[rtc-linux] Re: imx6ul: power-up using the RTC

From: Guy Shapiro <guy.shapiro@mobi-wize.com>
To: Alexandre Belloni <alexandre.belloni@free-electrons.com>,
	Sebastian Reichel <sre@kernel.org>
Cc: b20788@freescale.com, shawnguo@kernel.org,
	Frank.Li@freescale.com, kernel@pengutronix.de,
	fabio.estevam@nxp.com, a.zummo@towertech.it,
	linux-arm-kernel@lists.infradead.org, rtc-linux@googlegroups.com,
	linux-pm@vger.kernel.org
Subject: [rtc-linux] Re: imx6ul: power-up using the RTC
Date: Sun, 22 Jan 2017 12:46:24 +0200	[thread overview]
Message-ID: <7a2d9e48-9a58-7d79-bc36-05a9b790cce3@mobi-wize.com> (raw)
In-Reply-To: <20170120153459.tewyg54dcgqnpz2j@piout.net>

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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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