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

[Paul Cercueil <paul@xxxxxxxxxxxxxxx>]

Hi Alexandre,

Le mar., avril 19 2022 at 21:35:35 +0200, Alexandre Belloni 
<alexandre.belloni@xxxxxxxxxxx> 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@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.

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