linux-kernel - Re: [PATCH RFC] arm: l2x0: Leverage power saving features

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Message-ID: <20130304041927.GA14686@mudshark.cambridge.arm.com>
Date:	Mon, 4 Mar 2013 04:19:27 +0000
From:	Will Deacon <will.deacon@....com>
To:	Soren Brinkmann <soren.brinkmann@...inx.com>
Cc:	Michal Simek <michal.simek@...inx.com>,
	"monstr@...str.eu" <monstr@...str.eu>,
	Russell King <linux@....linux.org.uk>,
	Gregory CLEMENT <gregory.clement@...e-electrons.com>,
	Jason Cooper <jason@...edaemon.net>,
	Tony Lindgren <tony@...mide.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC] arm: l2x0: Leverage power saving features

Hello Soren,

On Fri, Mar 01, 2013 at 06:51:26PM +0000, Soren Brinkmann wrote:
> Enable the 'dynamic clock stop' and 'standby mode' features in the
> l2x0 disable path.
> 
> Signed-off-by: Soren Brinkmann <soren.brinkmann@...inx.com>
> ---
> Hi,
> 
> we are currently implementing a suspend to RAM like low power mode for
> Zynq.
> The code for entering suspend looks like this:
> 	outer_disable();                                                 
>         cpu_suspend(0, zynq_pm_suspend);                                 
>         outer_resume();
> 
> In our low power guidelines we mention the pl310's standby and clock stop
> feature and that they should be enabled for low power modes. The question
> arising here now is: Does the outer_disable() make these features redundant,
> or does it make sense to add it like shown in this patch?

The settings in the power control register are unrelated to CPU suspend
afaict. Instead, they are for semi-autonomous entry to non-destructive
low-power states (e.g. when the L2 is idle for n cycles, or all of the L1
masters have asserted WFI).

This raises the question of whether or not we should poke this register at
all (other than saving/restoring it across suspend, which *is* destructive).
I personally think that this stuff is better off being dealt with in
firmware and initialised there, which follows what we do for other registers
like this.

Will
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