linux-kernel - Re: [PATCH v3] driver core: add a debugfs entry to show deferred devices

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Date:   Sun, 8 Jul 2018 15:22:13 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Javier Martinez Canillas <javierm@...hat.com>
Cc:     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
Subject: Re: [PATCH v3] driver core: add a debugfs entry to show deferred
 devices

On Sun, Jul 08, 2018 at 03:21:07PM +0200, Greg Kroah-Hartman wrote:
> On Sun, Jul 08, 2018 at 02:32:56AM +0200, Javier Martinez Canillas wrote:
> > 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>
> > 
> > ---
> > 
> > Changes since RFC v1:
> > - Remove unneeded ret variable from deferred_devs_show()
> > 
> > Changes since RFC v2:
> > - Use DEFINE_SHOW_ATTRIBUTE() macro.
> > - Don't propagate debugfs_create_file() error.
> > - Remove IS_ENABLED(CONFIG_DEBUG_FS) guards.
> > - Drop RFC prefix.
> > 
> > Changes since v1:
> > - Better explain in the commit message why this patch is useful.
> > - Rename deferred_devices entry to devices_deferred.
> > - Add an exit function and remove the debugfs entry.
> > 
> > Changes since v2:
> > - Add Andy Shevchenko and Mark Brown Reviewed-by tag.
> > - Rebase on top of Greg's driver-core-next branch.
> > 
> > 
> >  drivers/base/dd.c | 30 ++++++++++++++++++++++++++++++
> >  1 file changed, 30 insertions(+)
> > 
> > diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> > index 6ea9c5cece7..140f534ee9c 100644
> > --- a/drivers/base/dd.c
> > +++ b/drivers/base/dd.c
> > @@ -16,6 +16,7 @@
> >   * Copyright (c) 2007-2009 Novell Inc.
> >   */
> >  
> > +#include <linux/debugfs.h>
> >  #include <linux/device.h>
> >  #include <linux/delay.h>
> >  #include <linux/dma-mapping.h>
> > @@ -53,6 +54,8 @@ static DEFINE_MUTEX(deferred_probe_mutex);
> >  static LIST_HEAD(deferred_probe_pending_list);
> >  static LIST_HEAD(deferred_probe_active_list);
> >  static atomic_t deferred_trigger_count = ATOMIC_INIT(0);
> > +static struct dentry *deferred_devices;
> > +static bool initcalls_done;
> 
> Wait, why add this variable?  No one uses it that I can see in this
> patch, doesn't it create a build warning?
> 
> You did test-build this, right?

I just tested it, it causes a build warning, which implies that you did
not test this :(

greg k-h