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

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Date:	Mon, 1 Feb 2016 08:22:45 +0000
From:	Lee Jones <lee.jones@...aro.org>
To:	Maxime Ripard <maxime.ripard@...e-electrons.com>
Cc:	André Przywara <andre.przywara@....com>,
	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

On Mon, 01 Feb 2016, Maxime Ripard wrote:
> 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.

Exactly.  This is a not a CLK_DRIVER_NOT_{AUTHORED|UPSTREAM} or
CLK_DRIVER_NOT_ENABLED implementation, it's for CLK_CRITICALs.
Critical clocks must _never_ be turned off, no matter what, else
something really bad will happen.  In our use-case, if the clocks are
turned of, it will be catastrophic to the running system.

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

Right, but I think you are missing part of the conversation.  Mike and
I had a face-to-face meeting in San Francisco last year.  The
conclusion was that the CLK_CRITICAL and CLK_HANDOVER solutions should
be separated.  Different handling, different code.  This submission
only solves the former problem.  I believe Mike was going to submit
and follow-up on the CLK_HANDOVER solution separately.

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