linux-kernel - Re: [PATCH v2 4/4] clk: dt: Introduce always-on clock domain documentation

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Date:	Thu, 19 Feb 2015 10:28:10 +0000
From:	Lee Jones <lee.jones@...aro.org>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:	Rob Herring <robherring2@...il.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Mike Turquette <mturquette@...aro.org>,
	Stephen Boyd <sboyd@...eaurora.org>, kernel@...inux.com,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [PATCH v2 4/4] clk: dt: Introduce always-on clock domain
 documentation

On Thu, 19 Feb 2015, Geert Uytterhoeven wrote:

> Hi Lee,
> 
> On Thu, Feb 19, 2015 at 11:11 AM, Lee Jones <lee.jones@...aro.org> wrote:
> > On Thu, 19 Feb 2015, Geert Uytterhoeven wrote:
> >> On Thu, Feb 19, 2015 at 10:42 AM, Lee Jones <lee.jones@...aro.org> wrote:
> >> >> What kind of clocks are these? What do they control?
> >> >> Memory controllers? Bus controllers?
> >> >>
> >> >> They must control some device(s), so there should be one or more device
> >> >> nodes in DT that reference these clocks.
> >> >> As soon as that information is in DT, support can be added to Linux to
> >> >> make sure the "critical" clocks stay enabled, either through a real driver,
> >> >> or through platform code.
> >> >
> >> > Some do, some don't.  For instance, we have one clock which controls
> >> > SPI and I2C that must not be turned off.  We discovered this then when
> >> > a suspend was attempted and the board refused to resume.  This clock
> >> > also runs one of the critical interconnects that runs from the a9.  It
> >> > would be wrong to remove the clk_disable() attempt from the SPI/I2C
> >> > drivers because the same IP on another board might be controlled by a
> >> > different clock which is able to be gated.
> >> >
> >> > There are also clocks which control other interconnects that are not
> >> > connected to any device drivers.  If we fail to take references for
> >> > them before clk_disable_unused() is called, again the board hangs.  We
> >> > even lose JTAG support.
> >>
> >> Interconnects are buses. Can't you represent those buses in the DT
> >> hierarchy, and give them clocks properties?
> >
> > So instead of this nice succinct, simple, cover all bases
> > (interconnects was just an example, there are bound to be others),
> > generic framework, you are suggesting to write drivers for devices
> > which other than "don't turn my clocks off", Linux can't actually see
> > or control?
> 
> DT describes the hardware, not behavior.

Okay so ...

/*
 * ICNs are not visible/controllable in Linux, but references to their
 * clocks must be obtained and retained or the platform will become
 * irrecoverably unresponsive.
 */
interconnects@0 {
       compatible = "always-on-clk-domain";
       clocks = <&clk_s_c0_flexgen CLK_ICN_SBC>,
                <&clk_s_c0_flexgen CLK_ICN_LMI>,
                <&clk_s_c0_flexgen CLK_ICN_CPU>,
                <&clk_s_c0_flexgen CLK_TX_ICN_DMU>;
};

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