linux-kernel - Re: [PATCH v3 0/4] clk: st: New always-on clock domain

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Date:	Thu, 2 Apr 2015 10:31:26 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Lee Jones <lee.jones@...aro.org>
Cc:	Mike Turquette <mturquette@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kernel@...inux.com, Stephen Boyd <sboyd@...eaurora.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [PATCH v3 0/4] clk: st: New always-on clock domain

Hi Lee,

On Thu, Mar 26, 2015 at 8:38 PM, Lee Jones <lee.jones@...aro.org> wrote:
> On Thu, 26 Mar 2015, Geert Uytterhoeven wrote:
>> On Thu, Mar 26, 2015 at 2:51 PM, Lee Jones <lee.jones@...aro.org> wrote:
>> > On Wed, 25 Mar 2015, Geert Uytterhoeven wrote:
>> >> On Mon, Mar 9, 2015 at 10:28 AM, Lee Jones <lee.jones@...aro.org> wrote:
>> >> > On Fri, 06 Mar 2015, Mike Turquette wrote:
>> >> >> This approach looks fine to me. In practice I think it is restricted to
>> >> >> hardware blocks that don't exist in DT yet (e.g. no driver, in the case
>> >> >> of your interconnect) and that restriction is probably for the best.
>> >> >
>> >> > Agreed.
>> >>
>> >> I think this restriction should be documented in the DT binding more clearly,
>> >> as adding a "clk-always-on" node prohibits you from handling the clock
>> >> correctly in
>> >> the future.
>> >
>> > Would you mind taking the time to explain what you think those
>> > limitations are?
>>
>> If you add a "clk-always-on" node, the clock will always on using that DT.
>> That will still be true later, when you get a better understanding of the
>> hardware, and might discover you're gonna need a driver for the currently
>> hidden core component that's driven by the clock, and may want to manage
>> that clock.
>
> So I have two points here.
>
> First point; I think you're looking at an older version of my set.
> The newer one can be found at [1] and no longer uses 'always-on'
> nodes.  Instead the 'clk-always-on' property is applied to the
> provider.  See the documentation patch [2] for more details.

Thanks, I was indeed looking at an old version.
Still, that doesn't change that the clock to not be disabled in specified
explicitly from DT.

> Second point; this binding is _not_ to be used as a hack because the
> hardware isn't understood.  Genuine uses are for clocks that must not
> be turned off ever, else bad things will happen.  If the hardware is
> not understood, use 'clk-disable-unused' on the kernel cmdline
> instead.

[...]

>> (The same is true for devices where the current driver isn't aware of the
>>  clock, and shouldn't be, but you still need to enable the clock until the
>>  driver has Runtime PM support (E.g. ARM GIC on shmobile, cfr.
>>  https://www.marc.info/?l=linux-pm&m=142670617929493&w=3 (good, now
>>  we have a bidirectional link between these two threads :-) Using a
>>  "clk-always-on" property there instead of adding a reference to the clock
>>  in the existing GIC device node would be just lying.)
>
> If this clock should _genuinely_ be always-on, then use my new
> binding in the clock controller node and the Clk framework will not
> turn it off.

It's supposed to be on when the application ARM core(s) is/are running.
Many SoCs also have smaller cores (SH, Cortex R or M), intended to
run a real-time OS. If the RT core is in charge, it may decide to shut down
the application ARM core(s), incl. supposedly always-on modules like
the ARM GIC.

I couldn't find a detailed block diagram of the STiH4xx SoCs, but at least
STiH416 has an "ST proprietary multi-compartmental security IP and DRM
processor".

> [1] https://lkml.org/lkml/2015/2/27/548
> [2] https://lkml.org/lkml/2015/2/27/551

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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