rtc: Fix offset calculation for .start_secs < 0

The comparison

        rtc->start_secs > rtc->range_max

has a signed left-hand side and an unsigned right-hand side.
So the comparison might become true for negative start_secs which is
interpreted as a (possibly very large) positive value.

As a negative value can never be bigger than an unsigned value
the correct representation of the (mathematical) comparison

        rtc->start_secs > rtc->range_max

in C is:

        rtc->start_secs >= 0 && rtc->start_secs > rtc->range_max

Use that to fix the offset calculation currently used in the
rtc-mt6397 driver.

Fixes: 989515647e783 ("rtc: Add one offset seconds to expand RTC range")
Signed-off-by: Alexandre Mergnat <amergnat@baylibre.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/r/20250428-enable-rtc-v4-2-2b2f7e3f9349@baylibre.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

diff --git a/drivers/rtc/class.c b/drivers/rtc/class.c
index b88cd4fb295bce9bad40aabff8bf04b2b32f9b97..b1a2be1f9e3b934e9bc5864006500e0b713d3f1f 100644 (file)
--- a/drivers/rtc/class.c
+++ b/drivers/rtc/class.c
@@ -326,7 +326,7 @@ static void rtc_device_get_offset(struct rtc_device *rtc)
         *
         * Otherwise the offset seconds should be 0.
         */
-       if (rtc->start_secs > rtc->range_max ||
+       if ((rtc->start_secs >= 0 && rtc->start_secs > rtc->range_max) ||
            rtc->start_secs + range_secs - 1 < rtc->range_min)
                rtc->offset_secs = rtc->start_secs - rtc->range_min;
        else if (rtc->start_secs > rtc->range_min)