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Date:   Sun, 4 Oct 2020 10:42:09 +0200
From:   Alexandre Belloni <alexandre.belloni@bootlin.com>
To:     Jonathan =?iso-8859-1?Q?Neusch=E4fer?= <j.neuschaefer@gmx.net>
Cc:     Uwe =?iso-8859-1?Q?Kleine-K=F6nig?= 
        <u.kleine-koenig@pengutronix.de>, linux-kernel@vger.kernel.org,
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        Linus Walleij <linus.walleij@linaro.org>,
        Thierry Reding <thierry.reding@gmail.com>,
        Fabio Estevam <festevam@gmail.com>, linux-rtc@vger.kernel.org,
        Arnd Bergmann <arnd@arndb.de>,
        Mauro Carvalho Chehab <mchehab+huawei@kernel.org>,
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        Daniel Palmer <daniel@0x0f.com>,
        Andy Shevchenko <andy.shevchenko@gmail.com>,
        Andreas Kemnade <andreas@kemnade.info>,
        NXP Linux Team <linux-imx@nxp.com>,
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        "David S. Miller" <davem@davemloft.net>
Subject: Re: [PATCH v3 5/7] rtc: New driver for RTC in Netronix embedded
 controller
Message-ID: <20201004084209.GV2804081@piout.net>
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 <20200925054424.snlr3lggnsv575wu@pengutronix.de>
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On 04/10/2020 03:43:23+0200, Jonathan Neusch?fer wrote:
> > > +static int ntxec_set_time(struct device *dev, struct rtc_time *tm)
> > > +{
> > > +	struct ntxec_rtc *rtc = dev_get_drvdata(dev);
> > > +	int res = 0;
> > > +
> > > +	res = regmap_write(rtc->ec->regmap, NTXEC_REG_WRITE_YEAR, ntxec_reg8(tm->tm_year - 100));
> > > +	if (res)
> > > +		return res;
> > > +
> > > +	res = regmap_write(rtc->ec->regmap, NTXEC_REG_WRITE_MONTH, ntxec_reg8(tm->tm_mon + 1));
> > > +	if (res)
> > > +		return res;
> > > +
> > > +	res = regmap_write(rtc->ec->regmap, NTXEC_REG_WRITE_DAY, ntxec_reg8(tm->tm_mday));
> > > +	if (res)
> > > +		return res;
> > > +
> > > +	res = regmap_write(rtc->ec->regmap, NTXEC_REG_WRITE_HOUR, ntxec_reg8(tm->tm_hour));
> > > +	if (res)
> > > +		return res;
> > > +
> > > +	res = regmap_write(rtc->ec->regmap, NTXEC_REG_WRITE_MINUTE, ntxec_reg8(tm->tm_min));
> > > +	if (res)
> > > +		return res;
> > > +
> > > +	return regmap_write(rtc->ec->regmap, NTXEC_REG_WRITE_SECOND, ntxec_reg8(tm->tm_sec));
> > 
> > I wonder: Is this racy? If you write minute, does the seconds reset to
> > zero or something like that? Or can it happen, that after writing the
> > minute register and before writing the second register the seconds
> > overflow and you end up with the time set to a minute later than
> > intended? If so it might be worth to set the seconds to 0 at the start
> > of the function (with an explaining comment).
> 
> The setting the minutes does not reset the seconds, so I think this race
> condition is possible. I'll add the workaround.
> 

Are you sure this happens? Usually, the seconds are not reset but the
internal 32768kHz counter is so you have a full second to write all the
registers.

> > .read_time has a similar race. What happens if minutes overflow between
> > reading NTXEC_REG_READ_DH and NTXEC_REG_READ_MS?
> 
> Yes, we get read tearing in that case. It could even propagate all the
> way to the year/month field, for example when the following time rolls
> over:
> 	   A   |  B  |  C
> 	2020-10-31 23:59:59
> 	2020-11-01 00:00:00
> 
> - If the increment happens after reading C, we get         2020-10-31 23:59:59
> - If the increment happens between reading B and C, we get 2020-10-31 23:00:00
> - If the increment happens between reading A and B, we get 2020-10-01 00:00:00
> - If the increment happens before reading A, we get        2020-11-01 00:00:00
> 
> ... both of which are far from correct.
> 
> To mitigate this issue, I think something like the following is needed:
> 
> - Read year/month
> - Read day/hour
> - Read minute/second
> - Read day/hour, compare with previously read value, restart on mismatch
> - Read year/month, compare with previously read value, restart on mismatch
> 
> The order of the last two steps doesn't matter, as far as I can see, but
> if I remove one of them, I can't catch all cases of read tearing.
> 

Are you also sure this happens?

Only one comparison is necessary, the correct order would be:

 - Read minute/second
 - Read day/hour
 - Read year/month
 - Read minute/second, compare

If day/hour changes but not minute/second, it would mean that it took at
least an hour to read all the registers. At this point, I think you have
other problems and the exact time doesn't matter anymore.

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