RFC, GIC based smp_cross_call cleanup suggestion

[Santosh Shilimkar]

On 4/3/2011 4:07 PM, Russell King - ARM Linux wrote:
> On Sat, Apr 02, 2011 at 10:47:57PM -0600, Grant Likely wrote:
>> On Sat, Apr 2, 2011 at 3:10 AM, Colin Cross<ccross at google.com>  wrote:
>>> On Sat, Apr 2, 2011 at 1:51 AM, Russell King - ARM Linux
>>> <linux at arm.linux.org.uk>  wrote:
>>>> On Fri, Apr 01, 2011 at 04:55:02PM -0600, Grant Likely wrote:
>>>>> On Fri, Apr 1, 2011 at 4:26 PM, John Linn<John.Linn at xilinx.com>  wrote:
>>>>>> I’m getting ready to submit a patch to add SMP to Xilinx code. I notice that
>>>>>> smp_cross_call for all GIC based platforms is duplicated across each
>>>>>> platform in smp.h.
>>>>>>
>>>>>>
>>>>>>
>>>>>> I thought I’d try to jump in to help with some cleanup, although I realize
>>>>>> it’s minimal, I have to start somewhere.
>>>>>>
>>>>>>
>>>>>>
>>>>>> What about moving the smp_cross_call for GIC based designs into gic.h?
>>>>>
>>>>> Go for it.  It's an obvious cleanup.
>>>>
>>>> That assumes that all SMP implementations will always have a GIC.  It
>>>> looks to me like this is conditional on shmobile, and so I don't think
>>>> its that trivial - maybe Paul or Magnus can first indicate why this is.
>>>
>>> OMAP4 may also require a custom smp_cross_call implementation if CPU
>>> idle is going to be supported in SMP - in CPU off idle modes, a GIC
>>> SGI will not wake the CPU, and a write directly to the CPU's power
>>> management controller or an external interrupt source would be
>>> required.
>>
>> What John proposes appears to be a pretty sane default though.  It
>> would make sense to move it to common code, and require explicit
>> action on the part of the subarch to compile it out for a different
>> behaviour.  Requiring each subarch to define it explicitly doesn't
>> seem optimal.
>
> Bear in mind that the GIC doesn't do any PM support, so the hardware
> folk are inventing their own IRQ controller to sit along side the GIC
> to provide that missing functionality.  What I expect will happen is
> that the GIC will be obsoleted and replaced by something integrating
> PM support.
>
> So we could end up in the situation where we need both GIC and something
> else in the kernel at the same time - especially if we persue the single
> kernel image thing.  Moving smp_cross_call() into gic.h would add an
> additional bar to that happening.
>
> A better solution may be to make smp_cross_call() be a function pointer
> which must be initialized as part of the platforms SMP initialization.
> That'd get rid of mach/smp.h entirely.
>
> Oh, and while looking at that, guess what annoyingly stands in the way
> of eliminating mach/smp.h ?  Yes, it's OMAP, because they've bunged some
> function prototypes into plat/smp.h.  (I've not cared about that in the
> patch below; I've deleted the entire thing, so it probably breaks OMAP.)
>
Sure, it does break OMAP4. These functions are there because they
are used/compiled only for SMP support.

If you plan to commit this change then I can move these to
other OMAP4 header file.

Regards
Santosh