[PATCH v4 1/2] x86/rtc: rewrite mach_get_cmos_time to delete duplicated code

[PATCH v4 1/2] x86/rtc: rewrite mach_get_cmos_time to delete duplicated code

[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 are equivalent. The major
differences are:

- mc146818_get_time() is better refined and handles 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 using mdelay(1) in every loop iteration.
  (However, there is my commit merged for Linux 5.20 / 6.0 to decrease
  this period to 100us:
commit d2a632a8a117 ("rtc: mc146818-lib: reduce RTC_UIP polling period")
  ),

- mach_get_cmos_time() assumed that the RTC year is >= 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
  and could hang if the RTC is broken or not present.

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.

mach_get_cmos_time() is really used only in read_persistent_clock64(),
which is called only in a few places in kernel/time/timekeeping.c .

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>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

---

v2:
  - use pr_err() in place of pr_err_ratelimited(). mach_get_cmos_time()
    is not called frequently, so ratelimiting is not necessary.
  - tweak the commit description.
v3:
  - update the commit description,
  - drop the cover letter.
v4:
  - slightly update the commit description.

Tested on 3 computers (on top of 5.19.0-rc5), on 5.19.1 tested less
strictly.

---
 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..1cadc8a15267 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("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 v4 2/2] 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 set only the minute 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>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>

---

v3:
  - add more people to the Cc list.
---
 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 1cadc8a15267..349046434513 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 e84ee5cdbd8c..57353519bc11 100644
--- a/arch/x86/kernel/x86_init.c
+++ b/arch/x86/kernel/x86_init.c
@@ -138,7 +138,7 @@ 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

Re: [PATCH v4 1/2] x86/rtc: rewrite mach_get_cmos_time to delete duplicated code

[Ingo Molnar]

* Mateusz Jończyk <mat.jonczyk@o2.pl> wrote:

> 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 are equivalent. The major
> differences are:
> 
> - mc146818_get_time() is better refined and handles 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 using mdelay(1) in every loop iteration.
>   (However, there is my commit merged for Linux 5.20 / 6.0 to decrease
>   this period to 100us:
> commit d2a632a8a117 ("rtc: mc146818-lib: reduce RTC_UIP polling period")
>   ),
> 
> - mach_get_cmos_time() assumed that the RTC year is >= 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
>   and could hang if the RTC is broken or not present.
> 
> 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.
> 
> mach_get_cmos_time() is really used only in read_persistent_clock64(),
> which is called only in a few places in kernel/time/timekeeping.c .

LGTM, I've added the following background to the commit description:

   These changes are not supposed to change behavior, but they are not 
   identity transformations either, as mc146818_get_time() is a better but 
   different implementation of the same logic - so regressions are possible 
   in principle.

Queued up both patches in tip:x86/timers and will push it out after 
testing.

Thanks!

	Ingo