Re: [PATCH 1/3] ARM: OMAP2+: 32k-counter: Use hwmod lookup to check presence of 32k timer

["Shilimkar, Santosh" <santosh.shilimkar@xxxxxx>]

On Wed, Mar 21, 2012 at 5:12 PM, Hiremath, Vaibhav <hvaibhav@xxxxxx> wrote:
> On Mon, Mar 19, 2012 at 17:45:32, Shilimkar, Santosh wrote:
>> On Monday 19 March 2012 05:14 PM, Ming Lei wrote:
>> > On Mon, Mar 19, 2012 at 7:11 PM, Hiremath, Vaibhav <hvaibhav@xxxxxx> wrote:
>> >>
>> >> I think you made very good point here. With the above patch, we are almost missing the capability of registering dmtimer as a clocksource for OMAP.
>> >> It will always use 32k-counter, and never fall back to dmtimer.
>> >>
>> >> Then the only options we have here is,
>> >>
>> >> 1) Register both the timers, 32k-counter and dmtimer for clocksource; let
>> >>   Kernel pick up best rating clocksource out of these two.
>> >>
>> >>   In case of OMAP1/2/3/4, kernel will use dmtimer, since it has better
>> >>   Rating. User can choose the 32k-counter clocksource via bootargs.
>> >>
>> >>   Impact: without bootargs for clocksource selection, kernel will choose
>> >>     dmtimer, impacting loss of time during suspend/resume.
>> >>
>> This is the right option. The problem is gptimer clocksource
>> doesn't work across power transitions and hence it is broken.
>>
>> Even for the perf, with PM enabled, dmtimer can't be used or it needs
>> to be used with 32KHz clock which makes it no better than sync timer.
>>
>> So here keeping 32K sync timer is default clocksource makes sense since
>> it is the only working and viable option.
>>
>> So what can be done is register both 32K and gptimer together but make
>> gptimer clocksource registration depends on PM enabled.
>>
>> This should solve all the needs I guess.
>>
>
> I am not sure this will work on all platforms, for example, AM33XX, where
> We do not have 32ksync timer available, but we need PM support. Also, I
> personally think, we should not again use compile time option here.
>
Why not ?
If 32ksync timer is not available, don't register it. Then in AM case
you just have one clock-source. I am against the idea of making
gptimer as the default and asking user to choose sync32k from
command-line.

Btw, if you need PM, how are you going to use GPTIMER
as a clocksource. Note sys-clock is generally stopped in
low power states. So that leaves you option with using
gptimer with 32K clock and in that case, GPTIMER
is not a better clock-source compare to 32K sync timer
and so shouldn't be the rating.

Regards
Santosh
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