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

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Date:	Mon, 2 Mar 2015 10:40:15 +0000
From:	Lee Jones <lee.jones@...aro.org>
To:	Jassi Brar <jaswinder.singh@...aro.org>
Cc:	Jassi Brar <jassisinghbrar@...il.com>,
	Devicetree List <devicetree@...r.kernel.org>,
	Mike Turquette <mturquette@...aro.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 Mon, 02 Mar 2015, Jassi Brar wrote:

> 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?

I would hope that the user would know that some communication channels
(interconnects in our case) would have to stay alive.  When everything
is turned off, the platform is considered powered down.  Most people
who are going to care about this stuff know that platforms consume
power when suspended.

I should think the user would be even more confused if the reference
count was >0, but there was no information regarding who is consuming
it.

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