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

[Santosh Shilimkar]

On Wednesday 13 March 2013 03:46 PM, Mark Rutland wrote:
> Hi Santosh,
> 
> On Wed, Mar 13, 2013 at 09:28:22AM +0000, Santosh Shilimkar wrote:
>> (Forgot to CC Thomas)
>>
>> On Wednesday 13 March 2013 02:36 PM, Santosh Shilimkar wrote:
>>> With recent arm broadcast time clean-up from Mark Rutland, the dummy
>>> broadcast device is always registered with timer subsystem. And since
>>> the rating of the dummy clock event is very high, it is preferred
>>> over a real broad-cast clock event.
>>>
>>> This is a change in behavior from past and not an intended
>>> one. So reduce the rating of the dummy clockevent so that
>>> real broadcast device is selected when available.
>>>
>>> Without this all the C states with C3STOP won't work since
>>> the broad cast notifier will take an abort.
>>>
>>> Cc: Mark Rutland <mark.rutland at arm.com>
>>> Cc: Russell King <linux at arm.linux.org.uk>
>>>
>>> Signed-off-by: Santosh Shilimkar <santosh.shilimkar at ti.com>
>>> ---
>>> Its a regression so hopefully can get into the 3.9-rcx. Noticed
>>> this one on A15 platform. A9 platform the issue may not be seen
>>> since the local timer check avoids dummy timer registration.
>>>
>> Some one pointed me to a fix made by Mark which was discussed
>> under '[BUG] ARM Architected timers appear broken in 3.9-rc1' subject.
>> That patch seems to be more of work around since the root of the
>> problem is incorrect dummy timer rating. Either way, both patches
>> fix the issue.
> 
> 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.
 
> 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.

> 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.


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.

> 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.

Thomas might be able to comment better.

Regards,
Santosh