linux-kernel - Re: [PATCH v4] driver core: add a debugfs entry to show deferred devices

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Message-ID: <20180710123648.GA4227@amd>
Date:   Tue, 10 Jul 2018 14:36:49 +0200
From:   Pavel Machek <pavel@....cz>
To:     Sebastian Reichel <sre@...nel.org>
Cc:     Javier Martinez Canillas <javierm@...hat.com>,
        linux-kernel@...r.kernel.org,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        Rob Herring <robh@...nel.org>, Mark Brown <broonie@...nel.org>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Peter Robinson <pbrobinson@...il.com>,
        linux-arm-kernel@...ts.infradead.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v4] driver core: add a debugfs entry to show deferred
 devices

Hi!

> > With Device Trees (DT), the dependencies of the devices are defined in the
> > DT, then the drivers parse that information to lookup the needed resources
> > that have as dependencies.
> > 
> > Since drivers and devices are registered in a non-deterministic way, it is
> > possible that a device that is a dependency has not been registered yet by
> > the time that is looked up.
> > 
> > In this case the driver that requires this dependency cannot probe and has
> > to defer it. So the driver core adds it to a list of deferred devices that
> > is iterated again every time that a new driver is probed successfully.
> > 
> > For debugging purposes it may be useful to know what are the devices whose
> > probe function was deferred. Add a debugfs entry showing that information.
> > 
> >   $ cat /sys/kernel/debug/devices_deferred
> >   48070000.i2c:twl@48:bci
> >   musb-hdrc.0.auto
> >   omapdrm.0
> > 
> > This information could be obtained partially by enabling debugging, but it
> > means that the kernel log has to be parsed and the probe deferral balanced
> > with the successes. This can be error probe and has to be done in a ad-hoc
> > manner by everyone who needs to debug these kind of issues.
> > 
> > Since the information is already known by the kernel, just show it to make
> > it easier to debug.
> > 
> > Signed-off-by: Javier Martinez Canillas <javierm@...hat.com>
> > Reviewed-by: Andy Shevchenko <andy.shevchenko@...il.com>
> > Reviewed-by: Mark Brown <broonie@...nel.org>
> 
> Reviewed-by: Sebastian Reichel <sebastian.reichel@...labora.co.uk>

Thanks for doing this.

Note that it rejects on current -rc4, but solution is trivial.

Tested-by: Pavel Machek <pavel@....cz>

Unfortunately, it does not help with pwm problem on Droid4, as nothing
is deffered:

user@...uan:/sys/bus/platform/drivers$ cat
/sys/kernel/debug/devices_deferred
user@...uan:/sys/bus/platform/drivers$

									Pavel



-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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