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 [thread overview]
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
next prev parent reply other threads:[~2020-10-04 8:42 UTC|newest]
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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