imx6ul: power-up using the RTC

[sre@kernel.org (Sebastian Reichel)]

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.

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 :/

-- Sebastian
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