linux-kernel - Re: [PATCH v2 2/8] driver core: add a deferred probe timeout

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Date:   Thu, 24 May 2018 14:45:48 -0500
From:   Rob Herring <robh@...nel.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Alexander Graf <agraf@...e.de>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Kevin Hilman <khilman@...nel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Joerg Roedel <joro@...tes.org>,
        Robin Murphy <robin.murphy@....com>,
        Mark Brown <broonie@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        devicetree@...r.kernel.org,
        Architecture Mailman List <boot-architecture@...ts.linaro.org>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 2/8] driver core: add a deferred probe timeout

On Thu, May 24, 2018 at 2:01 PM, Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
> On Thu, May 24, 2018 at 12:50:18PM -0500, Rob Herring wrote:
>> Deferring probe can wait forever on dependencies that may never appear
>> for a variety of reasons. This can be difficult to debug especially if
>> the console has dependencies or userspace fails to boot to a shell. Add
>> a timeout to retry probing without possibly optional dependencies and to
>> dump out the deferred probe pending list after retrying.
>>
>> This mechanism is intended for debug purposes. It won't work for the
>> console which needs to be enabled before userspace starts. However, if
>> the console's dependencies are resolved, then the kernel log will be
>> printed (as opposed to no output).
>>
>> Signed-off-by: Rob Herring <robh@...nel.org>
>> ---
>>  .../admin-guide/kernel-parameters.txt         |  7 +++++
>>  drivers/base/dd.c                             | 28 ++++++++++++++++++-
>>  2 files changed, 34 insertions(+), 1 deletion(-)
>>
>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>> index 11fc28ecdb6d..dd3f40b34a24 100644
>> --- a/Documentation/admin-guide/kernel-parameters.txt
>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>> @@ -809,6 +809,13 @@
>>                       Defaults to the default architecture's huge page size
>>                       if not specified.
>>
>> +     deferred_probe_timeout=
>> +                     [KNL] Set a timeout in seconds for deferred probe to
>> +                     give up waiting on dependencies to probe. Only specific
>> +                     dependencies (subsystems or drivers) that have opted in
>> +                     will be ignored. This option also dumps out devices
>> +                     still on the deferred probe list after retrying.
>
> Doesn't sound like a debugging-only option.  I can see devices enabling
> this when they figure out that's the only way their platform can boot :)

Here's some rope...

No doubt it can be abused. So are you saying don't call it a debug
option or hide it behind a config option? And for the latter, what's
one that distros don't just turn on?

Rob