imx6ul: power-up using the RTC

Guy Shapiro guy.shapiro at
Sun Jan 22 03:17:44 PST 2017

On 20/01/2017 17:34, Alexandre Belloni wrote:
> On 20/01/2017 at 13:12:10 +0100, Sebastian Reichel wrote :
>> Hi,
>> On Wed, Jan 18, 2017 at 04:49:11PM +0200, Guy Shapiro wrote:
>>> 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 :)
>> To summarize it looks like this?
>> | regmap_write       | 0x60 | broken                     |
>> | regmap_write       | 0x6b | works stable               |
>> | regmap_update_bits | 0x60 | works only with dbg prints |
>> For me it looks like the debug prints may delay the poweroff driver
>> long enough, that some other driver (rtc?) writes the 0x0b bits.
> Is snvs_rtc_alarm_irq_enable() waiting long enough? I'd say yes but you
> never know... You can also use the kernel tracin infrastructure to trace
> every accesses made to that regmap. That could give you a hint.
> At least you can try a regmap_read before returning from
> snvs_rtc_alarm_irq_enable() and see whether SNVS_LPCR_LPTA_EN and
> SNVS_LPCR_LPWUI_EN are still there.
I added a debug print on snvs_rtc_alarm_irq_enable. It reads and prints the
value of the control register after the write & sync operation.

Immediately after the write to /sys/class/rtc/rtc0/wakealarm the
register value is correct (0x2b).
However, during the shutdown process the function is called again via the
rtc_timer_do_work(). I still do not fully understand this flow. This call is
done with the enable parameter == 1, but my debug print shows 0x29,
meaning the LPTA_EN bit stays low.

My current guess is that the clear of LPTA_EN in snvs_rtc_set_alarm have
internal race with the afterward bit set.
I'll try to check this and update.

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