linux-kernel - Re: [PATCH 1/2] rtc: pch-rtc: add RTC driver for Intel Series PCH

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Message-Id: <20210820123410.46539-1-fr0st61te@gmail.com>
Date:   Fri, 20 Aug 2021 15:34:10 +0300
From:   Ivan Mikhaylov <fr0st61te@...il.com>
To:     fercerpav@...il.com
Cc:     a.zummo@...ertech.it, alexandre.belloni@...tlin.com,
        i.mikhaylov@...ro.com, linux-kernel@...r.kernel.org,
        openbmc@...ts.ozlabs.org
Subject: Re: [PATCH 1/2] rtc: pch-rtc: add RTC driver for Intel Series PCH

On Tue, Aug 15, 2021 at 01:52:42PM +0300, Paul Fertser wrote:
> 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?

Yes, from infromation that I know.

> > +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?

1. .use_single_read in regmap_config.
2. Maybe that is the right place for rollover check.

> > +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?

It just a synonym, does the same. I can change it but first it's need to
be decided if attribute would go or not.

Thanks.