linux-kernel - Re: [PATCH 3/4] rtc: Add support for configuring the UIP timeout for RTC reads

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Date:   Sat, 18 Nov 2023 20:06:59 +0100
From:   Mateusz Jończyk <mat.jonczyk@...pl>
To:     Mario Limonciello <mario.limonciello@....com>,
        Alessandro Zummo <a.zummo@...ertech.it>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>
Cc:     "open list:REAL TIME CLOCK (RTC) SUBSYSTEM" 
        <linux-rtc@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        linux-pm@...r.kernel.org, tobrohl@...il.com, aalsing@...il.com,
        Dhaval.Giani@....com, xmb8dsv4@...il.com, x86@...nel.org
Subject: Re: [PATCH 3/4] rtc: Add support for configuring the UIP timeout for
 RTC reads

Hello,

W dniu 17.11.2023 o 07:32, Mario Limonciello pisze:
> The UIP timeout is hardcoded to 10ms for all RTC reads, but in some
> contexts this might not be enough time. Add a timeout parameter to
> mc146818_get_time() and mc146818_get_time_callback().
> Make all callers use 10ms to ensure no functional changes.
>
> Signed-off-by: Mario Limonciello <mario.limonciello@....com>

Fixes: ec5895c0f2d8 ("rtc: mc146818-lib: extract mc146818_avoid_UIP")

If you would like to Cc: stable this patch,

commit d2a632a8a117 ("rtc: mc146818-lib: reduce RTC_UIP polling period")

is a prerequisite.

> ---
>  arch/alpha/kernel/rtc.c        |  2 +-
>  arch/x86/kernel/hpet.c         |  2 +-
>  arch/x86/kernel/rtc.c          |  2 +-
>  drivers/base/power/trace.c     |  2 +-
>  drivers/rtc/rtc-cmos.c         |  6 +++---
>  drivers/rtc/rtc-mc146818-lib.c | 31 +++++++++++++++++++++++--------
>  include/linux/mc146818rtc.h    |  3 ++-
>  7 files changed, 32 insertions(+), 16 deletions(-)
>
[snip]
> --- a/drivers/rtc/rtc-mc146818-lib.c
> +++ b/drivers/rtc/rtc-mc146818-lib.c
> @@ -8,26 +8,29 @@
>  #include <linux/acpi.h>
>  #endif
>  
> +#define UIP_RECHECK_DELAY		100	/* usec */
> +
>  /*
>   * Execute a function while the UIP (Update-in-progress) bit of the RTC is
> - * unset.
> + * unset. The timeout is configurable by the caller in ms.
>   *
>   * Warning: callback may be executed more then once.
>   */
>  bool mc146818_avoid_UIP(void (*callback)(unsigned char seconds, void *param),
> +			int timeout,
>  			void *param)
>  {
>  	int i;
>  	unsigned long flags;
>  	unsigned char seconds;
>  
> -	for (i = 0; i < 100; i++) {
> +	for (i = 0; i < USEC_PER_MSEC / UIP_RECHECK_DELAY * timeout; 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 100 usec for completion.
> +		 * 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 +40,7 @@ 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);
> -			udelay(100);
> +			udelay(UIP_RECHECK_DELAY);
>  			continue;
>  		}
>  
> @@ -56,7 +59,7 @@ 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);
> -			udelay(100);
> +			udelay(UIP_RECHECK_DELAY);
>  			continue;
>  		}
>  

I think that when reading the RTC is not finished in 100ms or so
(irrespective of the timeout parameter), the code should log
a warning / an error message.

Greetings,

Mateusz