imx6ul: power-up using the RTC

[Alexandre Belloni]

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.



-- 
Alexandre Belloni, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com