Re: [PATCH v3 3/4] rtc: Add support for configuring the UIP timeout for RTC reads

[Mario Limonciello]

On 11/27/2023 14:31, Mateusz Jończyk wrote:
> W dniu 27.11.2023 o 20:25, 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().
> >
> > If UIP timeout is configured by caller to be >=100 ms and a call
> > takes this long, log a warning.
> >
> > Make all callers use 10ms to ensure no functional changes.
> >
> > Cc: stable@xxxxxxxxxxxxxxx # 6.1.y
> > Fixes: ec5895c0f2d8 ("rtc: mc146818-lib: extract mc146818_avoid_UIP")
> > Signed-off-by: Mario Limonciello <mario.limonciello@xxxxxxx>
> > ---
> > v2->v3:
> >   * Logic adjustments
> >   * Clarify warning message
> > v1->v2:
> >   * Add a warning if 100ms or more
> >   * Add stable and fixes tags
> [snip]
> > diff --git a/drivers/rtc/rtc-mc146818-lib.c b/drivers/rtc/rtc-mc146818-lib.c
> > index 43a28e82674e..ab077dde397b 100644
> > --- a/drivers/rtc/rtc-mc146818-lib.c
> > +++ b/drivers/rtc/rtc-mc146818-lib.c
> > @@ -8,26 +8,31 @@
> >   #include <linux/acpi.h>
> >   #endif
> >   
> > +#define UIP_RECHECK_DELAY		100	/* usec */
> > +#define UIP_RECHECK_DELAY_MS		(USEC_PER_MSEC / UIP_RECHECK_DELAY)
> > +#define UIP_RECHECK_TIMEOUT_MS(x)	(x / UIP_RECHECK_DELAY_MS)
> > +
> >   /*
> >    * 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 < UIP_RECHECK_TIMEOUT_MS(timeout); i++) {
>
> Sorry, this will not work. UIP_RECHECK_DELAY_MS is 10, so
> UIP_RECHECK_TIMEOUT_MS(timeout) will be 1 for timeout=10. Should be
>
>        for (i = 0; UIP_RECHECK_TIMEOUT_MS(i) < timeout; i++) {
>
> With this, for i == 99, UIP_RECHECK_TIMEOUT_MS(i) = 9
> for i == 100, UIP_RECHECK_TIMEOUT_MS(i) = 10 and the loop correctly terminates.
>
> The macro should probably be renamed UIP_RECHECK_LOOPS_MS as it converts
> loop count to ms.

Got it; thanks for that.

> >   		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 +42,7 @@ 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 +61,7 @@ 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;
> >   		}
> >   
> > @@ -72,6 +77,10 @@ bool mc146818_avoid_UIP(void (*callback)(unsigned char seconds, void *param),
> >   		}
> >   		spin_unlock_irqrestore(&rtc_lock, flags);
> >   
> > +		if (i >= UIP_RECHECK_TIMEOUT_MS(100))
>
> Same, should be:
>
>            if (UIP_RECHECK_TIMEOUT_MS(i) >= 100)
>
> > +			pr_warn("Reading current time from RTC took around %d ms\n",
> > +				UIP_RECHECK_TIMEOUT_MS(i));
> > +
> >   		return true;
> >   	}
> >   	return false;
>
> [snip]
>
> Greetings,
>
> Mateusz