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From: Michael Turquette <mturquette@baylibre.com>
Subject: Re: [GIT PULL] On-demand device probing
Date: Mon, 26 Oct 2015 03:51:38 -0700
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To: "Rafael J. Wysocki" <rafael@kernel.org>, Mark Brown <broonie@kernel.org>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>, Rob Herring <robh+dt@kernel.org>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Tim Bird <tim.bird@sonymobile.com>, "frowand.list@gmail.com" <frowand.list@gmail.com>, Tomeu Vizoso <tomeu.vizoso@collabora.com>, Russell King <linux@arm.linux.org.uk>, Stephen Boyd <sboyd@codeaurora.org>, Vinod Koul <vinod.koul@intel.com>, Dan Williams <dan.j.williams@intel.com>, Linus Walleij <linus.walleij@linaro.org>, Alexandre Courbot <gnurou@gmail.com>, Thierry Reding <thierry.reding@gmail.com>, David Airlie <airlied@linux.ie>, =?utf-8?q?Terje_Bergstr=C3=B6m?= <tbergstrom@nvidia.com>, Stephen Warren <swarren@wwwdotorg.org>, Wolfram Sang <wsa@the-dreams.de>, Grant Likely <grant.likely@linaro.org>, Kishon Vijay Abraham I <kishon@ti.com>, Sebastian Reichel <sre@kernel.org>, Dmitry Eremin-Solenikov <dbary>
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Quoting Rafael J. Wysocki (2015-10-25 06:54:39)
> On Sun, Oct 25, 2015 at 12:06 AM, Mark Brown <broonie@kernel.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.

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

Regards,
Mike

> 
> If the dependency could only be discovered at the probe time, the
> order of things might be changed in response to letting the driver
> core know about it rather than "just in case", which should be more
> efficient.
> 
> Thanks,
> Rafael