linux-kernel - Re: [PATCH 1/7] rtc: Add support for limited alarm timer offsets

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Date:   Wed, 16 Aug 2023 16:57:30 +0200
From:   Alexandre Belloni <alexandre.belloni@...tlin.com>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Alessandro Zummo <a.zummo@...ertech.it>,
        Benson Leung <bleung@...omium.org>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        John Stultz <jstultz@...gle.com>,
        Stephen Boyd <sboyd@...nel.org>, linux-rtc@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Brian Norris <briannorris@...omium.org>
Subject: Re: [PATCH 1/7] rtc: Add support for limited alarm timer offsets

On 16/08/2023 06:39:30-0700, Guenter Roeck wrote:
> Some alarm timers are based on time offsets, not on absolute times.
> In some situations, the amount of time that can be scheduled in the
> future is limited. This may result in a refusal to suspend the system,
> causing substantial battery drain.
> 
> Some RTC alarm drivers remedy the situation by setting the alarm time
> to the maximum supported time if a request for an out-of-range timeout
> is made. This is not really desirable since it may result in unexpected
> early wakeups.
> 
> To reduce the impact of this problem, let RTC drivers report the maximum
> supported alarm timer offset. The code setting alarm timers can then
> decide if it wants to reject setting alarm timers to a larger value, if it
> wants to implement recurring alarms until the actually requested alarm
> time is met, or if it wants to accept the limited alarm time.
> 
> Only introduce the necessary variable into struct rtc_device.
> Code to set and use the variable will follow with subsequent patches.
> 
> Cc: Brian Norris <briannorris@...omium.org>
> Signed-off-by: Guenter Roeck <linux@...ck-us.net>
> ---
>  include/linux/rtc.h | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/include/linux/rtc.h b/include/linux/rtc.h
> index 1fd9c6a21ebe..b6d000ab1e5e 100644
> --- a/include/linux/rtc.h
> +++ b/include/linux/rtc.h
> @@ -146,6 +146,7 @@ struct rtc_device {
>  
>  	time64_t range_min;
>  	timeu64_t range_max;
> +	timeu64_t range_max_offset;

While range_min and range_max are for the wall clock time, I would
prefer using a name that would clearly mark this as an alarm related
variable.

>  	time64_t start_secs;
>  	time64_t offset_secs;
>  	bool set_start_time;
> -- 
> 2.39.2
> 

-- 
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com