Resend [PATCH 00/10] ARM: SMP initialization consolidation

[Rob Herring]

On 05/08/2011 04:42 AM, Russell King - ARM Linux wrote:
> On Tue, May 03, 2011 at 03:57:42PM +0200, Arnd Bergmann wrote:
>> On Tuesday 03 May 2011, Russell King - ARM Linux wrote:
>>> Such a SoC may wish to do things differently from the current approach
>>> (which is basically bring up all cores at boot) particularly as the
>>> performance gained from each CPU is far from identical.  So I've been
>>> nervous about moving the CPU map initialization into core code.
>>>
>>> Note that the low power CPU may not be the last in the set, so merely
>>> limiting the system to two CPUs is not the answer.
>>
>> If I read the patches correctly, it's still strictly opt-in per platform.
>
> Except where things are moved into the generic code, such as the setting
> of present CPUs.

Is there any harm in making the default be mark all cores present then 
let the platform code clear the present state if so desired? This was my 
intention.

>
>> When a platform wants to support the setup you mentioned or something
>> even stranger, all it would have to do is provide its own versions of
>> the SMP pen and scu init function, right?
>
> No.
>
> As I've already said in the past, I doubt that very many platforms actually
> need the SMP pen stuff.  There are two situations where you need that:
>
> 1. If you have more than two CPUs, _and_ you have no way to control which
>     of those CPUs are triggered to jump into the kernel.
> 2. If you have hotplug and you have no way to power down or reset the
>     CPUs.
>
> There seems to be few platforms which fall into (1) - most SMP
> implementations are based around two CPUs today so if you only have one
> secondary CPU, there's no point triggering it via a soft irq to enter the
> kernel, then hold it in a pen, then release it from the pen, etc.
>
> Those which seem to fall into (2) do so because they've cut'n'pasted the
> code from the Realview platforms, along with its bugs with the auxillary
> control register, which subseqently get tweaked.
>
> My feeling is that most of the hotplug implementations are just basic "this
> seems to approximately work" kind of things rather than someone taking a
> look at what the hardware supports and doing something more useful (such
> as powering down the CPU, placing it in reset, etc.)
>
> However, (2) is dependent on (1) as both share pen_release.  (2) needs to
> be sorted by platforms before the (1) can be eliminated from platform code.
>
> I'd much rather see the pen_release stuff being eliminated from platforms
> where it's not necessary, rather than being turned into a generic thing
> which everyone uses because its there.  Not only should this simplify the
> code, but also reduce the size of the platform SMP support code.
>

Thanks for the explanation. I now see that for my case I won't need the 
pen code at all.

What about the scu_init_cpus patch? You posted something similar as 
Stephen pointed out that I had missed. Do you plan to merge that?

Rob