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

From: Rob Lee
Date: Thu May 10 2012 - 10:11:16 EST

On Thu, May 10, 2012 at 7:41 AM, Sascha Hauer <s.hauer@xxxxxxxxxxxxxx> wrote:
> 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'll run some tests to check.


>> 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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