-----Original Message-----
From: Tomas Melin <redacted>
Sent: Thursday, January 8, 2026 6:21 PM
To: Alexandre Belloni <alexandre.belloni@bootlin.com>; Simek, Michal
[off-list ref]
Cc: linux-rtc@vger.kernel.org; linux-arm-kernel@lists.infradead.org; linux-
kernel@vger.kernel.org; Tomas Melin [off-list ref]
Subject: [PATCH v2 4/5] rtc: zynqmp: rework set_offset

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set_offset was using remainder of do_div as tick_mult which resulted in
wrong offset. Calibration value also assumed builtin calibration default.
Update fract_offset to correctly calculate the value for negative offset and
replace the for loop with division.

Signed-off-by: Tomas Melin <redacted>
---
 drivers/rtc/rtc-zynqmp.c | 33 +++++++++++++--------------------
 1 file changed, 13 insertions(+), 20 deletions(-)

diff --git a/drivers/rtc/rtc-zynqmp.c b/drivers/rtc/rtc-zynqmp.c index

6740c3aed1897d4b50a02c4823a746d9c2ae2655..d15c256e7ae56058ddc3884
9af6424cd29b8965e 100644

--- a/drivers/rtc/rtc-zynqmp.c
+++ b/drivers/rtc/rtc-zynqmp.c

@@ -208,13 +208,13 @@ static int xlnx_rtc_read_offset(struct device *dev,

long *offset)  static int xlnx_rtc_set_offset(struct device *dev, long offset)  {
        struct xlnx_rtc_dev *xrtcdev = dev_get_drvdata(dev);
-       unsigned long long rtc_ppb = RTC_PPB;
-       unsigned int tick_mult = do_div(rtc_ppb, xrtcdev->freq);
-       unsigned char fract_tick = 0;
+       int max_tick, tick_mult, fract_offset, fract_part;
        unsigned int calibval;
-       short int  max_tick;
-       int fract_offset;
+       int fract_data = 0;
+       int freq = xrtcdev->freq;

+       /* Tick to offset multiplier */
+       tick_mult = DIV_ROUND_CLOSEST(RTC_PPB, xrtcdev->freq);
        if (offset < RTC_MIN_OFFSET || offset > RTC_MAX_OFFSET)
                return -ERANGE;

@@ -223,29 +223,22 @@ static int xlnx_rtc_set_offset(struct device *dev,

long offset)

        /* Number fractional ticks for given offset */
        if (fract_offset) {
-               if (fract_offset < 0) {
-                       fract_offset = fract_offset + tick_mult;
+               fract_part = DIV_ROUND_UP(tick_mult, RTC_FR_MAX_TICKS);
+               fract_data = fract_offset / fract_part;
+               /* Subtract one from max_tick while adding fract_offset */
+               if (fract_offset < 0 && fract_data) {
                        max_tick--;
-               }
-               if (fract_offset > (tick_mult / RTC_FR_MAX_TICKS)) {
-                       for (fract_tick = 1; fract_tick < 16; fract_tick++) {
-                               if (fract_offset <=
-                                   (fract_tick *
-                                    (tick_mult / RTC_FR_MAX_TICKS)))
-                                       break;
-                       }
+                       fract_data += RTC_FR_MAX_TICKS;
                }
        }

        /* Zynqmp RTC uses second and fractional tick
         * counters for compensation
         */
-       calibval = max_tick + RTC_CALIB_DEF;
-
-       if (fract_tick)
-               calibval |= RTC_FR_EN;
+       calibval = max_tick + freq;

-       calibval |= (fract_tick << RTC_FR_DATSHIFT);
+       if (fract_data)
+               calibval |= (RTC_FR_EN | (fract_data <<
+ RTC_FR_DATSHIFT));

        writel(calibval, (xrtcdev->reg_base + RTC_CALIB_WR));


--
2.47.3