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

[Stephen Boyd <sboyd@codeaurora.org>]

On 10/27/2014 03:31 AM, Will Deacon wrote:
> Hi Stephen,
>
> On Tue, Oct 14, 2014 at 02:48:58PM +0100, Stephen Boyd wrote:
>> The subarchitecture field in the fpsid register is 7 bits wide on
>> ARM CPUs using the CPUID identification scheme, spanning bits 22
>> to 16. The topmost bit is used to designate that the
>> subarchitecture designer is not ARM when it is set to 1. On
>> non-CPUID scheme CPUs the subarchitecture field is only 4 bits
>> wide and the higher bits are used to indicate no double precision
>> support (bit 20) and the FTSMX/FLDMX format (bits 21-22).
>>
>> The VFP support code only looks at bits 19-16 to determine the
>> VFP version. On Qualcomm's processors (Krait and Scorpion) we
>> should see that we have HWCAP_VFPv3 but we don't because bit 22
>> is set to 1 to indicate that the subarchitecture is not
>> implemented by ARM and the rest of the bits are left as 0 because
>> this is the first subarchitecture that Qualcomm has designed.
>> Unfortunately we can't just widen the FPSID subarchitecture
>> bitmask to consider all the bits on a CPUID scheme because there
>> may be CPUs without the CPUID scheme that have VFP without double
>> precision support and then the version would be a very wrong and
>> large number. Instead, update the version detection logic to
>> consider if the CPU is using the CPUID scheme.
>>
>> 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?

>
> 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?

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