imx6ul: power-up using the RTC

Guy Shapiro guy.shapiro at
Sun Jan 22 03:15:20 PST 2017

Sorry for the bad mailer :(    Resending...

On 20/01/2017 14:12, 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 |

Yes, though I managed to add some useful debug prints while keeping the code

> For me it looks like the debug prints may delay the poweroff driver
> long enough, that some other driver (rtc?) writes the 0x0b bits.

The problem happens even if I add a long sleep between the RTC setting
and the power down command.

> FWIW we can't switch from regmap_write to regmap_update_bits in the
> syscon driver, since that may break other users. I guess we need to
> introduce a DT property providing the update mask. Unfortunately the
> register value property is already named mask :/

I agree. After fixing the bug, I'll start a discussion about this with
the DT list.

More information about the linux-arm-kernel mailing list