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

From: Alexandre Belloni
Date: Wed Jul 20 2016 - 05:02:32 EST

On 19/07/2016 at 16:56:56 -0700, Andrey Smirnov wrote :
> >> 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.
> Why? And I don't mean in a generic sense, but in this particular case.
> What is gained by not having this bit of configuration, whose only
> consumer is the driver, in the device tree file?

Because configuration doesn't belong to DT. DT is about hardware
description, not configuration.

> > And this choice is definitively not hardware related (as opposed to
> > the trickle charging that depends on the battery that is used on the
> > board).
> There's most certainly plenty of precedents of non hardware-related in
> device tree, first two that come to mind are "chosen" node and
> "local-mac-address" property and, granted, those might be
> exceptions/legacy bindings that are just there to stay, but even if
> you look at RTC subsystem rtc-cmos.txt, atmel,at91sam-rtc.txt and
> possibly rtc-st-lpc.txt are providing bindings that are not exactly
> hardware related.
> Rtc-cmos.txt is especially noticeable example since it literally does
> what I am trying to do -- allows the user to specify initial values to
> certain registers that would be initialized by the driver.

Well, the RTC subsystem has been left unmaintained for a while and it is
not because we made mistakes in the past that we have to make them

rtc-st-lpc is only hardware related. The mode it is used in is board
dependant. And I have a plan to change how the gpbr register is
allocated for the at91 RTT. I agree that rtc-cmos is an example of what
not to do.

> > 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.
> Correct me if I am wrong, but no such, at least semi-standardized, ABI
> exist as of today, correct? If that is so, then what you are saying is
> that the abstraction doesn't leak as long as you put it inside of a
> new abstraction that doesn't leak. I am not arguing that it is
> impossible to create a new one that would allow to hide hardware
> differences, I am positive it is, what I am arguing is that to do so
> is a lot of work for as far as I can see for no gain.

You are correct, that ABI doesn't exist and I'm asking to create it.
That is how kernel development happens.

> >
> >> > - 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.
> >
> How so? Consider the following two scenarios with assumption that the
> bootloader is broken:
> - Bits set wrong by bootloader, then corrected by kernel, device is
> powered off RTC consumes expected amount of current
> - Bits set wrong by bootloader, kernel does nothing, device is
> powered off RTC consumes more than anticipated and we drain the power
> storage device and loose time.
> What do you you think former is worse than latter?

Whether is is done in the kernel or in userspace doesn't change much
to that use case.

Alexandre Belloni, Free Electrons
Embedded Linux and Kernel engineering