[PATCH 00/12][RFC] Intial Kconfig Fragment Demo

[Amit Kucheria]

On 11 Mar 16, Nicolas Pitre wrote:
> On Fri, 11 Mar 2011, Amit Kucheria wrote:
> 
> > On Fri, Mar 11, 2011 at 12:32 PM, Dave Martin <dave.martin at linaro.org> wrote:
> > > On Wed, Mar 9, 2011 at 9:13 AM, Andy Green <andy at warmcat.com> wrote:
> > >> On 03/09/2011 09:04 AM, Somebody in the thread at some point said:
> > >>
> > >>>> I take it this magic of SMP or not is hidden in this config layering
> > >>>> scheme
> > >>>> you mentioned and it isn't really using the same config for say igep0020
> > >>>> and
> > >>>
> > >>> No, it is in the black depths of ARM assembly and TBH, it is voodoo to
> > >>> me. Nothing to do with kernel config as such. The SMP kernel, at
> > >>> runtime, (binary) patches itself to convert locking primitives to
> > >>> no-ops in the UP case. Or something to the effect.
> > >>
> > >> Hum my IGEP0020 config here has CONFIG_BROKEN_ON_SMP=y set so I guess this
> > >> is to do with what you mentioned.
> > >>
> > >>>> Panda.  In any event, there's a performance tradeoff running SMP kernel
> > >>>> on
> > >>>> uniprocessor to consider too.
> > >>>
> > >>> Apparently, with this one-time patching (per boot) there isn't such a
> > >>> tradeoff.
> > >>
> > >> OK thanks for the explanation.
> > >
> > > SMP-on-UP appears to be fully working nowadays.  We currently don't
> > > build a single SMP kernel for omap4 and omap3, but I've been playing
> > > with that and it's been shown to work.  Does anyone know whether we're
> > > planning to move to a single OMAP kernel?  Has anyone measured the
> > > performance delta?
> > >
> > > In principle, we could make this move; but there could be issues I'm
> > > not aware of.
> > >
> > > Note that the SMP-on-UP support is fairly minimal -- only those things
> > > which literally will fail on UP are patched out.
> > 
> > Do you have a pointer to the patches that enabled this support? SHA
> > ids are fine. I'm curious what the runtime patching voodoo looks like.
> 
> commit f00ec48fadf5e37e7889f14cff900aa70d18b644
> ARM: Allow SMP kernels to boot on UP systems
> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=f00ec48fad
> 
> We now have a similar clever trick for a runtime determined PHYS_OFFSET:
> 
> commit f47ecb85ba9c67cf6f6eba39c19cc528f783f5e7
> ARM: P2V: introduce phys_to_virt/virt_to_phys runtime patching
> http://ftp.arm.linux.org.uk/git/gitweb.cgi?p=linux-2.6-arm.git;a=commitdiff;h=f47ecb85ba
> 
> This one has been drafted for human understanding here:
> https://wiki.ubuntu.com/Specs/ARMSingleKernel
> (the "Optimized virt_to_phys() with a runtime determined PHYS_OFFSET" section)
> 

Thanks for the links Nico. And the wiki page. Very useful.

/Amit