linux-kernel - Re: [PATCH v2 1/4] arm64, thunder: Add Kconfig option for Cavium Thunder SoC Family

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Date:	Fri, 5 Sep 2014 12:05:52 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Robert Richter <robert.richter@...iumnetworks.com>
Cc:	Robert Richter <rric@...nel.org>,
	Will Deacon <will.deacon@....com>,
	Mark Rutland <mark.rutland@....com>,
	Rob Herring <robh+dt@...nel.org>,
	Arnd Bergmann <arnd@...db.de>,
	Catalin Marinas <catalin.marinas@....com>,
	Olof Johansson <olof@...om.net>,
	Radha Mohan Chintakuntla <rchintakuntla@...ium.com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 1/4] arm64, thunder: Add Kconfig option for Cavium
	Thunder SoC Family

On Fri, Sep 05, 2014 at 12:45:47PM +0200, Robert Richter wrote:
> On 05.09.14 10:32:40, Russell King - ARM Linux wrote:
> > On Fri, Sep 05, 2014 at 09:46:42AM +0200, Robert Richter wrote:
> > > From: Radha Mohan Chintakuntla <rchintakuntla@...ium.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@...ium.com>
> > > Signed-off-by: Robert Richter <rrichter@...ium.com>
> > > ---
> > >  arch/arm64/Kconfig | 6 ++++++
> > >  1 file changed, 6 insertions(+)
> > > 
> > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > > index fd4e81a4e1ce..77fb82b0f684 100644
> > > --- a/arch/arm64/Kconfig
> > > +++ b/arch/arm64/Kconfig
> > > @@ -134,6 +134,11 @@ source "kernel/Kconfig.freezer"
> > >  
> > >  menu "Platform selection"
> > >  
> > > +config ARCH_THUNDER
> > > +	bool "Cavium Inc. Thunder SoC Family"
> > > +	help
> > > +	  This enables support for Cavium's Thunder Family of SoCs.
> > > +
> > >  config ARCH_VEXPRESS
> > >  	bool "ARMv8 software model (Versatile Express)"
> > >  	select ARCH_REQUIRE_GPIOLIB
> > > @@ -256,6 +261,7 @@ config NR_CPUS
> > >  	range 2 32
> > >  	depends on SMP
> > >  	# These have to remain sorted largest to smallest
> > > +	default "32" if ARCH_THUNDER
> > >  	default "8"
> > 
> > Why do you need ARCH_THUNDER?  If it's just to change this default,
> 
> No, we need it just to enable all our drivers on the SoC. We want to
> enable the SoC by using defconfig + ARCH_THUNDER. As said in my other
> mail, I put it here to be able to base all other thunder driver patch
> sets on this initial base patch set. Otherwise this particular patch
> and also the dtb patch need to be shipped with all other driver patch
> sets. This might lead to duplicate submissions of the same patch.
> 
> With doing defconfig + ARCH_THUNDER we also want to enable the max
> number of cpus that is currently supported. I only enable 32 cpus
> since booting more cpus is untested. There might be problems at the 32
> cpu boundary. Just setting it to 64 does not mean a kernel will
> actually boot more than 32 cpus. But if it will be ack'ed, I would be
> fine to set NR_CPUS to 32 or 64 in general and independent from
> ARCH_THUNDER.
> 
> For simplicity I better drop setting NR_CPUS in this patch.

So, ARM64 will get a big long list of "config ARCH_foo" options just
to stuff lots of broken select statements into the configuration.  Yes,
this may have been the norm with ARM, but it's turned out to be more
of a problem than a solution, especially as it keeps causing Kconfig
warnings when things change in the rest of the kernel tree.

The same is true with defconfigs - Linus threatened to delete all ARM
defconfigs except one at one point.

As I said below, this isn't how stuff is dealt with on x86.

What I'm questioning here is the entire ethos of "have an ARCH_foo
configuration which sets stuff up for platform foo".

> > please don't bother - we don't do this kind of thing on x86, so why
> > should it be done here?  Just ensure that NR_CPUS is appropriately
> > adjusted.  Alternatively, look at how x86 deals with this (with
> > X86_BIGSMP / MAXSMP).

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