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

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Date:   Tue, 10 Jul 2018 12:01:52 +0200
From:   Sebastian Reichel <sre@...nel.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,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v4] driver core: add a debugfs entry to show deferred
 devices

Hi,

On Sun, Jul 08, 2018 at 03:34:59PM +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>

Reviewed-by: Sebastian Reichel <sebastian.reichel@...labora.co.uk>

-- Sebastian

> ---
> 
> 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.
> 
> Changes since v3:
> - Fixed a build warning caused by a wrong conflict resolution on v3.
> 
> 
>  drivers/base/dd.c | 29 +++++++++++++++++++++++++++++
>  1 file changed, 29 insertions(+)
> 
> diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> index 6ea9c5cece7..e85705e8440 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 +54,7 @@ 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;
>  
>  /*
>   * In some cases, like suspend to RAM or hibernation, It might be reasonable
> @@ -199,6 +201,24 @@ void device_unblock_probing(void)
>  	driver_deferred_probe_trigger();
>  }
>  
> +/*
> + * deferred_devs_show() - Show the devices in the deferred probe pending list.
> + */
> +static int deferred_devs_show(struct seq_file *s, void *data)
> +{
> +	struct device_private *curr;
> +
> +	mutex_lock(&deferred_probe_mutex);
> +
> +	list_for_each_entry(curr, &deferred_probe_pending_list, deferred_probe)
> +		seq_printf(s, "%s\n", dev_name(curr->device));
> +
> +	mutex_unlock(&deferred_probe_mutex);
> +
> +	return 0;
> +}
> +DEFINE_SHOW_ATTRIBUTE(deferred_devs);
> +
>  /**
>   * deferred_probe_initcall() - Enable probing of deferred devices
>   *
> @@ -208,6 +228,9 @@ void device_unblock_probing(void)
>   */
>  static int deferred_probe_initcall(void)
>  {
> +	deferred_devices = debugfs_create_file("devices_deferred", 0444, NULL,
> +					       NULL, &deferred_devs_fops);
> +
>  	driver_deferred_probe_enable = true;
>  	driver_deferred_probe_trigger();
>  	/* Sort as many dependencies as possible before exiting initcalls */
> @@ -216,6 +239,12 @@ static int deferred_probe_initcall(void)
>  }
>  late_initcall(deferred_probe_initcall);
>  
> +static void __exit deferred_probe_exit(void)
> +{
> +	debugfs_remove_recursive(deferred_devices);
> +}
> +__exitcall(deferred_probe_exit);
> +
>  /**
>   * device_is_bound() - Check if device is bound to a driver
>   * @dev: device to check
> -- 
> 2.17.1
> 

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