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The reference manuals of both the RX8803 and RV8803 dictate that
"[On V2F/VLF = ] all registers must be initialized".
The RV-8803 application manual (rev. 1.6) further specifies that crossing
V_LOW2 threshold enables flag V2F and triggers a Power-On reset.
According to table 3.11 in the document, all control registers are
defined to sensible values.
However, The Epson RX-8803 doesn't offer the same guarantees.
It explicitly states:
During the initial power-up, the TEST bit is reset to "0" and the VLF
bit is set to "1".
∗ At this point, all other register values are _undefined_, so be sure to
perform a reset before using the module.
Commit d3700b6b6479 ("rtc: rv8803: Stop the clock while setting the time")
also had this rationale:
Indeed, all the registers must be initialized if the voltage has been
lower than VLOW2 (triggering V2F), but not low enough to trigger a POR.
We should follow the advice and initialize all applicable registers.
We can group the registers into 3 groups:
A) Already correctly handled registers:
* 0B-0Ch | Timer Counter | unused and disabled by clearing TE in 0Dh
* 0Dh | Extension Reg | already initialized in rv8803_regs_configure
* 0Eh | Flag Reg | handled in IRQ handler, except for VLF, VDET
* 0Eh | VLF, VDET | cleared in ->set_time
* 10h | 100th Seconds | Already reset via RESET bit
* 20-21h | Capture Buffer | holds timestamp unused by driver
* 2Fh | Event Control | resets automatically
B) Registers that are hardware initialized on POR, but not on VLF:
* 0Fh | Control Reg
* 2Ch | OSC Offset
C) RAM that is undefined on voltage loss:
* 00-06h | Date/Time
* 07h | RAM
* 08-0Ah | Alarm
This means we should initialize after VLF the registers in group B
(RV8803_CTRL and RV8803_OSC_OFFSET).
Group C is all-zero after voltage loss on the RV-8803, but undefined on
the RX-8803. This is ok for Date/Time because ->get_time returns an
error code for as long as the voltage loss flag is active. It's cleared
on ->set_time however. Zeroing both RAM and alarm ensures a fixed value
is read afterwards.
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220426071056.1187235-4-s.hauer@pengutronix.de
c27fee16fa