[PATCH 5/5] arm: exynos: Add MCPM call-back functions

On Thu, Apr 17, 2014 at 04:20:26PM +0100, Nicolas Pitre wrote:

On Thu, 17 Apr 2014, Dave Martin wrote:

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On Thu, Apr 17, 2014 at 12:41:22AM +0530, Abhilash Kesavan wrote:

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The fact that there is no C interface for enabling ACE ports is
deliberate.  For CPUs connected to ACE and managed via MCPM,
it is incorrect to enable CCI via C code, since the safe window
is the window during which all outbound CPUs have reached CLUSTER_DOWN
and all inbound CPUs have not turned their MMU on yet (and thus cannot
execute any general Linux C code).

There might be scenarios involving GPUs and other non-CPU devices
connected to ACE ports where the device cannot enable CCI snoops
for itself -- but this would require a holding-pen protocol to enable
the device to wait and signal a CPU to enable CCI snoops on the device's
behalf before the device proceeds.  It is not the correct solution for
CPU clusters attached to ACE, precisely because we can be more efficient
in that case.

In fact, because you implement a power_up_setup method that calls
cci_enable_port_for_self, CCI snoops are actually enabled twice, making
the above code appear redundant.   Have I missed something?

When a cluster is being turned off the snoops for both the clusters
are being turned off. When the other cluster comes back the snoops are
being turned on for the incoming cluster via power_up_setup and here
for the other cluster. As previously mentioned, I will be dropping
this change.

That's a fair point.  If there is only one cluster alive, turning off
snoops for it should be safe, because there is no second cluster for
it to maintain coherency with.

But that's not that simple as I explained in a previous email.  If the 
other cluster has gone down via cpuidle, it may come back up at any 
moment without warning.  We do have the infrastructure in place to cope 
with possible races handling the CCI within a cluster.  We do not have 
anything for cross cluster races.  And before we do, it is necessary to 
know if it is worth it.

Agreed.  It could be done, perhaps by the approach I already considered
for handling multilevel clusters, but it is far from trivial, and I
would like to see some measurement of the potential benefit before
getting into it.

Cheers
---Dave