Re: [PATCH v4 08/25] kernel: Add combined power-off+restart handler call chain API

["Michał Mirosław" <mirq-linux@rere.qmqm.pl>]

On Mon, Nov 29, 2021 at 12:04:01AM +0300, Dmitry Osipenko wrote:
> 28.11.2021 03:43, Michał Mirosław пишет:
> > On Fri, Nov 26, 2021 at 09:00:44PM +0300, Dmitry Osipenko wrote:
> >> SoC platforms often have multiple ways of how to perform system's
> >> power-off and restart operations. Meanwhile today's kernel is limited to
> >> a single option. Add combined power-off+restart handler call chain API,
> >> which is inspired by the restart API. The new API provides both power-off
> >> and restart functionality.
> >>
> >> The old pm_power_off method will be kept around till all users are
> >> converted to the new API.
> >>
> >> Current restart API will be replaced by the new unified API since
> >> new API is its superset. The restart functionality of the sys-off handler
> >> API is built upon the existing restart-notifier APIs.
> >>
> >> In order to ease conversion to the new API, convenient helpers are added
> >> for the common use-cases. They will reduce amount of boilerplate code and
> >> remove global variables. These helpers preserve old behaviour for cases
> >> where only one power-off handler is expected, this is what all existing
> >> drivers want, and thus, they could be easily converted to the new API.
> >> Users of the new API should explicitly enable power-off chaining by
> >> setting corresponding flag of the power_handler structure.
> > [...]
> > 
> > Hi,
> > 
> > A general question: do we really need three distinct chains for this?
> 
> Hello Michał,
> 
> At minimum this makes code easier to follow.
> 
> > Can't there be only one that chain of callbacks that get a stage
> > (RESTART_PREPARE, RESTART, POWER_OFF_PREPARE, POWER_OFF) and can ignore
> > them at will? Calling through POWER_OFF_PREPARE would also return
> > whether that POWER_OFF is possible (for kernel_can_power_off()).
> 
> I'm having trouble with parsing this comment. Could you please try to
> rephrase it? I don't see how you could check whether power-off handler
> is available if you'll mix all handlers together.

If notify_call_chain() would be fixed to return NOTIFY_OK if any call
returned NOTIFY_OK, then this would be a clear way to gather the
answer if any of the handlers will attempt the final action (reboot or
power off).

> 
> > I would also split this patch into preparation cleanups (like wrapping
> > pm_power_off call with a function) and adding the notifier-based
> > implementation.
> 
> What's the benefit of this split up will be? Are you suggesting that it
> will ease reviewing of this patch or something else?

Mainly to ease review, as the wrapping will be a no-op, but the addition
of notifier chain changes semantics a bit.

Best Regards
Michał Mirosław