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

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Date:   Sun, 8 Jul 2018 15:21:07 +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 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?

greg k-h