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SubjectRe: [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@redhat.com>
> Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
> Reviewed-by: Mark Brown <broonie@kernel.org>

Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.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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