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

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Date:   Tue, 19 Apr 2022 22:00:32 +0200
From:   Alexandre Belloni <alexandre.belloni@...tlin.com>
To:     Paul Cercueil <paul@...pouillou.net>
Cc:     Alessandro Zummo <a.zummo@...ertech.it>, list@...ndingux.net,
        linux-rtc@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mips@...r.kernel.org
Subject: Re: [PATCH 3/5] rtc: jz4740: Reset scratchpad register on power loss

On 19/04/2022 20:48:54+0100, Paul Cercueil wrote:
> Hi Alexandre,
> 
> Le mar., avril 19 2022 at 21:35:35 +0200, Alexandre Belloni
> <alexandre.belloni@...tlin.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@...pouillou.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?
> 

Well, -EINVAL is returned when the time is invalid, this should be
enough. I'm not actually sure what is the issue you are trying to fix
here.

> 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
> 
> 

-- 
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com