[PATCH v3 0/5] Enable RTC for the MT6357

[Alexandre Mergnat]

The RTC subsystem in the Linux kernel has long had issues handling dates
before January 1, 1970, particularly for hardware with base years before
the Unix epoch. This patch series fixes these issues, focusing on the
MediaTek MT635x PMIC RTC implementations.

The core problem is that MediaTek MT635x PMIC RTCs use some defined years
before 1970 which are negative values after conversion. These differences led
to inconsistencies and bugs when the hardware's native time representation was
converted to the kernel's time64_t format, especially for dates prior to 1970.

The first patch adds MT6357 support to the MT6359 RTC driver. The second patch
fixes the fundamental time conversion functions in the RTC subsystem to properly
handle negative time64_t values (pre-1970 dates). The third patch adds
explicit type casts between signed time64_t and unsigned timeu64_t to
fix comparison bugs that were causing validation problems.

With the core functionality fixed, the fourth patch removes hardcoded start time
parameters from the MT6397 driver and instead relies on the start-year
property from device tree. Finally, the fifth patch updates the DTS files to
specify the correct start-year values for all MediaTek RTCs.

These changes make the kernel correctly handle the full range of dates
supported by the hardware. This matters for embedded systems using
these MediaTek PMICs, which may require accurate representation of
time across a wide range of years, including before 1970.

Signed-off-by: Alexandre Mergnat <amergnat at baylibre.com>
---
Changes in v3:
- Rebase on top of rtc-6.16
- Added explicit start-year property in DTSIs for MT6357, MT6358, and
  MT6359 PMIC RTCs to ensure consistent values between hardware
  registers and the RTC framework.
- Removed hardcoded offset and start_secs parameter in mt6397 driver
  in favor of using the DTS start-year property.
- Fixed type comparison issues between signed time64_t and unsigned
  range values to correctly handle dates before 1970.
- Added proper handling of negative time values (pre-1970 dates) in
  time conversion functions.
- Modified rtc_time64_to_tm() to correctly handle negative timestamp
  values.
- Removed the tm_year < 70 restriction in rtc_valid_tm() to allow
  pre-1970 dates to be validated correctly .
- Link to v2: https://lore.kernel.org/all/20250109-enable-rtc-v2-0-d7ddc3e73c57@baylibre.com/

Changes in v2:
- Split the patch to have:
  - Add MT6357 support
  - Fix hwclock issue
- Handle the year offset in another way, but the V1 way still viable.
- Link to v1: https://lore.kernel.org/r/20250109-enable-rtc-v1-0-e8223bf55bb8@baylibre.com

---
Alexandre Mergnat (5):
      rtc: mt6359: Add mt6357 support
      rtc: Add handling of pre-1970 dates in time conversion functions
      rtc: Fix the RTC time comparison issues adding cast
      rtc: mt6397: Remove start time parameters
      arm64: dts: mediatek: Set RTC start year property

 arch/arm64/boot/dts/mediatek/mt6357.dtsi |  1 +
 arch/arm64/boot/dts/mediatek/mt6358.dtsi |  1 +
 arch/arm64/boot/dts/mediatek/mt6359.dtsi |  1 +
 drivers/rtc/class.c                      |  6 ++---
 drivers/rtc/interface.c                  |  8 +++----
 drivers/rtc/lib.c                        | 38 +++++++++++++++++++++++++++-----
 drivers/rtc/rtc-mt6397.c                 |  3 +--
 7 files changed, 43 insertions(+), 15 deletions(-)
---
base-commit: 424dfcd441f035769890e6d1faec2081458627b9
change-id: 20250109-enable-rtc-b2ff435af2d5

Best regards,
-- 
Alexandre Mergnat <amergnat at baylibre.com>