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Date:   Tue, 13 Jun 2023 10:58:54 +0200
From:   Alexandre Belloni <alexandre.belloni@bootlin.com>
To:     Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc:     Alessandro Zummo <a.zummo@towertech.it>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        devicetree@vger.kernel.org, Rob Herring <robh+dt@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Conor Dooley <conor+dt@kernel.org>, linux-rtc@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 6/8] rtc: isl12022: trigger battery level detection
 during probe
Message-ID: <2023061308585481c7cb97@mail.local>
References: <20230612113059.247275-1-linux@rasmusvillemoes.dk>
 <20230612113059.247275-7-linux@rasmusvillemoes.dk>
 <20230612141518c2119835@mail.local>
 <f3dc01bc-cdd1-ab0c-5891-083f6d255a4c@rasmusvillemoes.dk>
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On 13/06/2023 09:44:55+0200, Rasmus Villemoes wrote:
> On 12/06/2023 16.15, Alexandre Belloni wrote:
> > On 12/06/2023 13:30:56+0200, Rasmus Villemoes wrote:
> >> Since the meaning of the SR_LBAT85 and SR_LBAT75 bits are different in
> >> battery backup mode, they may very well be set after power on, and
> >> stay set for up to a minute (i.e. until the battery detection in VDD
> >> mode happens when the seconds counter hits 59). This would mean that
> >> userspace doing a ioctl(RTC_VL_READ) early on could get a false
> >> positive.
> >>
> >> The battery level detection can also be triggered by explicitly
> >> writing a 1 to the TSE bit in the BETA register. Do that once during
> >> boot (well, probe), and emit a single warning to the kernel log if the
> >> battery is already low.
> >>
> >> Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> >> ---
> >>  drivers/rtc/rtc-isl12022.c | 19 ++++++++++++++++++-
> >>  1 file changed, 18 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/rtc/rtc-isl12022.c b/drivers/rtc/rtc-isl12022.c
> >> index 1b6659a9b33a..690dbb446d1a 100644
> >> --- a/drivers/rtc/rtc-isl12022.c
> >> +++ b/drivers/rtc/rtc-isl12022.c
> >> @@ -280,8 +280,25 @@ static void isl12022_set_trip_levels(struct device *dev)
> >>  	mask = ISL12022_REG_VB85_MASK | ISL12022_REG_VB75_MASK;
> >>  
> >>  	ret = regmap_update_bits(regmap, ISL12022_REG_PWR_VBAT, mask, val);
> >> -	if (ret)
> >> +	if (ret) {
> >>  		dev_warn(dev, "unable to set battery alarm levels: %d\n", ret);
> >> +		return;
> >> +	}
> >> +
> >> +	ret = regmap_write_bits(regmap, ISL12022_REG_BETA,
> >> +				ISL12022_BETA_TSE, ISL12022_BETA_TSE);
> >> +	if (ret) {
> >> +		dev_warn(dev, "unable to trigger battery level detection: %d\n", ret);
> > 
> > This is too verbose, there is no action for the user upon getting this
> > message.
> 
> OK.
> 
> > Setting TSE also enables temperature compensation, which may be an
> > undesirable side effect. Shouldn't this be reverted if necessary?
> 
> Well, I can't imagine the board designer not wanting/expecting
> temperature compensation to be enabled since they've spent the $$ on
> using a part with that capability. Also, we anyway set TSE if
> CONFIG_HWMON so that the TEMP registers get updated once per minute.
> 
> If you insist I'll do the proper logic to set it back to 0 if it wasn't
> set beforehand, but I prefer to just keep it as-is.
> 

Ok, fine

> > 
> >> +		return;
> >> +	}
> >> +
> >> +	ret = isl12022_read_sr(regmap);
> >> +	if (ret < 0) {
> >> +		dev_warn(dev, "unable to read status register: %d\n", ret);
> >> +	} else if (ret & (ISL12022_SR_LBAT85 | ISL12022_SR_LBAT75)) {
> >> +		dev_warn(dev, "battery voltage is below %u%%\n",
> >> +			 (ret & ISL12022_SR_LBAT75) ? 75 : 85);
> > 
> > This message is useless, I'd drop the whole block.
> 
> I only added this as "compensation" for ripping out the warning in 1/8,
> since I assumed somebody actually wanted at least one warning in the
> kernel log if the battery is low.
> 

No need, I had a patch removing the message anyway.

> I/we are not going to scrape dmesg but will use the ioctl() to monitor
> the battery, so I'm perfectly happy to just remove this. That will also
> make the question of how long to wait after setting TSE moot, since
> certainly userspace won't be able to issue the ioctl() soon enough to
> see stale values in the LBAT bits.
> 

Exactly.

> Rasmus
> 

-- 
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com