Re: [PATCH] OMAP3: PM: Dynamic calculation of SDRC clock stabilization delay

[Kevin Hilman <khilman@deeprootsystems.com>]

Romit Dasgupta <romit@ti.com> writes:

> Kevin Hilman wrote:
>> Kevin Hilman <khilman@deeprootsystems.com> writes:
>> 
>> [...]
>> 
>>> PMC code is ARM generic and already largely exists in other places
>>> (oprofile for one.)  So the use of the PMC will need to be generalized
>>> as well as be shown not to interfere with other users (as raised
>>> already by Jean Pihet.)
>> 
>> Someone is already trying to generalize a PMC interface.  You might want
>> to follow this thread:
>> 
>> http://lists.infradead.org/pipermail/linux-arm-kernel/2009-December/005898.html
>> 
>
> I tried to find the code in LO but I think it is not yet present. 

Right, it is still being discussed.

> So with that in mind may I propose that the pmc functions we
> introduce in plat-omap will be present as __init code (so that we
> ensure no one uses it after the system finishes booting up) as long
> as the pmc infrastructure is not available for non-Oprofile uses? 

Personally, I don't like this idea.

IMHO, the PMC is not OMAP specific and does not belong in plat-omap
even as init code.  Either generalize and use existing PMC
infrastructure, or use a different timer.

You didn't answer my other question about whether or not a GPtimer
could be used for this.  You could quickly request/program/free a
GPtimer for this too using the omap_dm_timer* API.

> As I mentioned in an earlier thread, the pmc is used and stopped
> very early during the kernel boot and AFAICT there should not be any
> problem (probably we need to check its behavior on EMU/HS devices).

This isn't very convicing.  

Also, it's not just oprofile we have to be worried about.  The
perf/trace infrastructure arm is also using the PMC.  It will not be
uncommon to use perf on early boot/init and the use of the PMC in this
patch will clearly interfere with that.

In summary, the PMC is a shared resource and should be treated as such
using common code and common APIs.

Kevin