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

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Date:   Thu, 24 May 2018 21:01:26 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Rob Herring <robh@...nel.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, devicetree@...r.kernel.org,
        boot-architecture@...ts.linaro.org,
        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 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 :)

thanks,

greg k-h