[PATCH] arm64/sve: Document firmware support requirements in Kconfig

[Dave Martin]

On Fri, Mar 23, 2018 at 05:12:34PM +0000, Catalin Marinas wrote:
> On Wed, Mar 21, 2018 at 11:54:49AM +0000, Dave P Martin wrote:
> > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > index 7381eeb..e6fa47b 100644
> > --- a/arch/arm64/Kconfig
> > +++ b/arch/arm64/Kconfig
> > @@ -1104,6 +1104,23 @@ config ARM64_SVE
> >  
> >  	  To enable use of this extension on CPUs that implement it, say Y.
> >  
> > +	  Note that for architectural reasons, firmware _must_ implement SVE
> > +	  support when running on SVE capable hardware.  The required support
> > +	  is present in:
> > +
> > +	    * version 1.5 and later of the ARM Trusted Firmware
> > +	    * the AArch64 boot wrapper since commit 5e1261e08abf
> > +	      ("bootwrapper: SVE: Enable SVE for EL2 and below").
> > +
> > +	  For other firmware implementations, consult the firware documentation
> > +	  or vendor.
> > +
> > +	  If you need the kernel to boot on SVE-capable hardware with broken
> > +	  firmware, you may need to say N here until you get your firmware
> > +	  fixed.  Otherwise, you may experience firmware panics or lockups when
> > +	  booting the kernel.  If unsure and you are not observing these
> > +	  symptoms, you should assume that it is safe to say Y.
> 
> It's a shame that we can't detect this architecturally but the
> alternative would be to ask the firmware to describe the presence of SVE
> via DT or ACPI (which I don't particularly like since it's a CPU feature
> rather than an SoC one).

That was my view also.  We could have done that, but it would set an
undesirable precedent, given that this shouldn't apply to real
platforms. 

We could revisit this later if necessary.

> I'm personally fine with the warning in Kconfig.
> 
> Acked-by: Catalin Marinas <catalin.marinas at arm.com>

Thanks, I'll repost with the typo fix pointed out by Suzuki.

Cheers
---Dave