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

[Javier Martinez Canillas <javierm@redhat.com>]

On 07/08/2018 03:22 PM, Greg Kroah-Hartman wrote:
> 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@redhat.com>
>>> Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
>>> Reviewed-by: Mark Brown <broonie@kernel.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?
>>

Argh, sorry about that. It was my bad when doing the conflict resolution. This
variable got removed by commit 0a50f61c4fb ("drivers: base: initcall_debug logs
for driver probe times") that made the patch to not apply cleanly anymore.

>> You did test-build this, right?
> 
> I just tested it, it causes a build warning, which implies that you did
> not test this :(
>

I did but missed the build warning.

> greg k-h
>

Best regards,
-- 
Javier Martinez Canillas
Software Engineer - Desktop Hardware Enablement
Red Hat