linux-kernel - Re: [Question] rtc wake behavior and sysfs

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Date:   Mon, 5 Oct 2020 21:47:01 -0400
From:   Peter Geis <pgwipeout@...il.com>
To:     Alexandre Belloni <alexandre.belloni@...tlin.com>
Cc:     a.zummo@...ertech.it, "Rafael J. Wysocki" <rjw@...ysocki.net>,
        len.brown@...el.com, pavel@....cz,
        Greg KH <gregkh@...uxfoundation.org>,
        linux-rtc@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-tegra@...r.kernel.org
Subject: Re: [Question] rtc wake behavior and sysfs

On Mon, Oct 5, 2020 at 6:29 PM Alexandre Belloni
<alexandre.belloni@...tlin.com> wrote:
>
> On 05/10/2020 09:13:08-0400, Peter Geis wrote:
> > Good Morning,
> >
> > While testing suspend to ram on the Ouya, I encountered an interesting
> > issue with the rtc-tps65910 driver.
> > Attempting to use rtc-wake on the default configuration returned:
> > rtcwake: /dev/rtc0 not enabled for wakeup events
> > This is due to:
> > eb5eba4ef722 drivers/rtc/rtc-tps65910.c: enable/disable wake in suspend/resume
> > This commit changed this driver's behavior to not enable wakeup by
> > default, but enables it when entering sleep mode.
> > This seems to be odd behavior to me.
> > Looking at a few other rtc drivers show they simply enable themselves
> > as wakeup sources by default.
> >
> > I also found the sysfs entries are at /sys/devices/ ..
> > /tps65910-rtc/power but are missing at /sys/class/rtc/rtc0/power/
> >
> > I have two questions.
> >  - Should the sysfs wakeup entries be missing at /sys/class/rtc/rtc0/power/ ?
>
> I would be in /sys/class/rtc/rtc0/device/power
>
> >  - Shouldn't a rtc be enabled as a wakeup source by default?
> >
>
> The short answer is no, the reason being that not all RTCs are connected
> to an IRQ or a pin that can wakeup or start the platform. What should be
> done is enabling wakeup only when interrupts are available or the
> wakeup-source property is in the rtc device tree node.

Thank you for your explanation.

So it would seem we have two issues.
- The sysfs wakeup entries are not populating in
/sys/class/rtc/rtc0/power when they are populating in
/sys/devices/<..>/tps65910-rtc/power.
- The wakeup-source property is not being applied to the tps65910-rtc
sub-device when it is applied in the tps65910 devicetree node.

Should wakeup-source handling be the tps65910-rtc driver's
responsibility, or is that a limitation of the driver core that needs
to be fixed?

>
> --
> Alexandre Belloni, Bootlin
> Embedded Linux and Kernel engineering
> https://bootlin.com