Re: [GIT PULL] On-demand device probing

[Tomeu Vizoso <tomeu.vizoso@collabora.com>]

On 22 October 2015 at 02:54, Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
> On Tuesday, October 20, 2015 06:21:55 PM Tomeu Vizoso wrote:
>> On 20 October 2015 at 18:04, Alan Stern <stern@rowland.harvard.edu> wrote:
>> > On Tue, 20 Oct 2015, Mark Brown wrote:
>> >
>> >> On Tue, Oct 20, 2015 at 10:40:03AM -0400, Alan Stern wrote:
>> >>
>> >> > Furthermore, that applies only to devices that use synchronous suspend.
>> >> > Async suspend is becoming common, and there the only restrictions are
>> >> > parent-child relations plus whatever explicit requirements that drivers
>> >> > impose by calling device_pm_wait_for_dev().
>> >>
>> >> Hrm, this is the first I'd noticed that feature though I see the initial
>> >> commit dates from January.
>> >
>> > Async suspend and device_pm_wait_for_dev() were added in January 2010,
>> > not 2015!
>> >
>> >>  It looks like most of the users are PCs at
>> >> the minute but we should be using it more widely for embedded things,
>> >> there's definitely some cases I'm aware of where it will allow us to
>> >> remove some open coding.
>> >>
>> >> It does seem like we want to be feeding dependency information we
>> >> discover for probing way into the suspend dependencies...
>> >
>> > Rafael has been thinking about a way to do this systematically.
>> > Nothing concrete has emerged yet.
>>
>> This iteration of the series would make this quite easy, as
>> dependencies are calculated before probes are attempted:
>>
>> https://lkml.org/lkml/2015/6/17/311
>
> Well, if you know how to represent "links" between devices, the mechanism
> introduced here doesn't really add much value, because in that case the
> core knows what the dependencies are in the first place and can only
> defer the probes that have to be deferred.

By "here" you mean what you are proposing for ordering device
suspends, or on-demand probing?

If you meant that probing on-demand is unneeded if we already have
dependency information, I agree with you and that's why I only pushed
forward on-demand, as the approach linked above introduced some
duplication when inferring the dependencies. Maybe that could be
avoided without too much refactoring.

In any case, Thierry mentioned the other day in #tegra that one could
also collect dependency information as a follow up to the on-demand
series by calling device_depend() or such instead of
of_device_probe().

Regards,

Tomeu

> Thanks,
> Rafael
>
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