Re: [PATCH 03/13] RTC: ds1307: Add DS1341 specific power-saving options

From: Alexandre Belloni
Date: Tue Jul 19 2016 - 18:47:36 EST

On 21/06/2016 at 19:34:56 -0700, Andrey Smirnov wrote :
> On Tue, Jun 21, 2016 at 2:07 PM, Alexandre Belloni
> <alexandre.belloni@xxxxxxxxxxxxxxxxxx> wrote:
> > On 21/06/2016 at 15:49:04 -0500, Rob Herring wrote :
> >> So wouldn't you want to set one mode while running and the lower power
> >> mode while suspended? I'm trying to understand the frequency of changing
> >> this. If it is always one setting for a board, then yes it belongs in
> >> DT. If it is a user decision, then it probably shouldn't be in DT.
> >>
> >> Seeing as these are reused, I've probably already had this discussion...
> >>
> >
> > I would agree with Rob here. It may be better to provide a sysfs
> > interface to configure that particular behavior.
> I don't see any value in doing that, could you give me a realistic
> example of a scenario in which a user would want to spend some of
> uptime with RTC oscillator fault detection/glitch filtering disabled
> and then enable it?

Well, the issue is not being dynamic, it is differentiating between
hardware description and user configuration. Configuration must not be in
DT. And this choice is definitively not hardware related (as opposed to
the trickle charging that depends on the battery that is used on the

> > This is usually ok because the use case is:
> > - the RTC is not configured, time has never been set
> > - time is set for the first time
> > - the user can set the oscillator mode/detection/...
> Unfortunately exposing that feature using sysfs gives you a leaky
> abstraction and your userspace instead of using a generic RTC starts
> using DS1341 RTC. So to accommodate for that a user would have to:
> a) Write + integrate a userspace tool to set the mode (which IMHO is
> decided upon once and doesn't change)
> b) If a board design is new and there's a chance of moving this chip
> to a different I2C bus, the code above would have to account for that
> and not hardcore sysfs path
> c) If board's BSP is intended to be used in multiple generations of a
> product, not all of which would use DS1341, it would be necessary to
> accommodate for that by either more code in the tool or an additional
> BSP build configuration variant

Well, it doesn't leak abstraction as long as all the RTC that are able
to disable the oscillator failure detection use the same ABI.

> > - on subsequent reboots, the mode is kept alongside the time and date
> >
> This assumes that your bootloader leaves those mode bits alone.

Well, if that is not the case, the bootloader as to be fixed anyway and
silently changing the configuration back using DT is probably worse.

> > I would advise against trying to set a mode automatically in the driver
> > because you may have unexpected power cuts and it may then let the RTC
> > consume more power than what you really want.
> >
> I fell like I am not understanding you correctly. Why would moving
> configuration decision logic into userspace improve the situation in
> case of unexpected power loss?

I was reacting to Rob's idea to running in the lower power mode when
suspend. I'm pretty sure this is not a good idea. I'd leave the whole
policy to userspace.

Alexandre Belloni, Free Electrons
Embedded Linux and Kernel engineering