linux-kernel - Re: [PATCH v3 2/3] ARM: imx: Add imx5 cpuidle driver

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Date:	Thu, 10 May 2012 14:41:30 +0200
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	Rob Lee <rob.lee@...aro.org>
Cc:	kernel@...gutronix.de, shawn.guo@...aro.org,
	amit.kucheria@...aro.org, daniel.lezcano@...aro.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linaro-dev@...ts.linaro.org, patches@...aro.org, jj@...osbits.net
Subject: Re: [PATCH v3 2/3] ARM: imx: Add imx5 cpuidle driver

On Wed, May 09, 2012 at 09:27:02AM -0500, Rob Lee wrote:
> Sascha,
> 
> >
> > This clk_get should go away here and be moved somewhere to
> > initialization. Also, if getting this clock fails we can still
> > do regular cpu_do_idle. Additionally, if clk_get fails, we'll
> > have a ERR_PTR value in gpc_dvfs_clk in which case the
> > gpc_dvfs_clk == NULL won't trigger next time you are here and
> > then you'll enable a nonexisting clock below.
> >
> 
> Agree.  I'd prefer to enable this clock during intialization and just
> leave it running.  It is supposed to be a very low power clock and I
> couldn't measuring any power difference with and without it being
> enabled

Ok, even better.

> >
> > I wonder why you don't add the default ARM_CPUIDLE_WFI_STATE_PWR state.
> > The above is something different, right? It has a greater exit latency
> > than ARM_CPUIDLE_WFI_STATE_PWR, so why don't we add it here aswell?
> 
> Yes and no.  Yes this is a different state but no, it doesn't have a
> significantly greater exit latency, or at least a large enough exit
> latency to warrant an extra state in my opinion.  According to the
> i.MX5 documentation, the extra exit time beyond basic WFI required for
> the  "WAIT_UNCLOCKED_POWER_OFF" state is 500ns (this is due to a
> difference in i.MX5 hardware implementation compared to all other ARM
> platforms).  In reality, it did require a few more microseconds to
> perform in my testing just based on the extra register writes in
> mx5_cpu_lp_set().  I'd like to clean up mx5_cpu_lp_set() and add a
> global variable to track the previous state and to just exit out if
> the new state is the same as the old.

Do you think it's worth it? You buy skipping the read with an additional
test.

> I could do this cleanup as part of this patchset if you prefer that.

Yes please. Cleanups before adding new features is always a good reason
to apply a patch series ;)

Sascha

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