependency of
CONFIG_MIPS_MALTA and CONFIG_CPU_LOONGSON2EF.

The should be safe as:

- malta_defconfig already uses a standard RTC CMOS driver
  (CONFIG_RTC_DRV_CMOS=y). The Malta board has an Intel 82371EB (PIIX4E)
  south bridge with the CMOS RTC, so should work correctly with the
  modification,

- Loongson2e and 2f apparently use the VIA686B south bridge and the AMD
  CS5536 south bridge respectively (at least according to Kconfig). I
  have checked datasheets of both and these appear to be MC146818
  software compatible.

Signed-off-by: Mateusz Jończyk <mat.jonczyk@o2.pl>

--

Tested only in an emulator for Malta.
---
 arch/mips/Kconfig                     |  2 ++
 arch/mips/include/asm/mc146818-time.h | 34 ++++++---------------------
 2 files changed, 9 insertions(+), 27 deletions(-)

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index 1e48184ecf1e..4df5589b3d35 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -563,6 +563,7 @@ config MIPS_MALTA
 	select MIPS_L1_CACHE_SHIFT_6
 	select MIPS_MSC
 	select PCI_GT64XXX_PCI0
+	select RTC_MC146818_LIB
 	select SMP_UP if SMP
 	select SWAP_IO_SPACE
 	select SYS_HAS_CPU_MIPS32_R1
@@ -1837,6 +1838,7 @@ config CPU_LOONGSON2EF
 	select CPU_SUPPORTS_64BIT_KERNEL
 	select CPU_SUPPORTS_HIGHMEM
 	select CPU_SUPPORTS_HUGEPAGES
+	select RTC_MC146818_LIB
 
 config CPU_LOONGSON32
 	bool
diff --git a/arch/mips/include/asm/mc146818-time.h b/arch/mips/include/asm/mc146818-time.h
index 4e07914e94e6..ac52a30b4161 100644
--- a/arch/mips/include/asm/mc146818-time.h
+++ b/arch/mips/include/asm/mc146818-time.h
@@ -8,41 +8,21 @@
 #ifndef __ASM_MC146818_TIME_H
 #define __ASM_MC146818_TIME_H
 
-#include <linux/bcd.h>
 #include <linux/mc146818rtc.h>
 #include <linux/time.h>
 
+#ifdef CONFIG_RTC_MC146818_LIB
 static inline time64_t mc146818_get_cmos_time(void)
 {
-	unsigned int year, mon, day, hour, min, sec;
-	unsigned long flags;
+	struct rtc_time tm;
 
-	spin_lock_irqsave(&rtc_lock, flags);
-
-	do {
-		sec = CMOS_READ(RTC_SECONDS);
-		min = CMOS_READ(RTC_MINUTES);
-		hour = CMOS_READ(RTC_HOURS);
-		day = CMOS_READ(RTC_DAY_OF_MONTH);
-		mon = CMOS_READ(RTC_MONTH);
-		year = CMOS_READ(RTC_YEAR);
-	} while (sec != CMOS_READ(RTC_SECONDS));
-
-	if (!(CMOS_READ(RTC_CONTROL) & RTC_DM_BINARY) || RTC_ALWAYS_BCD) {
-		sec = bcd2bin(sec);
-		min = bcd2bin(min);
-		hour = bcd2bin(hour);
-		day = bcd2bin(day);
-		mon = bcd2bin(mon);
-		year = bcd2bin(year);
+	if (mc146818_get_time(&tm, 1000)) {
+		pr_err("Unable to read current time from RTC\n");
+		return 0;
 	}
-	spin_unlock_irqrestore(&rtc_lock, flags);
-	if (year < 70)
-		year += 2000;
-	else
-		year += 1900;
 
-	return mktime64(year, mon, day, hour, min, sec);
+	return rtc_tm_to_time64(&tm);
 }
+#endif /* CONFIG_RTC_MC146818_LIB */
 
 #endif /* __ASM_MC146818_TIME_H */
-- 
2.25.1

Re: [PATCH 0/4] mips: CMOS RTC refactoring

[Thomas Bogendoerfer]

On Sun, Jul 13, 2025 at 12:04:30PM +0200, Mateusz Jończyk wrote:
> Hello,
> 
> A while ago I wrote some patches that refactor a bit the RTC CMOS support on the
> MIPS architecture. These were in response to a bug report about locking
> problems during suspend / resume:
> 
> Link: https://lore.kernel.org/all/CAP-bSRZ0CWyZZsMtx046YV8L28LhY0fson2g4EqcwRAVN1Jk+Q@mail.gmail.com/
> 
> Back then, I was trying to convert the rtc_lock to a raw spinlock. This proved
> to be unnecessary and harmful, but in the process I removed some dead and
> duplicated code that touches the MC146818 (the chip of the CMOS RTC) for
> the MIPS architecture.
> 
> These patches were tested only in an emulator, as I don't have the hardware.
> They are roughly in the order of increasing risk.
> 
> Greetings,
> Mateusz
> 
> Mateusz Jończyk (4):
>   mips: remove unused function mc146818_set_rtc_mmss
>   mips/mach-rm: remove custom mc146818rtc.h file
>   mips: remove redundant macro mc146818_decode_year
>   mips/malta,loongson2ef: use generic mc146818_get_time function
> 
>  arch/mips/Kconfig                             |   2 +
>  .../include/asm/mach-generic/mc146818rtc.h    |   4 -
>  arch/mips/include/asm/mach-jazz/mc146818rtc.h |   2 -
>  .../mips/include/asm/mach-malta/mc146818rtc.h |   2 -
>  arch/mips/include/asm/mach-rm/mc146818rtc.h   |  21 ----
>  arch/mips/include/asm/mc146818-time.h         | 105 ++----------------
>  6 files changed, 9 insertions(+), 127 deletions(-)
>  delete mode 100644 arch/mips/include/asm/mach-rm/mc146818rtc.h
> 
> 
> base-commit: d7b8f8e20813f0179d8ef519541a3527e7661d3a
> -- 
> 2.25.1

series applied to mips-next.

Thomas.

-- 
Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
good idea.                                                [ RFC1925, 2.3 ]