linux-kernel - Re: [Regression] Missing device nodes for ETR, ETF and STM after CONFIG_UEVENT

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Date:   Fri, 26 Jul 2019 15:54:18 +0530
From:   Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Geert Uytterhoeven <geert+renesas@...der.be>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Leo Yan <leo.yan@...aro.org>, linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [Regression] Missing device nodes for ETR, ETF and STM after
 CONFIG_UEVENT_HELPER=n

On 7/26/2019 3:49 PM, Greg Kroah-Hartman wrote:
> On Fri, Jul 26, 2019 at 03:44:40PM +0530, Sai Prakash Ranjan wrote:
>> On 7/26/2019 3:14 PM, Sai Prakash Ranjan wrote:
>>> On 7/26/2019 2:11 PM, Greg Kroah-Hartman wrote:
>>>> On Fri, Jul 26, 2019 at 01:50:27PM +0530, Sai Prakash Ranjan wrote:
>>>>> On 7/26/2019 12:34 PM, Greg Kroah-Hartman wrote:
>>>>>> On Fri, Jul 26, 2019 at 11:49:19AM +0530, Sai Prakash Ranjan wrote:
>>>>>>> Hi,
>>>>>>>
>>>>>>> When trying to test my coresight patches, I found that etr,etf and stm
>>>>>>> device nodes are missing from /dev.
>>>>>>
>>>>>> I have no idea what those device nodes are.
>>>>>>
>>>>>>> Bisection gives this as the bad commit.
>>>>>>>
>>>>>>> 1be01d4a57142ded23bdb9e0c8d9369e693b26cc is the first bad commit
>>>>>>> commit 1be01d4a57142ded23bdb9e0c8d9369e693b26cc
>>>>>>> Author: Geert Uytterhoeven <geert+renesas@...der.be>
>>>>>>> Date:   Thu Mar 14 12:13:50 2019 +0100
>>>>>>>
>>>>>>>        driver: base: Disable CONFIG_UEVENT_HELPER by default
>>>>>>>
>>>>>>>        Since commit 7934779a69f1184f ("Driver-Core:
>>>>>>> disable /sbin/hotplug by
>>>>>>>        default"), the help text for the /sbin/hotplug fork-bomb says
>>>>>>>        "This should not be used today [...] creates a
>>>>>>> high system load, or
>>>>>>>        [...] out-of-memory situations during bootup".
>>>>>>> The rationale for this
>>>>>>>        was that no recent mainstream system used this
>>>>>>> anymore (in 2010!).
>>>>>>>
>>>>>>>        A few years later, the complete uevent helper
>>>>>>> support was made optional
>>>>>>>        in commit 86d56134f1b67d0c ("kobject: Make support
>>>>>>> for uevent_helper
>>>>>>>        optional.").  However, if was still left enabled
>>>>>>> by default, to support
>>>>>>>        ancient userland.
>>>>>>>
>>>>>>>        Time passed by, and nothing should use this
>>>>>>> anymore, so it can be
>>>>>>>        disabled by default.
>>>>>>>
>>>>>>>        Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>
>>>>>>>        Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>>>>>>>
>>>>>>>     drivers/base/Kconfig | 1 -
>>>>>>>     1 file changed, 1 deletion(-)
>>>>>>>
>>>>>>>
>>>>>>> Any idea on this?
>>>>>>
>>>>>> That means that who ever created those device nodes is relying on udev
>>>>>> to do this, and is not doing the correct thing within the kernel and
>>>>>> using devtmpfs.
>>>>>>
>>>>>> Any pointers to where in the kernel those devices are trying to be
>>>>>> created?
>>>>>>
>>>>>
>>>>> Somewhere in drivers/hwtracing/coresight/* probably. I am not sure,
>>>>> Mathieu/Suzuki would be able to point you to the exact code.
>>>>>
>>>>> Also just to add on some more details, I am using *initramfs*
>>>>
>>>> Are you using devtmpfs for your /dev/ mount?
>>>>
>>>
>>> I am not mounting devtmpfs. However
>>>
>>>    CONFIG_DEVTMPFS=y
>>>    CONFIG_DEVTMPFS_MOUNT=y
>>>
>>
>> Ok my initramfs is using mdev:
>>
>> */sbin/mdev -s*
>>
>> This somehow is not mounting etr, etf, stm devices when uevent-helper is
>> disabled. Anyways as Suzuki mentioned, using devtmpfs does fix the issue.
> 
> Last I looked (many years ago) mdev requires uevent-helper in order for
> it to work.  I recommend that if you rely on mdev to keep that option
> enabled, or to just use devtmpfs and udev :)
> 

Ok that explains, its not a hard requirement for me to use mdev. I just 
happen to stumble upon this issue. Thanks for your time :)

Thanks,
Sai

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