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@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?
>
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
