[v2,1/4] arm64, thunder: Add Kconfig option for Cavium Thunder SoC Family

From: Radha Mohan Chintakuntla <rchintakuntla@cavium.com>

Increase maximum numbers of cpus to 32. This relates to current
maximal possible cpu number. Increasing this to 64 cpus will be a
separate patch not part of this enablement patches.

Signed-off-by: Radha Mohan Chintakuntla <rchintakuntla@cavium.com>
Signed-off-by: Robert Richter <rrichter@cavium.com>
---
 arch/arm64/Kconfig | 6 ++++++
 1 file changed, 6 insertions(+)

On Fri, Sep 05, 2014 at 08:46:42AM +0100, Robert Richter wrote:
> From: Radha Mohan Chintakuntla <rchintakuntla@cavium.com>
> 
> Increase maximum numbers of cpus to 32. This relates to current
> maximal possible cpu number. Increasing this to 64 cpus will be a
> separate patch not part of this enablement patches.

Just out of interest, does raising the current maximum limit actually break
any existing code? If not, then doing this as two patches doesn't seem worth
it.

Will
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On 05.09.14 09:39:32, Will Deacon wrote:
> On Fri, Sep 05, 2014 at 08:46:42AM +0100, Robert Richter wrote:
> > From: Radha Mohan Chintakuntla <rchintakuntla@cavium.com>
> > 
> > Increase maximum numbers of cpus to 32. This relates to current
> > maximal possible cpu number. Increasing this to 64 cpus will be a
> > separate patch not part of this enablement patches.
> 
> Just out of interest, does raising the current maximum limit actually break
> any existing code? If not, then doing this as two patches doesn't seem worth
> it.

Increasing to 64 should be fine from the perspective of cpu mask
implementation. Memory foot print should be the same already as this
uses long which is 64 bit. So this wouldn't hurt.

However, I felt a bit uncomfortable having a dependency here to
enabling 64 cpus and getting this patch set upstream. Support for more
than 32 cpus is not well tested yet and there still might be problems
with e.g. interrupt delivery or topology.

If increasing to 32 cpus as an intermediate step isn't the best
approach here, we can live with the current defaults too. I would then
just remove that change from the patch.

For follow on driver patch sets it would be good to have ARCH_THUNDER
upstream as this option will be shared between all drivers.

-Robert
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On Fri, Sep 05, 2014 at 10:21:35AM +0100, Robert Richter wrote:
> On 05.09.14 09:39:32, Will Deacon wrote:
> > On Fri, Sep 05, 2014 at 08:46:42AM +0100, Robert Richter wrote:
> > > From: Radha Mohan Chintakuntla <rchintakuntla@cavium.com>
> > > 
> > > Increase maximum numbers of cpus to 32. This relates to current
> > > maximal possible cpu number. Increasing this to 64 cpus will be a
> > > separate patch not part of this enablement patches.
> > 
> > Just out of interest, does raising the current maximum limit actually break
> > any existing code? If not, then doing this as two patches doesn't seem worth
> > it.
> 
> Increasing to 64 should be fine from the perspective of cpu mask
> implementation. Memory foot print should be the same already as this
> uses long which is 64 bit. So this wouldn't hurt.
> 
> However, I felt a bit uncomfortable having a dependency here to
> enabling 64 cpus and getting this patch set upstream. Support for more
> than 32 cpus is not well tested yet and there still might be problems
> with e.g. interrupt delivery or topology.

All I mean is, if the kernel doesn't explode on existing systems by changing
the upper limit to 64, then we should do that. If you're not comfortable
that the Thunder code can handle that, then leave the thunder default as 32,
like you do in the current patch. It just seems odd not to change the
maximum number, since it's an arbitrary limit from my perspective.

Will
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On Fri, Sep 05, 2014 at 10:25:08AM +0100, Will Deacon wrote:
> On Fri, Sep 05, 2014 at 10:21:35AM +0100, Robert Richter wrote:
> > On 05.09.14 09:39:32, Will Deacon wrote:
> > > On Fri, Sep 05, 2014 at 08:46:42AM +0100, Robert Richter wrote:
> > > > From: Radha Mohan Chintakuntla <rchintakuntla@cavium.com>
> > > > 
> > > > Increase maximum numbers of cpus to 32. This relates to current
> > > > maximal possible cpu number. Increasing this to 64 cpus will be a
> > > > separate patch not part of this enablement patches.
> > > 
> > > Just out of interest, does raising the current maximum limit actually break
> > > any existing code? If not, then doing this as two patches doesn't seem worth
> > > it.
> > 
> > Increasing to 64 should be fine from the perspective of cpu mask
> > implementation. Memory foot print should be the same already as this
> > uses long which is 64 bit. So this wouldn't hurt.
> > 
> > However, I felt a bit uncomfortable having a dependency here to
> > enabling 64 cpus and getting this patch set upstream. Support for more
> > than 32 cpus is not well tested yet and there still might be problems
> > with e.g. interrupt delivery or topology.
> 
> All I mean is, if the kernel doesn't explode on existing systems by changing
> the upper limit to 64, then we should do that. If you're not comfortable
> that the Thunder code can handle that, then leave the thunder default as 32,
> like you do in the current patch. It just seems odd not to change the
> maximum number, since it's an arbitrary limit from my perspective.

FWIW my Juno happily boots with a NR_CPUS=64 kernel.

Mark.
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On 05.09.14 10:36:47, Mark Rutland wrote:
> On Fri, Sep 05, 2014 at 10:25:08AM +0100, Will Deacon wrote:
> > All I mean is, if the kernel doesn't explode on existing systems by changing
> > the upper limit to 64, then we should do that. If you're not comfortable
> > that the Thunder code can handle that, then leave the thunder default as 32,
> > like you do in the current patch. It just seems odd not to change the
> > maximum number, since it's an arbitrary limit from my perspective.
> 
> FWIW my Juno happily boots with a NR_CPUS=64 kernel.

It looks like if it's fine to set the general default to NR_CPUS=64
even if the kernel might not yet boot more than 32 cpus. Will make
this change separately and drop it in this patch.

See also my other reply to Russell in this thread.

-Robert
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