linux-kernel - Re: [PATCH v2] driver core: add a debugfs entry to show deferred devices

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Date:   Sun, 8 Jul 2018 02:31:49 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
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 v2] driver core: add a debugfs entry to show deferred
 devices

Hi Greg,

On 07/07/2018 05:59 PM, Greg Kroah-Hartman wrote:
> On Thu, Jun 28, 2018 at 12:06:56AM +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: Mark Brown <broonie@...nel.org>
> 
> This doesn't apply to my tree anymore :(
>

I see, I made sure that it applied on top of linux-next.
 
> Can you rebase and resend?
>

I guess you want me to rebase on top of your driver-core-next branch. I think
that linux-next should pull that branch instead of the driver-core-linus one:

https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/Next/Trees#n29

> thanks,
> 
> greg k-h
> 

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