linux-kernel - RE: [PATCH v2 0/5] rtc: zynqmp: fixes for read and set offset

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Message-ID:
 <LV5PR12MB9804F0A8A3BF51D2821C1ED1928FA@LV5PR12MB9804.namprd12.prod.outlook.com>
Date: Wed, 14 Jan 2026 15:44:37 +0000
From: "T, Harini" <Harini.T@....com>
To: Tomas Melin <tomas.melin@...sala.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 v2 0/5] rtc: zynqmp: fixes for read and set offset

[Public]

Write +34335 ppb → Read +34335 ppb (0 ppb error)
Write -34335 ppb → Read -34326 ppb (9 ppb error)
Asymmetry: 9 ppb

The 9 ppb error is acceptable as it's well within the hardware's ±1908 ppb
quantization limit (4-bit fractional resolution)

Tested-by: Harini T <harini.t@....com>

Thanks,
Harini T

> -----Original Message-----
> From: Tomas Melin <tomas.melin@...sala.com>
> Sent: Thursday, January 8, 2026 6:21 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 v2 0/5] rtc: zynqmp: fixes for read and set offset
>
> Caution: This message originated from an External Source. Use proper
> caution when opening attachments, clicking links, or responding.
>
>
> Add improvements for read and set offset functions.
> The basic functionality is still the same, but offset correction values are now
> updated to match with expected.
>
> The RTC calibration value operates with full ticks, and fractional ticks which
> are a 1/16 of a full tick.
> The 16 lowest bits in the calibration registers are for the full ticks and value
> matches the external oscillator in Hz. Through that, the maximum and
> minimum offset values can be calculated dynamically, as they depend on the
> input frequency used.
>
> For docs on the calibration register, see https://docs.amd.com/r/en-
> US/ug1087-zynq-ultrascale-registers/CALIB_READ-RTC-Register
>
> Due to rounding errors (different number of fract ticks), offset readback will
> differ slightly depending on if the offset is negative or positive.
>
> For example
> $ echo 34335 > offset
> $ cat offset
> 34335
> $ echo -34335 > offset
> $ cat offset
> -34326
>
> Signed-off-by: Tomas Melin <tomas.melin@...sala.com>
> ---
> Changes in v2:
> - Add commit introducing check for calibration value overflow
> - Update comments
> - Align data types across set and read
> - Rename fract_tick as fract_data conforming to data sheet
> - Further improve on set offset calculation logic
> - Link to v1: https://lore.kernel.org/r/20251201-zynqmp-rtc-updates-v1-0-
> 33875c1e385b@...sala.com
>
> ---
> Tomas Melin (5):
>       rtc: zynqmp: correct frequency value
>       rtc: zynqmp: check calibration max value
>       rtc: zynqmp: rework read_offset
>       rtc: zynqmp: rework set_offset
>       rtc: zynqmp: use dynamic max and min offset ranges
>
>  drivers/rtc/rtc-zynqmp.c | 74 ++++++++++++++++++++++++++---------------------
> -
>  1 file changed, 40 insertions(+), 34 deletions(-)
> ---
> base-commit: cd635e33b0113287c94021be53d2a7c61a1614e9
> change-id: 20251201-zynqmp-rtc-updates-d260364cc01b
>
> Best regards,
> --
> Tomas Melin <tomas.melin@...sala.com>
>