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