Re: [PATCH] rtc: s3c-rtc: Avoid using broken ALMYEAR register

[Alexandre Belloni <alexandre.belloni@bootlin.com>]

On 13/11/2018 13:15:33+0100, Marek Szyprowski wrote:
> Hi Krzysztof,
> 
> On 2018-11-13 12:50, Krzysztof Kozlowski wrote:
> > On Tue, 13 Nov 2018 at 12:33, Marek Szyprowski <m.szyprowski@samsung.com> wrote:
> >> (RTC,ALM)YEAR registers of Exynos built-in RTC device contains 3 BCD
> >> characters. s3c-rtc driver uses only 2 lower of them and supports years
> >> from 2000..2099 range. The third BCD value is typically set to 0, but it
> >> looks that handling of it is broken in the hardware. It sometimes
> >> defaults to a random (even non-BCD) value. This is not an issue
> >> for handling RTCYEAR register, because bcd2bin() properly handles only
> >> 8bit values (2 BCD characters, the third one is skipped). The problem
> >> is however with ALMYEAR register and proper RTC alarm operation. When
> >> YEAREN bit is set for the configured alarm, RTC hardware triggers alarm
> >> only when ALMYEAR and RTCYEAR matches. This usually doesn't happen
> >> because of the random noise on the third BCD character.
> >>
> >> Fix this by simply skipping setting ALMYEAR register in alarm
> >> configuration. This workaround fixes broken alarm operation on Exynos
> >> built-in rtc device. My tests revealed that the issue happens on the
> >> following Exynos series: 3250, 4210, 4412, 5250 and 5410.
> > Does it mean that alarm set for 20.11.2019 will be triggered also this
> > year on 20.11.2018 (and any other year as well)?
> Yes, this will trigger one year earlier.
> The question is if this is really and issue in typical use cases...
> 

Note that the core will already set the alarm again if it fired too
early. So this is way better than not firing at all.

-- 
Alexandre Belloni, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com