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

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Date:	Thu, 2 Apr 2015 10:09:19 +0530
From:	Jassi Brar <jassisinghbrar@...il.com>
To:	Michael Turquette <mturquette@...aro.org>
Cc:	Jassi Brar <jaswinder.singh@...aro.org>,
	Lee Jones <lee.jones@...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 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);

cheers!
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