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

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Date:	Mon, 2 Mar 2015 11:25:28 +0100
From:	Maxime Coquelin <maxime.coquelin@...com>
To:	Lee Jones <lee.jones@...aro.org>,
	Jassi Brar <jassisinghbrar@...il.com>
CC:	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>,
	Jassi Brar <jaswinder.singh@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [STLinux Kernel] [PATCH 3/4] clk: Provide always-on clock support


On 03/02/2015 11:18 AM, Lee Jones 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.  This solution is saying "one of the drivers is still
> consuming this clock", instead, in the original implementation we're
> saying "we know there are no consumers of this clock, but keep it on
> anyway due to [insert reason here]".
>
So maybe introducing a new "CLK_DISABLE_NEVER" flag will be more
explicit than hacking around "CLK_IGNORE_UNUSED" one?

BR,
Maxime
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