Messages in this thread | | | Date | Tue, 19 Apr 2022 20:48:54 +0100 | From | Paul Cercueil <> | Subject | Re: [PATCH 3/5] rtc: jz4740: Reset scratchpad register on power loss |
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Hi Alexandre,
Le mar., avril 19 2022 at 21:35:35 +0200, Alexandre Belloni <alexandre.belloni@bootlin.com> a écrit : > 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@crapouillou.net> >> --- >> 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.
So what is the best thing to do then?
Cheers, -Paul
>> + } >> + >> 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
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