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

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Date:	Sat, 28 Feb 2015 08:52:23 +0100
From:	Maxime Coquelin <maxime.coquelin@...com>
To:	Lee Jones <lee.jones@...aro.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>
CC:	<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

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.

BR,
Maxime

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