linux-kernel - Re: [PATCH 4/4] rtc: zynqmp: use dynamic max and min offset ranges

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Message-ID: <ea85c8d6-ead8-4fda-905d-909968de5056@vaisala.com>
Date: Wed, 10 Dec 2025 14:25:27 +0200
From: Tomas Melin <tomas.melin@...sala.com>
To: "T, Harini" <Harini.T@....com>,
 Alexandre Belloni <alexandre.belloni@...tlin.com>,
 "Simek, Michal" <michal.simek@....com>
Cc: "linux-rtc@...r.kernel.org" <linux-rtc@...r.kernel.org>,
 "linux-arm-kernel@...ts.infradead.org"
 <linux-arm-kernel@...ts.infradead.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/4] rtc: zynqmp: use dynamic max and min offset ranges

Hi,

On 09/12/2025 21:28, T, Harini wrote:
> [Public]
> 
> Hi,
> 
>> -----Original Message-----
>> From: Tomas Melin <tomas.melin@...sala.com>
>> Sent: Monday, December 1, 2025 6:20 PM
>> To: Alexandre Belloni <alexandre.belloni@...tlin.com>; Simek, Michal
>> <michal.simek@....com>
>> Cc: linux-rtc@...r.kernel.org; linux-arm-kernel@...ts.infradead.org; linux-
>> kernel@...r.kernel.org; Tomas Melin <tomas.melin@...sala.com>
>> Subject: [PATCH 4/4] rtc: zynqmp: use dynamic max and min offset ranges
>>
>> Caution: This message originated from an External Source. Use proper
>> caution when opening attachments, clicking links, or responding.
>>
>>
>> Maximum and minimum offsets in ppb that can be handled are dependent
>> on the rtc clock frequency and what can fit in the 16-bit register field.
>>
>> Signed-off-by: Tomas Melin <tomas.melin@...sala.com>
>> ---
>>  drivers/rtc/rtc-zynqmp.c | 8 +++-----
>>  1 file changed, 3 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/rtc/rtc-zynqmp.c b/drivers/rtc/rtc-zynqmp.c index
>> 3bc8831ba2c4c4c701a49506b67ae6174f3ade3d..0cebc99b15a6de2440a60afc
>> 2bd1769eccfa84b3 100644
>> --- a/drivers/rtc/rtc-zynqmp.c
>> +++ b/drivers/rtc/rtc-zynqmp.c
>> @@ -44,8 +44,6 @@
>>  #define RTC_FR_MASK            0xF0000
>>  #define RTC_FR_MAX_TICKS       16
>>  #define RTC_PPB                        1000000000LL
>> -#define RTC_MIN_OFFSET         -32768000
>> -#define RTC_MAX_OFFSET         32767000
>>
>>  struct xlnx_rtc_dev {
>>         struct rtc_device       *rtc;
>> @@ -215,12 +213,12 @@ static int xlnx_rtc_set_offset(struct device *dev,
>> long offset)
>>
>>         /* ticks to reach RTC_PPB */
>>         tick_mult = DIV_ROUND_CLOSEST(RTC_PPB, xrtcdev->freq);
>> -       if (offset < RTC_MIN_OFFSET || offset > RTC_MAX_OFFSET)
>> -               return -ERANGE;
>> -
>>         /* Number ticks for given offset */
>>         max_tick = div_s64_rem(offset, tick_mult, &fract_offset);
>>
>> +       if (freq + max_tick > RTC_TICK_MASK || (freq + max_tick < 1))
> The check 'freq + max_tick < 1' should be '<2' to prevent writing 0 to the calibration register when fract_offset < 0 causes max_tick--.
> Example: freq=32767, max_tick=-32766 passes (sum=1), but after decrement becomes calibval=0.
calibval=0 is not documented as invalid calibration value. AFAIS it
would mean a frequency of 1Hz. Can You provide more info on this?

Thanks,
Tomas


>> +               return -ERANGE;
>> +
>>         /* Number fractional ticks for given offset */
>>         if (fract_offset) {
>>                 /* round up here so we stay below a full tick */
>>
>> --
>> 2.47.3
>>
> 
> Thanks,
> Harini T
>