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SubjectRe: [PATCH 3/5] rtc: jz4740: Reset scratchpad register on power loss
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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