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

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Message-ID: <20150402071027.GW9447@x1>
Date:	Thu, 2 Apr 2015 08:10:27 +0100
From:	Lee Jones <lee.jones@...aro.org>
To:	Jassi Brar <jassisinghbrar@...il.com>
Cc:	Michael Turquette <mturquette@...aro.org>,
	Jassi Brar <jaswinder.singh@...aro.org>,
	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

On Thu, 02 Apr 2015, Jassi Brar wrote:

> On Wed, Apr 1, 2015 at 7:12 AM, Michael Turquette <mturquette@...aro.org> wrote:
> > 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.
> >
> Yeah it needs to be done at higher level,
> - clk->core->enable_count = 1;
> - clk->core->prepare_count = 1;
> + clk_prepare_enable(clk);

FYI: https://lkml.org/lkml/2015/4/1/267

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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