[PATCH v2 3/5] rtc: zynqmp: rework read_offset

[T, Harini]

[Public]

Reviewed-by: Harini T <harini.t at amd.com>

Thanks,
Harini T

> -----Original Message-----
> From: Tomas Melin <tomas.melin at vaisala.com>
> Sent: Thursday, January 8, 2026 6:21 PM
> To: Alexandre Belloni <alexandre.belloni at bootlin.com>; Simek, Michal
> <michal.simek at amd.com>
> Cc: linux-rtc at vger.kernel.org; linux-arm-kernel at lists.infradead.org; linux-
> kernel at vger.kernel.org; Tomas Melin <tomas.melin at vaisala.com>
> Subject: [PATCH v2 3/5] rtc: zynqmp: rework read_offset
>
> Caution: This message originated from an External Source. Use proper
> caution when opening attachments, clicking links, or responding.
>
>
> read_offset() was using static frequency for determining the tick offset. It was
> also using remainder from do_div() operation as tick_mult value which
> caused the offset to be incorrect.
>
> At the same time, rework function to improve readability.
> It is worth noting, that due to rounding errors, the offset readback will differ
> slightly for positive and negative calibration values.
>
> Signed-off-by: Tomas Melin <tomas.melin at vaisala.com>
> ---
>  drivers/rtc/rtc-zynqmp.c | 25 ++++++++++++++++---------
>  1 file changed, 16 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/rtc/rtc-zynqmp.c b/drivers/rtc/rtc-zynqmp.c index
> caacce3725e2ef3803ea42d40e77ceaeb7d7b914..6740c3aed1897d4b50a02c4
> 823a746d9c2ae2655 100644
> --- a/drivers/rtc/rtc-zynqmp.c
> +++ b/drivers/rtc/rtc-zynqmp.c
> @@ -178,21 +178,28 @@ static void xlnx_init_rtc(struct xlnx_rtc_dev
> *xrtcdev)  static int xlnx_rtc_read_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 int calibval;
> +       unsigned int calibval, fract_data, fract_part;
> +       int freq = xrtcdev->freq;
> +       int max_tick, tick_mult;
>         long offset_val;
>
> +       /* Tick to offset multiplier */
> +       tick_mult = DIV_ROUND_CLOSEST(RTC_PPB, freq);
> +
>         calibval = readl(xrtcdev->reg_base + RTC_CALIB_RD);
>         /* Offset with seconds ticks */
> -       offset_val = calibval & RTC_TICK_MASK;
> -       offset_val = offset_val - RTC_CALIB_DEF;
> -       offset_val = offset_val * tick_mult;
> +       max_tick = calibval & RTC_TICK_MASK;
> +       offset_val = max_tick - freq;
> +       /* Convert to ppb */
> +       offset_val *= tick_mult;
>
>         /* Offset with fractional ticks */
> -       if (calibval & RTC_FR_EN)
> -               offset_val += ((calibval & RTC_FR_MASK) >> RTC_FR_DATSHIFT)
> -                       * (tick_mult / RTC_FR_MAX_TICKS);
> +       if (calibval & RTC_FR_EN) {
> +               fract_data = (calibval & RTC_FR_MASK) >> RTC_FR_DATSHIFT;
> +               fract_part = DIV_ROUND_UP(tick_mult, RTC_FR_MAX_TICKS);
> +               offset_val += (fract_part * fract_data);
> +       }
> +
>         *offset = offset_val;
>
>         return 0;
>
> --
> 2.47.3
>