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Message-ID: <YoQd5g8Y8vCmmuv4@mail.local>
Date: Wed, 18 May 2022 00:12:54 +0200
From: Alexandre Belloni <alexandre.belloni@...tlin.com>
To: Mateusz Jończyk <mat.jonczyk@...pl>
Cc: linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
"H. Peter Anvin" <hpa@...or.com>,
Alessandro Zummo <a.zummo@...ertech.it>,
Prarit Bhargava <prarit@...hat.com>
Subject: Re: [PATCH v2 resend 1/2] x86/rtc: rewrite mach_get_cmos_time to
delete duplicated code
On 17/05/2022 22:21:30+0200, Mateusz Jończyk 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 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 mdelay(1) 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.
>
> As a bonus, mach_get_cmos_time() after the modification does not hang
> indefinitely if the CMOS RTC is 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@...pl>
Acked-by: 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: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: x86@...nel.org
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: Alessandro Zummo <a.zummo@...ertech.it>
> Cc: Alexandre Belloni <alexandre.belloni@...tlin.com>
> Cc: Prarit Bhargava <prarit@...hat.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 commit description.
> ---
> 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
>
--
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com
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