I'm trying to use the low power RTC of the i.MX6UL to start from
power-off state.

I started by hopefully running: # echo +30 >
/sys/class/rtc/rtc0/wakealarm && shutdown -h now The system was
powered down, but it didn't come up after 30 seconds as expected.

So I dug into the datasheet and the source...
To activate the power on alarm, the flags SRTC_ENV, LPTA_EN and LPWUI_EN on
the SNVS_LP Control register (LPCR) should be asserted. The wakeup time
should be written to the SNVS_LP Time alarm register (LPTA). The code that
does this is on drivers/rtc/rtc-snvs.c:snvs_rtc_set_alarm().

The first problem I found was with the use of the syscon-poweroff driver.
The "Turn off System Power" flag is part of the same register (LPCR). The
current code of syscon-poweroff set the register to the "mask" property from
the device tree on power off, overriding all the existing flags.
After setting the "mask" property on the device tree to 0x6b instead of
0x60 (asserting the mentioned bits), the system do power up on timer, as
expected.

However, I didn't like the idea of keeping those flags on even when no one
set the alarm.
As a quick test, I modified the syscon-poweroff driver to ignore the bits
that are not on the mask:

diff --git a/drivers/power/reset/syscon-poweroff.c

b/drivers/power/reset/syscon-poweroff.c
index b683383..a5da02b 100644

--- a/drivers/power/reset/syscon-poweroff.c
+++ b/drivers/power/reset/syscon-poweroff.c

@@ -33,7 +33,7 @@ static u32 mask;

static void syscon_poweroff(void)
{
        /* Issue the poweroff */
-       regmap_write(map, offset, mask);
+       regmap_update_bits(map, offset, mask, mask);
        mdelay(1000);


After applying this fix, the wake up alarm didn't work. Strangely, when I
added some debug prints to investigate the case, it worked again
(sometimes... depends on the exact places I add the prints).
I suspect that some other driver clears the flags during the power down, but
I couldn't find such driver.

Do you have any clue what code may change this register during the shutdown
process? Any other insights are welcomed as well :)