linux-kernel - Re: [PATCH 3/3] clk: Provide OF helper to mark clocks as CRITICAL

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Date:	Tue, 2 Feb 2016 13:39:58 +0000
From:	Andre Przywara <andre.przywara@....com>
To:	Maxime Ripard <maxime.ripard@...e-electrons.com>
Cc:	Lee Jones <lee.jones@...aro.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	kernel@...inux.com, s.hauer@...gutronix.de, sboyd@...eaurora.org,
	geert@...ux-m68k.org, mturquette@...libre.com,
	maxime.coquelin@...com
Subject: Re: [PATCH 3/3] clk: Provide OF helper to mark clocks as CRITICAL

Hi Maxime,

On 01/02/16 06:32, Maxime Ripard wrote:
> Hi Andre,
> 
> On Wed, Jan 27, 2016 at 11:51:45PM +0000, André Przywara wrote:
>> Hi,
>>
>> On 18/01/16 14:28, Lee Jones wrote:
>>> This call matches clocks which have been marked as critical in DT
>>> and sets the appropriate flag.  These flags can then be used to
>>> mark the clock core flags appropriately prior to registration.
>>
>> I like the idea of having a generic property very much. Also this solves
>> a problem I have in a very elegant way.
> 
> Not really. It has a significant set of drawbacks that we already
> detailed in the initial thread, which are mostly related to the fact
> that the clocks are to be left on is something that totally depends on
> the software support in the kernel. Some clocks should be reported as
> critical because they are simply missing a driver for it, some should
> be because the driver for it as not been compiled, some should because
> we don't have the proper clocks drivers yet for one of their
> downstream clocks.
> 
> Basically, it all boils down to this: some clocks should never ever be
> shutdown because <hardware reason>, and I believe it's the case Lee is
> in. But most of the current code that would use it might, and might
> even need at some point to shut down such a clock.

I was bascically interested in pushing the critical-clock property into
DT to solve that cumbersome clk-sunxi init scheme - which you have fixed
now in a much better way (thanks for that, btw.)
For that particular case the CPU clock really looks like being actually
critical in the hardware sense - no-one maybe except the mgmt core
should turn the one single CPU clock source off.

So I wonder if we should document this "for hardware reasons only" and
still have that property in DT?
At the weekend I coded something into the generic DT clock code to let
it parse for basically every clock node - without a particular driver
needing to ask for it.
If this sounds useful to you I can post that one.

Cheers,
Andre.
> 
> Mike's solution with the flags + handover was solving all this, I'm
> not sure why he's not pushed it forward.
> 
> Maxime
>