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

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Message-ID: <20150302081606.GC31325@x1>
Date:	Mon, 2 Mar 2015 08:16:06 +0000
From:	Lee Jones <lee.jones@...aro.org>
To:	Maxime Coquelin <maxime.coquelin@...com>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	mturquette@...aro.org, sboyd@...eaurora.org, kernel@...inux.com,
	devicetree@...r.kernel.org
Subject: Re: [STLinux Kernel] [PATCH 3/4] clk: Provide always-on clock support

On Sat, 28 Feb 2015, Maxime Coquelin wrote:

> Hi Lee,
> 
> On 02/27/2015 10:14 PM, Lee Jones 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.
> 
> Please correct me if I'm wrong, but your patch does not fix the
> issue you had initially.
> Let's take an example:
> A clock is critical for the system, and should never be gated, so
> you add the CLK_IGNORE_UNUSED
> flag so that it is not disabled by clk_disable_unused() procedure.
> The same clock is also used by other IPs, for example spi 0 instance.
> When starting a spi transfer, clk_enable() is called on this clock,
> so its usecount becomes 1.
> Once transfer done, clk_disable() is called, usecount becomes 0 and
> the clock gets disabled: system freeze.

You're right.  I also need to extend clk_core_disable() to take notice
of CLK_IGNORE_UNUSED.

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