linux-kernel - Re: sysfs deferred

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Date:   Thu, 12 Jan 2017 20:46:21 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Rob Herring <robh@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ben Hutchings <ben.hutchings@...ethink.co.uk>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Mark Brown <broonie@...nel.org>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Thierry Reding <thierry.reding@...il.com>
Subject: Re: sysfs deferred_probe attribute

Hi Rob,

On Thu, Jan 12, 2017 at 6:27 PM, Rob Herring <robh@...nel.org> wrote:
> I just noticed that we have a new device attribute 'deferred_probe'
> added in 4.10 with this commit:
>
> commit 6751667a29d6fd64afb9ce30567ad616b68ed789
> Author: Ben Hutchings <ben.hutchings@...ethink.co.uk>
> Date:   Tue Aug 16 14:34:18 2016 +0100
>
>     driver core: Add deferred_probe attribute to devices in sysfs
>
>     It is sometimes useful to know that a device is on the deferred probe
>     list rather than, say, not having a driver available.  Expose this
>     information to user-space.
>
>     Signed-off-by: Ben Hutchings <ben.hutchings@...ethink.co.uk>
>     Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>
>
> It seems like a bad idea to add an ABI for an internal kernel feature.
> When/if we replace deferred probe with something better based on
> functional dependencies are we going to keep this attr around? Or
> remove it and assume no userspace uses it? Perhaps it should be hidden
> behind CONFIG_DEBUG or just make a debugfs file that lists the
> deferred list. Then you wouldn't have to hunt for what got deferred.

FWIW, I had just created a "check-deferred-probe" script that does

    find /sys -name deferred_probe -print0 | xargs -0 grep -v '^0$'

A list would be even better, from the point of view of the user.
As I haven't looked at the implementation, I don't know what impact
that would have on the system due to e.g. locking.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds