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

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Message-ID: <20201005222953.GD2804081@piout.net>
Date:   Tue, 6 Oct 2020 00:29:53 +0200
From:   Alexandre Belloni <alexandre.belloni@...tlin.com>
To:     Peter Geis <pgwipeout@...il.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 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.

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