Re: [PATCH v3 03/12] rtc: rzn1: fix weekday underflow when alarm crosses month boundary

[Wolfram Sang <wsa+renesas@sang-engineering.com>]

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On Mon, Jul 06, 2026 at 06:51:29PM +0100, Prabhakar wrote:
> From: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
> 
> In rzn1_rtc_set_alarm(), the driver attempts to calculate the weekday
> for an alarm by computing the day delta between the alarm time and the
> current time:
> 
>     days_ahead = tm->tm_mday - tm_now.tm_mday;
>     wday = (tm_now.tm_wday + days_ahead) % 7;
> 
> However, if an alarm is scheduled for the beginning of the next month
> while the current time is at the end of the month (e.g., current day is
> 31, alarm day is 1), `tm->tm_mday - tm_now.tm_mday` results in a negative
> value (-30). Since `days_ahead` is an unsigned int, this underflows to a
> large positive number, leading to an incorrect `wday` being written to
> the RZN1_RTC_ALW register. As a result, the alarm fails to fire.
> 
> Fix this by utilizing the already computed `alarm` time64_t timestamp.
> Convert it back into an rtc_time struct via rtc_time64_to_tm(), which
> automatically handles month boundaries and correctly populates the
> `tm_wday` field.
> 
> Fixes: b5ad1bf00d2c4 ("rtc: rzn1: Add alarm support")
> Cc: stable@vger.kernel.org
> Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>

These messages look a bit LLM-generated. Is this true? What about the
code?

> -	days_ahead = tm->tm_mday - tm_now.tm_mday;
> -	wday = (tm_now.tm_wday + days_ahead) % 7;
> +	rtc_time64_to_tm(alarm, &alarm_tm);

This conversion is superfluous, we already have 'tm'? So, I think we can
skip the whole conversion block here and use?

	writel(BIT(tm->tm_wday), rtc->base + RZN1_RTC_ALW);

Only lightly tested, please double check.


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