linux-kernel - Re: [PATCH 3/4] clk: Provide always-on clock support

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Message-ID: <20150401014218.25195.61119@quantum>
Date:	Tue, 31 Mar 2015 18:42:18 -0700
From:	Michael Turquette <mturquette@...aro.org>
To:	Jassi Brar <jaswinder.singh@...aro.org>,
	"Lee Jones" <lee.jones@...aro.org>
Cc:	"Jassi Brar" <jassisinghbrar@...il.com>,
	"Devicetree List" <devicetree@...r.kernel.org>, kernel@...inux.com,
	"Stephen Boyd" <sboyd@...eaurora.org>,
	"lkml" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 3/4] clk: Provide always-on clock support

Quoting Jassi Brar (2015-03-02 02:28:44)
> On 2 March 2015 at 15:48, Lee Jones <lee.jones@...aro.org> wrote:
> > On Mon, 02 Mar 2015, Jassi Brar wrote:
> >
> >> On Mon, Mar 2, 2015 at 2:06 PM, Lee Jones <lee.jones@...aro.org> wrote:
> >> > On Sat, 28 Feb 2015, Jassi Brar wrote:
> >> >
> >> >> On 28 February 2015 at 02:44, Lee Jones <lee.jones@...aro.org> wrote:
> >> >> > Lots of platforms contain clocks which if turned off would prove fatal.
> >> >> > The only way to recover from these catastrophic failures is to restart
> >> >> > the board(s).  Now, when a clock is registered with the framework it is
> >> >> > compared against a list of provided always-on clock names which must be
> >> >> > kept ungated.  If it matches, we enable the existing CLK_IGNORE_UNUSED
> >> >> > flag, which will prevent the common clk framework from attempting to
> >> >> > gate it during the clk_disable_unused() procedure.
> >> >> >
> >> >> If a clock is critical on a certain board, it could be got+enabled
> >> >> during early boot so there is always a user.
> >> >
> >> > I tried this.  There was push-back from the DT maintainers.
> >> >
> >> >   http://lists.infradead.org/pipermail/linux-arm-kernel/2015-February/324417.html
> >> >
> >> Thanks, I wasn't aware of the history.
> >>
> >> >> To be able to do that from DT, maybe add a new, say, CLK_ALWAYS_ON
> >> >> flag could be made to initialize the clock with one phantom user
> >> >> already. Or just reuse the CLK_IGNORE_UNUSED?
> >> >
> >> > How is that different to what this set is doing?
> >> >
> >> The phantom user - that's there but none can see it.
> >>
> >> How about?
> >>
> >> +       of_property_for_each_string(np, "clock-always-on", prop, clkname) {
> >> +               clk = __clk_lookup(clkname);
> >> +               if (!clk)
> >> +                       continue;
> >> +
> >> +               clk->core->enable_count = 1;
> >> +               clk->core->prepare_count = 1;
> >> +       }
> >
> > This is only fractionally different from the current implementation.
> >
> > I believe the current way it slightly nicer, as we don't have to fake
> > the user count.
> >
> Well... the user is indeed there, isn't it? It's just not known to
> Linux. So 'fake' isn't most applicable here.
> Otherwise you might have to stub out some existing and future
> functions for CLK_IGNORE_UNUSED. And how do we explain to userspace
> which would see power drawn but no user of the clock?

Jassi,

This is broken. What if the parent of this clock has
{enable,prepare}_count of zero? The way we propagate these refcounts up
the tree would fall over.

Regards,
Mike

> 
> Anways, I am OK either way.
> 
> Cheers!
> Jassi
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