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

[Alexandre Belloni <alexandre.belloni@bootlin.com>]

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.

> +	}
> +
>  	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