On 18/04/2022 19:49:31+0100, Paul Cercueil wrote:
> On power loss, reading the RTC value would fail as the scratchpad lost
> its magic value, until the hardware clock was set once again.
>
> To avoid that, reset the RTC value to Epoch in the probe if we detect
> that the scratchpad lost its magic value.
>
> Signed-off-by: Paul Cercueil <paul@xxxxxxxxxxxxxxx>
> ---
> drivers/rtc/rtc-jz4740.c | 24 +++++++++++++++++++++---
> 1 file changed, 21 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/rtc/rtc-jz4740.c b/drivers/rtc/rtc-jz4740.c
> index 119baf168b32..aac5f68bf626 100644
> --- a/drivers/rtc/rtc-jz4740.c
> +++ b/drivers/rtc/rtc-jz4740.c
> @@ -42,6 +42,9 @@
> /* Magic value to enable writes on jz4780 */
> #define JZ_RTC_WENR_MAGIC 0xA55A
>
> +/* Value written to the scratchpad to detect power losses */
> +#define JZ_RTC_SCRATCHPAD_MAGIC 0x12345678
> +
> #define JZ_RTC_WAKEUP_FILTER_MASK 0x0000FFE0
> #define JZ_RTC_RESET_COUNTER_MASK 0x00000FE0
>
> @@ -134,10 +137,11 @@ static int jz4740_rtc_ctrl_set_bits(struct jz4740_rtc *rtc, uint32_t mask,
> static int jz4740_rtc_read_time(struct device *dev, struct rtc_time *time)
> {
> struct jz4740_rtc *rtc = dev_get_drvdata(dev);
> - uint32_t secs, secs2;
> + uint32_t secs, secs2, magic;
> int timeout = 5;
>
> - if (jz4740_rtc_reg_read(rtc, JZ_REG_RTC_SCRATCHPAD) != 0x12345678)
> + magic = jz4740_rtc_reg_read(rtc, JZ_REG_RTC_SCRATCHPAD);
> + if (magic != JZ_RTC_SCRATCHPAD_MAGIC)
> return -EINVAL;
>
> /* If the seconds register is read while it is updated, it can contain a
> @@ -169,7 +173,8 @@ static int jz4740_rtc_set_time(struct device *dev, struct rtc_time *time)
> if (ret)
> return ret;
>
> - return jz4740_rtc_reg_write(rtc, JZ_REG_RTC_SCRATCHPAD, 0x12345678);
> + return jz4740_rtc_reg_write(rtc, JZ_REG_RTC_SCRATCHPAD,
> + JZ_RTC_SCRATCHPAD_MAGIC);
> }
>
> static int jz4740_rtc_read_alarm(struct device *dev, struct rtc_wkalrm *alrm)
> @@ -307,6 +312,7 @@ static int jz4740_rtc_probe(struct platform_device *pdev)
> struct jz4740_rtc *rtc;
> unsigned long rate;
> struct clk *clk;
> + uint32_t magic;
> int ret, irq;
>
> rtc = devm_kzalloc(dev, sizeof(*rtc), GFP_KERNEL);
> @@ -369,6 +375,18 @@ static int jz4740_rtc_probe(struct platform_device *pdev)
> /* Each 1 Hz pulse should happen after (rate) ticks */
> jz4740_rtc_reg_write(rtc, JZ_REG_RTC_REGULATOR, rate - 1);
>
> + magic = jz4740_rtc_reg_read(rtc, JZ_REG_RTC_SCRATCHPAD);
> + if (magic != JZ_RTC_SCRATCHPAD_MAGIC) {
> + /*
> + * If the scratchpad doesn't hold our magic value, then a
> + * power loss occurred. Reset to Epoch.
> + */
> + struct rtc_time time;
> +
> + rtc_time64_to_tm(0, &time);
> + jz4740_rtc_set_time(dev, &time);
Don't do that, this defeats the purpose of detecting when the power is
lost. Returning a known bogus time is the worst thing you can do here.
> + }
> +
> ret = devm_rtc_register_device(rtc->rtc);
> if (ret)
> return ret;
> --
> 2.35.1
>
--
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com
