[PATCH] ARM: smp: Allow real broadcast device selection instead of always dummy

On Wednesday 13 March 2013 05:55 PM, Mark Rutland wrote:

On Wed, Mar 13, 2013 at 11:24:01AM +0000, Santosh Shilimkar wrote:

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On Wednesday 13 March 2013 03:46 PM, Mark Rutland wrote:

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Hi Santosh,

[..]

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Is the problem that the dummy timer is being registered as the broadcast
source, or that it is selected as a local timer in preference of real timers?

Dummy timer is preferred over real broadcast timer.

Ok.

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If it is the former, Then I believe my patch solve the issue more generally -
if you happen to register a dummy timer before other timers, it will become the
broadcast source. Regardless of how temporary this is, it should never happen,
and lowering the rating of the dummy won't fix this.

Well by the time we need active broadcast functionality, clock-events are
already chosen if the ratings are appropriate.

That's a fair point. I still think it's worth having the check in the core
broadcast code - rejecting dummy timers is always a sensible thing to do, and
it prevents future crashes if new timers are added without sensible rating
values.

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If it is the latter, then this patch would ensure that a real timer with a
rating over 100 is selected in preference to the dummy, which is certainly what
we want. The proposed generic dummy timer in Stephen Boyd's local timer API
removal series [1] similarly uses a low rating to ensure that real timers are
selected in preference to an always-registered dummy. I note that the
architected timer has a higher rating (450) than the dummy (400), so this
shouldn't currently be a problem.

Because we always register dummy broadcast on ARM now 
and with higher rating, it is picked as broadcast source. We definitely don't
want such a behavior when we have real broadcast device is available.

Agreed. We *never* want to pick a dummy timer as a broadcast source, as this
never makes sense.

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With your patch, we are trying to avoid the registration which goes
against the the whole idea of registering it always and picking
the right clock-event based on rating by clock-event core.
The clock-event core except the proper ratings to be provided based on
the capability of the timer source and its resolution and in that case
dummy should have the lowest rating which is what I tried to patch.

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Have I missed something?

Not much. I was just looking at x86 code as well after your email
to see how the LAPIC issue is handled. They seems to also have
correct rating to take care of the selection.

Probably we can merge both the fixes but from clock-event core
code perspective, the ratings fix is more than enough.

I do agree it'd be worth lowering the dummy timer's rating to ensure it doesn't
override a real timer elsewhere. 

Yep. Can I add you acked-by tag then for $subject patch ?
Would be good to get this one merged as well.

Regards
Santosh