linux-kernel - Re: [PATCH v2 2/3] ARM: vfp: Fix VFPv3 hwcap detection on CPUID based cpus

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Message-ID: <20141028121154.GA12136@arm.com>
Date:	Tue, 28 Oct 2014 12:11:55 +0000
From:	Will Deacon <will.deacon@....com>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	Russell King <linux@....linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Rob Clark <robdclark@...il.com>
Subject: Re: [PATCH v2 2/3] ARM: vfp: Fix VFPv3 hwcap detection on CPUID
 based cpus

On Mon, Oct 27, 2014 at 07:50:42PM +0000, Stephen Boyd wrote:
> On 10/27/2014 03:31 AM, Will Deacon wrote:
> > On Tue, Oct 14, 2014 at 02:48:58PM +0100, Stephen Boyd wrote:
> >> If the CPU is using CPUID scheme, use the MVFR registers to
> >> determine what version of VFP is supported. We already do this
> >> for VFPv4, so do something similar for VFPv3 and look for single
> >> or double precision support in MVFR0. Otherwise fall back to
> >> using FPSID to detect VFP suppport on non-CPUID scheme CPUs. We
> >> know that VFPv3 is only present in CPUs that have support for the
> >> CPUID scheme so this should be equivalent.
> > This looks correct to me, but it raises a bigger question about the
> > suitability of hwcaps for describing features of the instruction set.
> 
> Great. Can I get your reviewed-by on this patch please?

Sure. There's a spelling mistake ("arhitecture") which you should fix,
but the code looks ok.

> > With the extended CPUID scheme, there are a whole bunch of different
> > instruction set features that are reported and bundling arbitrary subsets of
> > them into hwcaps such as `VFPv4' doesn't feel like the right thing to do in
> > the long run. It also doesn't seem to match where the architecture is going.
> >
> > Perhaps it would be better to consider exposing the ID registers to
> > userspace in some manner? This could be done either via an undef handler, or
> > using the vdso. We would add a (final) hwcap advertising this cpuid support.
> > For big/little systems, the kernel would need to expose a suitable subset of
> > the features (we already have the sanity checking code from Rutland).
> >
> > I'd certainly like to explore that route for arm64, before we start adding a
> > bunch of fine-grained capabilities.
> 
> I have an RFC for the undef handler written up, except for the
> big/little thing. Let me post it. Is there anyone from the userspace
> side that can be on Cc?

Off the top of my head:

  Mans Rullgard (already replied to this thread)
  Peter Maydell <peter.maydell@...aro.org> [QEMU]
  Jacob Bramley <jacob.bramley@....com> [JITs]
  Kyrylo Tkachov <kyrylo.tkachov@....com> [GCC]

(CC Rutland for the big/little bits too)

Will
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