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Date:   Sun, 15 Aug 2021 01:52:42 +0300
From:   Paul Fertser <fercerpav@gmail.com>
To:     Ivan Mikhaylov <i.mikhaylov@yadro.com>
Cc:     Alessandro Zummo <a.zummo@towertech.it>,
        Alexandre Belloni <alexandre.belloni@bootlin.com>,
        openbmc@lists.ozlabs.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] rtc: pch-rtc: add RTC driver for Intel Series PCH
Message-ID: <20210814225242.GY15173@home.paul.comp>
References: <20210810154436.125678-1-i.mikhaylov@yadro.com>
 <20210810154436.125678-2-i.mikhaylov@yadro.com>
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On Tue, Aug 10, 2021 at 06:44:35PM +0300, Ivan Mikhaylov wrote:
> +config RTC_DRV_PCH
> +	tristate "PCH RTC driver"
> +	help
> +	  If you say yes here you get support for the Intel Series PCH

I'm afraid this is really lacking some specification of devices that
are supported. Is it really everything that Intel currently calls PCH?

> +static int pch_rtc_read_time(struct device *dev, struct rtc_time *tm)
> +{
> +	struct i2c_client *client = to_i2c_client(dev);
> +	struct pch *pch = i2c_get_clientdata(client);
> +	unsigned char rtc_data[NUM_TIME_REGS] = {0};
> +	int rc;
> +
> +	rc = regmap_bulk_read(pch->regmap, PCH_REG_SC, rtc_data, NUM_TIME_REGS);
> +	if (rc < 0) {
> +		dev_err(dev, "fail to read time reg(%d)\n", rc);
> +		return rc;
> +	}

Citing 26.7.2.3 from C620 (Lewisburg/Purley) datasheet:

"The PCH SMBus slave interface only supports Byte Read operation. The
external SMBus master will read the RTC time bytes one after
another. It is software’s responsibility to check and manage the
possible time rollover when subsequent time bytes are read.

For example, assuming the RTC time is 11 hours: 59 minutes: 59
seconds. When the external SMBus master reads the hour as 11, then
proceeds to read the minute, it is possible that the rollover happens
between the reads and the minute is read as 0. This results in 11
hours: 0 minutes instead of the correct time of 12 hours: 0 minutes.
Unless it is certain that rollover will not occur, software is
required to detect the possible time rollover by reading multiple
times such that the read time bytes can be adjusted accordingly if
needed."

Should this be taken additional care of somehow?

> +static ssize_t force_off_store(struct device *dev,
> +			       struct device_attribute *attr,
> +			       const char *buf, size_t count)
> +{
> +	struct i2c_client *client = to_i2c_client(dev);
> +	struct pch *pch = i2c_get_clientdata(client);
> +	unsigned long val;
> +	int rc;
> +
> +	if (kstrtoul(buf, 10, &val))
> +		return -EINVAL;
> +
> +	if (val) {
> +		/* 0x02 host force off */

I wonder why you write "host force off" while the C620 datasheet calls
it "Unconditional Power Down", does your PCH manual use different
naming?

In any case this doesn't belong to an RTC driver, as previously noted.

-- 
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