rtc: isl12022: remove wrong warning for low battery level

There are multiple problems with this warning.

First of all, it triggers way too often, in fact nearly on every boot,
because the SR_LBAT85/SR_LBAT75 bits have another meaning when in
battery backup mode. Quoting from the data sheet:

  LOW BATTERY INDICATOR 85% BIT (LBAT85)

  In Normal Mode (VDD), this bit indicates when the battery level has
  dropped below the pre-selected trip levels. [...] The LBAT85
  detection happens automatically once every minute when seconds
  register reaches 59.

  In Battery Mode (VBAT), this bit indicates the device has entered
  into battery mode by polling once every 10 minutes. The LBAT85
  detection happens automatically once when the minute register
  reaches x9h or x0h minutes.

Similar wording applies to the LBAT75 bit.

This means that if the device is powered off for more than 10 minutes,
the LBAT85 bit is guaranteed to be set. Upon power-on, unless we're
close enough to the end of a minute and/or the boot is slow enough
that the second register passes 59, the LBAT85 bit is still set when
the kernel (or early userspace) reads the RTC to set the system's
wallclock time.

Another minor problem is with the bit logic. If the 75% level is
reached, logically we're also below 85%, so both bits would most
likely be set. So even if the battery is below 75%, the warning would
still say "voltage dropped below 85%".

A third problem is that the driver and current DT binding offer no way
to indicate the nominal battery level and/or settings of the Battery
Level Monitor Trip Bits. Since the default value of the VB85TP[2:0] and
VB75TP[2:0] bits are 000, this means the actual setting of the
LBAT85/LBAT75 bits in VDD mode doesn't happen until the battery is below
2.125V/1.875V, which for a standard 3V battery is way too late.

A fourth problem is emitting this warning from ->read_time:
util-linux' hwclock will, in the absence of support for getting an
interrupt when the seconds counter is updated, issue
ioctl(RTC_RD_TIME) in a busy-loop until it sees a change in the
seconds field. In that case, if the battery low bits are set (either
genuinely, more than a minute after boot, due to the battery actually
being low, or as above, bogusly shortly after boot), the kernel log is
swamped with hundreds of identical warnings.

Subsequent patches will add such bindings and driver support, and also
proper support for RTC_VL_READ. For now, remove the broken warning.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Link: https://lore.kernel.org/r/20230615105826.411953-2-linux@rasmusvillemoes.dk
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

1 files changed, 0 insertions, 6 deletions

diff --git a/drivers/rtc/rtc-isl12022.c b/drivers/rtc/rtc-isl12022.c
index a613257d1574..a39aea90230f 100644
--- a/drivers/rtc/rtc-isl12022.c
+++ b/drivers/rtc/rtc-isl12022.c
@@ -141,12 +141,6 @@ static int isl12022_rtc_read_time(struct device *dev, struct rtc_time *tm)
 	if (ret)
 		return ret;
 
-	if (buf[ISL12022_REG_SR] & (ISL12022_SR_LBAT85 | ISL12022_SR_LBAT75)) {
-		dev_warn(dev,
-			 "voltage dropped below %u%%, date and time is not reliable.\n",
-			 buf[ISL12022_REG_SR] & ISL12022_SR_LBAT85 ? 85 : 75);
-	}
-
 	dev_dbg(dev,
 		"raw data is sec=%02x, min=%02x, hr=%02x, mday=%02x, mon=%02x, year=%02x, wday=%02x, sr=%02x, int=%02x",
 		buf[ISL12022_REG_SC],