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

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Message-ID: <4332dbf1-76be-434a-a65a-d4686cb4d2fd@amd.com>
Date:   Mon, 27 Nov 2023 14:37:45 -0600
From:   Mario Limonciello <mario.limonciello@....com>
To:     Mateusz Jończyk <mat.jonczyk@...pl>,
        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,
        dhaval.giani@...il.com, Dave Hansen <dave.hansen@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v3 3/4] rtc: Add support for configuring the UIP timeout
 for RTC reads

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@...r.kernel.org # 6.1.y
>> Fixes: ec5895c0f2d8 ("rtc: mc146818-lib: extract mc146818_avoid_UIP")
>> Signed-off-by: Mario Limonciello <mario.limonciello@....com>
>> ---
>> 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
> 
>