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From: Alexandre Belloni <alexandre.belloni@bootlin.com>
To: "Jonathan Neuschäfer" <j.neuschaefer@gmx.net>
Cc: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>,
	linux-kernel@vger.kernel.org, "Heiko Stuebner" <heiko@sntech.de>,
	linux-pwm@vger.kernel.org,
	"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>,
	"Sam Ravnborg" <sam@ravnborg.org>,
	"Daniel Palmer" <daniel@0x0f.com>,
	"Andy Shevchenko" <andy.shevchenko@gmail.com>,
	"Andreas Kemnade" <andreas@kemnade.info>,
	"NXP Linux Team" <linux-imx@nxp.com>,
	devicetree@vger.kernel.org,
	"Stephan Gerhold" <stephan@gerhold.net>,
	allen <allen.chen@ite.com.tw>,
	"Sascha Hauer" <s.hauer@pengutronix.de>,
	"Lubomir Rintel" <lkundrak@v3.sk>,
	"Rob Herring" <robh+dt@kernel.org>,
	"Lee Jones" <lee.jones@linaro.org>,
	linux-arm-kernel@lists.infradead.org,
	"Alessandro Zummo" <a.zummo@towertech.it>,
	"Mark Brown" <broonie@kernel.org>,
	"Pengutronix Kernel Team" <kernel@pengutronix.de>,
	"Heiko Stuebner" <heiko.stuebner@theobroma-systems.com>,
	"Josua Mayer" <josua.mayer@jm0.eu>,
	"Shawn Guo" <shawnguo@kernel.org>,
	"David S. Miller" <davem@davemloft.net>
Subject: Re: [PATCH v3 5/7] rtc: New driver for RTC in Netronix embedded controller
Date: Sun, 4 Oct 2020 10:42:09 +0200
Message-ID: <20201004084209.GV2804081@piout.net> (raw)
In-Reply-To: <20201004014323.GD500800@latitude>

On 04/10/2020 03:43:23+0200, Jonathan Neuschäfer 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

  reply index

Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-24 19:24 [PATCH v3 0/7] Netronix embedded controller driver for Kobo and Tolino ebook readers Jonathan Neuschäfer
2020-09-24 19:24 ` [PATCH v3 1/7] dt-bindings: Add vendor prefix for Netronix, Inc Jonathan Neuschäfer
2020-10-12  7:19   ` Uwe Kleine-König
2020-10-12 11:48     ` Lee Jones
2020-09-24 19:24 ` [PATCH v3 2/7] dt-bindings: mfd: Add binding for Netronix embedded controller Jonathan Neuschäfer
2020-09-29 18:52   ` Rob Herring
2020-09-24 19:24 ` [PATCH v3 3/7] mfd: Add base driver " Jonathan Neuschäfer
2020-09-25  9:29   ` Andy Shevchenko
2020-09-25 21:32     ` Jonathan Neuschäfer
2020-09-29 16:37       ` Andy Shevchenko
2020-10-02 22:54         ` Jonathan Neuschäfer
2020-09-24 19:24 ` [PATCH v3 4/7] pwm: ntxec: Add driver for PWM function in Netronix EC Jonathan Neuschäfer
2020-09-25  6:30   ` Uwe Kleine-König
2020-09-27 21:10     ` Jonathan Neuschäfer
2020-09-28  8:35       ` Uwe Kleine-König
2020-10-02 23:36         ` Jonathan Neuschäfer
2020-10-03  8:17           ` Andy Shevchenko
2020-09-24 19:24 ` [PATCH v3 5/7] rtc: New driver for RTC in Netronix embedded controller Jonathan Neuschäfer
2020-09-24 20:40   ` Andreas Kemnade
2020-10-02 23:41     ` Jonathan Neuschäfer
2020-09-25  5:44   ` Uwe Kleine-König
2020-10-04  1:43     ` Jonathan Neuschäfer
2020-10-04  8:42       ` Alexandre Belloni [this message]
2020-10-11 19:09         ` Jonathan Neuschäfer
2020-10-05  7:35       ` Uwe Kleine-König
2020-09-25  9:36   ` Alexandre Belloni
2020-09-25 22:00     ` Jonathan Neuschäfer
2020-09-24 19:24 ` [PATCH v3 6/7] MAINTAINERS: Add entry for " Jonathan Neuschäfer
2020-09-25  5:08 ` [PATCH v3 7/7] ARM: dts: imx50-kobo-aura: Add " Jonathan Neuschäfer
2020-10-07  7:46   ` Krzysztof Kozlowski
2020-10-11 19:42     ` Jonathan Neuschäfer

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