linux-kernel - Re: [PATCH v2] clk: Propagate prepare and enable when reparenting orphans

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Date:	Wed, 19 Nov 2014 21:15:41 -0800
From:	Doug Anderson <dianders@...omium.org>
To:	Mike Turquette <mturquette@...aro.org>
Cc:	Heiko Stuebner <heiko@...ech.de>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] clk: Propagate prepare and enable when reparenting orphans

Mike,

On Wed, Nov 19, 2014 at 6:30 PM, Mike Turquette <mturquette@...aro.org> wrote:
> Quoting Doug Anderson (2014-11-07 17:06:58)
>> With the existing code, if you find a parent for an orhpan that has
>> already been prepared / enabled, you won't enable the parent.  That
>> can cause later problems since the clock tree isn't in a consistent
>> state.  Fix by propagating the prepare and enable.
>>
>> NOTE: this does bring up the question about whether the enable of the
>> orphan actually made sense.  If the orphan's parent wasn't enabled by
>> default (by the bootloader or the default state of the hardware) then
>> the original enable of the orphan probably didn't do what the caller
>> though it would.  Some users of the orphan might have preferred an
>> EPROBE_DEFER be returned until we had a full path to a root clock.
>> This patch doesn't address those concerns and really just syncs up the
>> state.
>
> -ECANOFWORMS
>
> I'm thinking about this patch but I haven't quite made up my mind. It is
> reasonable, but also some nice kind of error might be preferable when
> preparing/enabling an orphaned clock.
>
> Under what conditions might a clock be orphaned? An obvious example is
> just bad luck during the thundering herd of clock registrations from a
> driver. In this case deferring the probe might be a good idea.
>
> However what about the case where a loadable module provides the parent
> clock? It might be a long time before the orphan clocks gets picked up
> from the orphanage. Is deferring probe the right thing here as well?

I will defer to your wisdom here.  I agree that these are the two
primary solutions and I've picked one, but I have no idea which will
be more of a PITA in the long run.

Note: I'm not sure that anyone expects EPROBE_DEFER to be returned
from a clk_enable() (do they?).  It almost seems like the right answer
is to fail to allow anyone to clk_get() any clock that doesn't have a
path to root.


I will say that without this patch or the EPROBE_DEFER solution we
have a clear bug.  You can get into a situation where a clock is
enabled/prepared but its parent isn't.

-Doug
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