linux-kernel - Re: [GIT PULL] On-demand device probing

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Message-ID: <CAJZ5v0imLPyq8nxZrqvjXopzD=aBUCp+weBPuJ7Emk9=EQBEnw@mail.gmail.com>
Date:	Tue, 27 Oct 2015 00:37:28 +0100
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Michael Turquette <mturquette@...libre.com>
Cc:	"Rafael J. Wysocki" <rafael@...nel.org>,
	Mark Brown <broonie@...nel.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Rob Herring <robh+dt@...nel.org>,
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Subject: Re: [GIT PULL] On-demand device probing

On Mon, Oct 26, 2015 at 11:51 AM, Michael Turquette
<mturquette@...libre.com> wrote:
> Quoting Rafael J. Wysocki (2015-10-25 06:54:39)
>> On Sun, Oct 25, 2015 at 12:06 AM, Mark Brown <broonie@...nel.org> wrote:
>> > On Sat, Oct 24, 2015 at 04:17:12PM +0200, Rafael J. Wysocki wrote:
>> >
>> >> Well, I'm not quite sure why exactly everyone is so focused on probing here.
>> >
>> > Probe deferral is really noisy even if it's working fine on a given
>> > system so it's constantly being highlighted to people in a way that
>> > other issues aren't if you're not directly having problems.
>> >
>> > There's also the understanding people had that the order things get
>> > bound changes the ordering for some of the other cases (perhaps it's a
>> > good idea to do that, it seems likely to be sensible?).
>>
>> But it really doesn't do that.  Also making it do so doesn't help much
>> in the cases where things can happen asynchronously (system
>> suspend/resume, runtime PM).
>>
>> If, instead, there was a way to specify a functional dependency at the
>> device registration time, it might be used to change the order of
>> everything relevant, including probe.  That should help to reduce the
>> noise you're referring to.
>
> Taking it a step further, if functional dependencies were understood at
> link-time then we could optimize link order as well. There are probably
> lots of optimizations if we only made the effort to understand these
> dependencies earlier.

Do you mean the kernel link time or something else?

At least in some cases the dependency information won't be known at
that time, so we need a way to record a dependency at the time it
becomes visible to us anyway.

> Constructing the device/resource dependency graph before the device ever
> boots sounds interesting to me.

That's only practical if you build the kernel for a specific device.
If you want a generic binary that can work with multiple different
devices, that graph may very well be different for each of them.

Thanks,
Rafael
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