[PATCH 0/3] rtc,x86: RTC CMOS refactoring

[PATCH 0/3] rtc,x86: RTC CMOS refactoring

[Mateusz Jończyk]

Hello,

The main aim of this series is to remove duplicated code in
mach_get_cmos_time() in arch/x86/kernel/rtc.c . This function performed
the same thing as mc146818_get_time() - reading the time from the CMOS
RTC - but used a different algorithm. So modify it to use standard
mc146818_get_time().

First, to make these functions more similar to each other (and also for
another reason described in the patch), reduce the polling period in
mc146818_get_time(). Then, rewrite mach_get_cmos_time().

The last patch renames a function in arch/x86/kernel/rtc.c.

I have tested this on three different computers.

Greetings,
Mateusz

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>

Mateusz Jończyk (3):
  rtc-mc146818-lib: reduce RTC_UIP polling period
  x86/rtc: rewrite mach_get_cmos_time
  x86/rtc: rename mach_set_rtc_mmss

 arch/x86/include/asm/mc146818rtc.h |  2 +-
 arch/x86/kernel/rtc.c              | 63 +++++-------------------------
 arch/x86/kernel/x86_init.c         |  2 +-
 drivers/rtc/rtc-mc146818-lib.c     |  8 ++--
 4 files changed, 15 insertions(+), 60 deletions(-)


base-commit: cfb92440ee71adcc2105b0890bb01ac3cddb8507
-- 
2.25.1

[PATCH 1/3] rtc-mc146818-lib: reduce RTC_UIP polling period

[Mateusz Jończyk]

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>
---
 drivers/rtc/rtc-mc146818-lib.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/rtc/rtc-mc146818-lib.c b/drivers/rtc/rtc-mc146818-lib.c
index ae9f131b43c0..ae4b19e0a981 100644
--- 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;
 		}
 
-- 
2.25.1

Re: (subset) [PATCH 1/3] rtc-mc146818-lib: reduce RTC_UIP polling period

[alexandre.belloni]

From: Alexandre Belloni <alexandre.belloni@bootlin.com>

On Fri, 25 Feb 2022 22:50:09 +0100, Mateusz Jończyk wrote:
> 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.
> 
> [...]

Applied, thanks!

[1/3] rtc-mc146818-lib: reduce RTC_UIP polling period
      commit: 4b94a798e1ca59f4614f5ff6b94fe0c287412b9b

Best regards,
-- 
Alexandre Belloni <alexandre.belloni@bootlin.com>

[PATCH 2/3] x86/rtc: rewrite mach_get_cmos_time

[Mateusz Jończyk]

There are functions in drivers/rtc/rtc-mc146818-lib.c that handle
reading from / writing to the CMOS RTC clock. mach_get_cmos_time() in
arch/x86/kernel/rtc.c did not use them and was mostly a duplicate of
mc146818_get_time(). Modify mach_get_cmos_time() to use
mc146818_get_time() and remove the duplicated code.

mach_get_cmos_time() used a different algorithm than
mc146818_get_time(), but these functions were equivalent. The major
differences were:

- mc146818_get_time() is better refined: it was updated in
  commit 05a0302c3548 ("rtc: mc146818: Prevent reading garbage")
  to take account of various edge conditions,

- when the UIP ("Update in progress") bit of the RTC is set,
  mach_get_cmos_time() was busy waiting with cpu_relax() while
  mc146818_get_time() is now using udelay(100) in every loop iteration,

- mach_get_cmos_time() assumed that the RTC year must be >= 2000, which
  may not be true on some old boxes with a dead battery,

- mach_get_cmos_time() was holding the rtc_lock for a long time.

The RTC writing counterpart, mach_set_rtc_mmss() is already using
mc146818_get_time() from drivers/rtc. This was done in
        commit 3195ef59cb42 ("x86: Do full rtc synchronization with ntp")
It appears that mach_get_cmos_time() was simply forgotten.

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>
---
 arch/x86/kernel/rtc.c | 59 +++++--------------------------------------
 1 file changed, 7 insertions(+), 52 deletions(-)

diff --git a/arch/x86/kernel/rtc.c b/arch/x86/kernel/rtc.c
index 586f718b8e95..9f251b65219f 100644
--- a/arch/x86/kernel/rtc.c
+++ b/arch/x86/kernel/rtc.c
@@ -4,11 +4,8 @@
  */
 #include <linux/platform_device.h>
 #include <linux/mc146818rtc.h>
-#include <linux/acpi.h>
-#include <linux/bcd.h>
 #include <linux/export.h>
 #include <linux/pnp.h>
-#include <linux/of.h>
 
 #include <asm/vsyscall.h>
 #include <asm/x86_init.h>
@@ -20,15 +17,12 @@
 /*
  * This is a special lock that is owned by the CPU and holds the index
  * register we are working with.  It is required for NMI access to the
- * CMOS/RTC registers.  See include/asm-i386/mc146818rtc.h for details.
+ * CMOS/RTC registers.  See arch/x86/include/asm/mc146818rtc.h for details.
  */
 volatile unsigned long cmos_lock;
 EXPORT_SYMBOL(cmos_lock);
 #endif /* CONFIG_X86_32 */
 
-/* For two digit years assume time is always after that */
-#define CMOS_YEARS_OFFS 2000
-
 DEFINE_SPINLOCK(rtc_lock);
 EXPORT_SYMBOL(rtc_lock);
 
@@ -62,8 +56,7 @@ int mach_set_rtc_mmss(const struct timespec64 *now)
 
 void mach_get_cmos_time(struct timespec64 *now)
 {
-	unsigned int status, year, mon, day, hour, min, sec, century = 0;
-	unsigned long flags;
+	struct rtc_time tm;
 
 	/*
 	 * If pm_trace abused the RTC as storage, set the timespec to 0,
@@ -74,51 +67,13 @@ void mach_get_cmos_time(struct timespec64 *now)
 		return;
 	}
 
-	spin_lock_irqsave(&rtc_lock, flags);
-
-	/*
-	 * If UIP is clear, then we have >= 244 microseconds before
-	 * RTC registers will be updated.  Spec sheet says that this
-	 * is the reliable way to read RTC - registers. If UIP is set
-	 * then the register access might be invalid.
-	 */
-	while ((CMOS_READ(RTC_FREQ_SELECT) & RTC_UIP))
-		cpu_relax();
-
-	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);
-
-#ifdef CONFIG_ACPI
-	if (acpi_gbl_FADT.header.revision >= FADT2_REVISION_ID &&
-	    acpi_gbl_FADT.century)
-		century = CMOS_READ(acpi_gbl_FADT.century);
-#endif
-
-	status = CMOS_READ(RTC_CONTROL);
-	WARN_ON_ONCE(RTC_ALWAYS_BCD && (status & RTC_DM_BINARY));
-
-	spin_unlock_irqrestore(&rtc_lock, flags);
-
-	if (RTC_ALWAYS_BCD || !(status & RTC_DM_BINARY)) {
-		sec = bcd2bin(sec);
-		min = bcd2bin(min);
-		hour = bcd2bin(hour);
-		day = bcd2bin(day);
-		mon = bcd2bin(mon);
-		year = bcd2bin(year);
+	if (mc146818_get_time(&tm)) {
+		pr_err_ratelimited("Unable to read current time from RTC\n");
+		now->tv_sec = now->tv_nsec = 0;
+		return;
 	}
 
-	if (century) {
-		century = bcd2bin(century);
-		year += century * 100;
-	} else
-		year += CMOS_YEARS_OFFS;
-
-	now->tv_sec = mktime64(year, mon, day, hour, min, sec);
+	now->tv_sec = rtc_tm_to_time64(&tm);
 	now->tv_nsec = 0;
 }
 
-- 
2.25.1

[PATCH 3/3] x86/rtc: rename mach_set_rtc_mmss

[Mateusz Jończyk]

Once upon a time, before
commit 3195ef59cb42 ("x86: Do full rtc synchronization with ntp")
in 2013, the function used to set only the minutes and seconds registers
of the CMOS RTC. This is no longer true, so rename the function to
mach_set_cmos_time.

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>
---
 arch/x86/include/asm/mc146818rtc.h | 2 +-
 arch/x86/kernel/rtc.c              | 4 ++--
 arch/x86/kernel/x86_init.c         | 2 +-
 3 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/mc146818rtc.h b/arch/x86/include/asm/mc146818rtc.h
index 97198001e567..6115bb3d5795 100644
--- a/arch/x86/include/asm/mc146818rtc.h
+++ b/arch/x86/include/asm/mc146818rtc.h
@@ -95,7 +95,7 @@ static inline unsigned char current_lock_cmos_reg(void)
 unsigned char rtc_cmos_read(unsigned char addr);
 void rtc_cmos_write(unsigned char val, unsigned char addr);
 
-extern int mach_set_rtc_mmss(const struct timespec64 *now);
+extern int mach_set_cmos_time(const struct timespec64 *now);
 extern void mach_get_cmos_time(struct timespec64 *now);
 
 #define RTC_IRQ 8
diff --git a/arch/x86/kernel/rtc.c b/arch/x86/kernel/rtc.c
index 9f251b65219f..24ca997a210e 100644
--- a/arch/x86/kernel/rtc.c
+++ b/arch/x86/kernel/rtc.c
@@ -27,13 +27,13 @@ DEFINE_SPINLOCK(rtc_lock);
 EXPORT_SYMBOL(rtc_lock);
 
 /*
- * In order to set the CMOS clock precisely, set_rtc_mmss has to be
+ * In order to set the CMOS clock precisely, mach_set_cmos_time 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.
  */
-int mach_set_rtc_mmss(const struct timespec64 *now)
+int mach_set_cmos_time(const struct timespec64 *now)
 {
 	unsigned long long nowtime = now->tv_sec;
 	struct rtc_time tm;
diff --git a/arch/x86/kernel/x86_init.c b/arch/x86/kernel/x86_init.c
index 7d20c1d34a3c..927924a319a7 100644
--- a/arch/x86/kernel/x86_init.c
+++ b/arch/x86/kernel/x86_init.c
@@ -133,7 +133,7 @@ struct x86_platform_ops x86_platform __ro_after_init = {
 	.calibrate_cpu			= native_calibrate_cpu_early,
 	.calibrate_tsc			= native_calibrate_tsc,
 	.get_wallclock			= mach_get_cmos_time,
-	.set_wallclock			= mach_set_rtc_mmss,
+	.set_wallclock			= mach_set_cmos_time,
 	.iommu_shutdown			= iommu_shutdown_noop,
 	.is_untracked_pat_range		= is_ISA_range,
 	.nmi_init			= default_nmi_init,
-- 
2.25.1