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

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Date:   Tue, 19 Apr 2022 21:53:17 +0100
From:   Paul Cercueil <paul@...pouillou.net>
To:     Alexandre Belloni <alexandre.belloni@...tlin.com>
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



Le mar., avril 19 2022 at 22:00:32 +0200, Alexandre Belloni 
<alexandre.belloni@...tlin.com> a écrit :
> 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.

htop fails to start and tells me:
"No btime in /proc/stat: No such file or directory"

until the date is reset. So I was assuming it was a case of the jz4740 
driver not being correct and breaking userspace.

Cheers,
-Paul


> 
>>  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