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Date:   Wed, 18 May 2022 00:12:54 +0200
From:   Alexandre Belloni <alexandre.belloni@bootlin.com>
To:     Mateusz =?utf-8?Q?Jo=C5=84czyk?= <mat.jonczyk@o2.pl>
Cc:     linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        Alessandro Zummo <a.zummo@towertech.it>,
        Prarit Bhargava <prarit@redhat.com>
Subject: Re: [PATCH v2 resend 1/2] x86/rtc: rewrite mach_get_cmos_time to
 delete duplicated code
Message-ID: <YoQd5g8Y8vCmmuv4@mail.local>
References: <20220517202131.379939-1-mat.jonczyk@o2.pl>
 <20220517202131.379939-2-mat.jonczyk@o2.pl>
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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@o2.pl>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

> 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>
> Cc: Prarit Bhargava <prarit@redhat.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