[PATCH] arm: Fix mounting root on omaps with CPU_V6 and CPU_V7

[Catalin Marinas]

On Wed, 2010-03-17 at 19:11 +0000, Tony Lindgren wrote:
> * Catalin Marinas <catalin.marinas at arm.com> [100317 11:04]:
> > On Wed, 2010-03-17 at 17:57 +0000, Tony Lindgren wrote:
> > > Here's an updated version of this patch with more details.
> > >
> > > Looks like VFPv3 is only available on V7:
> > >
> > > http://www.arm.com/products/processors/technologies/vector-floating-point.php
> >
> > But does it cause any problem if the feature is enabled in the kernel?
> > The vfp_init() code should check for its presence and set the hwcap
> > accordingly.
> 
> Yeah, it causes the problem posted in the patch description. I took a
> quick look at it and at least the VFPFMRX in vfpmacros.h for CONFIG_VFPv3
> is a problem.

This would indeed need more checking to avoid reading some registers
which aren't present on ARMv6.

I think the main problem with just falling back to VFPv2 is the lack of
NEON support even if the CPU supports it.

> Also I think we would need to have separate vfp_get_double functions
> in vfphw.S for VFPv2 and 3 that get used based on the features.

I don't think that's causing problems (or at least we can identify where
the function gets called for higher VFP registers). Even with VFPv3, you
may not have the D16-D31 registers.

> > > HAS_TLS reg is only on ARM11 starting with r1p0:
> > >
> > > http://infocenter.arm.com/help/topic/com.arm.doc.ddi0211k/Babeihid.html
> > >
> > > So that explains why it won't work on omap2420 as it's r0p2.
> >
> > Same here, would it work with dynamic detection?
> 
> Hmm I believe here the problem is __switch_to in entry-armv.S.
> I don't think we want to dynamically test it every time.. Or
> at least it would have to be optimized out in most cases.

But if you disable this, you won't be able to use an SMP build on both
v6 and v7. Anyway, I don't think that dynamically checking this would
introduce performance penalties, the __switch_to code is pretty complex
already with all the notifier calls.

We may also have optimised user space that reads the TLS register
directly rather than going through the kuser helper, in which case we
would need a kernel built only for ARMv7 (maybe that's acceptable in
this situation).

-- 
Catalin