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From: "Jonathan Neuschäfer" <j.neuschaefer@gmx.net>
To: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>
Cc: "Jonathan Neuschäfer" <j.neuschaefer@gmx.net>,
	linux-kernel@vger.kernel.org,
	"Alexandre Belloni" <alexandre.belloni@bootlin.com>,
	"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 03:43:23 +0200
Message-ID: <20201004014323.GD500800@latitude> (raw)
In-Reply-To: <20200925054424.snlr3lggnsv575wu@pengutronix.de>


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On Fri, Sep 25, 2020 at 07:44:24AM +0200, Uwe Kleine-König wrote:
> Hello Jonathan,
> 
> On Thu, Sep 24, 2020 at 09:24:53PM +0200, Jonathan Neuschäfer wrote:
> > +#define NTXEC_REG_WRITE_YEAR	0x10
> > +#define NTXEC_REG_WRITE_MONTH	0x11
> > +#define NTXEC_REG_WRITE_DAY	0x12
> > +#define NTXEC_REG_WRITE_HOUR	0x13
> > +#define NTXEC_REG_WRITE_MINUTE	0x14
> > +#define NTXEC_REG_WRITE_SECOND	0x15
> > +
> > +#define NTXEC_REG_READ_YM	0x20
> > +#define NTXEC_REG_READ_DH	0x21
> > +#define NTXEC_REG_READ_MS	0x23
> 
> Is this an official naming? I think at least ..._MS is a poor name.
> Maybe consider ..._MINSEC instead and make the other two names a bit longer
> for consistency?

It's inofficial (the vendor kernel uses 0x10 etc. directly).
I'll pick longer names.

> > +static int ntxec_read_time(struct device *dev, struct rtc_time *tm)
> > +{
> > +	struct ntxec_rtc *rtc = dev_get_drvdata(dev);
> > +	unsigned int value;
> > +	int res;
> > +
> > +	res = regmap_read(rtc->ec->regmap, NTXEC_REG_READ_YM, &value);
> > +	if (res < 0)
> > +		return res;
> > +
> > +	tm->tm_year = (value >> 8) + 100;
> > +	tm->tm_mon = (value & 0xff) - 1;
> > +
> > +	res = regmap_read(rtc->ec->regmap, NTXEC_REG_READ_DH, &value);
> > +	if (res < 0)
> > +		return res;
> > +
> > +	tm->tm_mday = value >> 8;
> > +	tm->tm_hour = value & 0xff;
> > +
> > +	res = regmap_read(rtc->ec->regmap, NTXEC_REG_READ_MS, &value);
> > +	if (res < 0)
> > +		return res;
> > +
> > +	tm->tm_min = value >> 8;
> > +	tm->tm_sec = value & 0xff;
> > +
> > +	return 0;
> > +}
> > +
> > +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.

> .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.

> > +static struct platform_driver ntxec_rtc_driver = {
> > +	.driver = {
> > +		.name = "ntxec-rtc",
> > +	},
> > +	.probe = ntxec_rtc_probe,
> 
> No .remove function?

I don't think it would serve a purpose in this driver. There are no
device-specific resources to release (no clocks to unprepare, for
example).


Thanks,
Jonathan Neuschäfer

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  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 [this message]
2020-10-04  8:42       ` Alexandre Belloni
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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