linux-kernel - Re: [PATCH] ARM: vfp: fix fpsid register subarchitecture field mask width

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Date:	Mon, 25 Feb 2013 19:01:11 -0800
From:	Stephen Boyd <sboyd@...eaurora.org>
To:	Will Deacon <will.deacon@....com>
CC:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
	Steve Muckle <smuckle@...eaurora.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ARM: vfp: fix fpsid register subarchitecture field mask
 width

On 02/25/13 03:18, Will Deacon wrote:
> On Fri, Feb 22, 2013 at 11:46:18PM +0000, Stephen Boyd wrote:
>> On 2/22/2013 10:27 AM, Will Deacon wrote:
>>> What value do you have in fpsid? As far as I can tell, the
>>> subarchitecture bits 6:0 should start at 0x40 for you, right?
>> Yes it does.
> Ok, good. Could you share the different subarchitecture encodings that you
> have please? (assumedly some/all of these are compatible with a variant of
> VFP).

Definitely all Krait processors have 0x40 for the subarchitecture
encoding. I need to check our Scorpions but I'm fairly certain they also
have 0x40.

>
>>> I can see cases for changing this code, I just don't see why it would go
>>> wrong in the case you're describing.
>> VFP_arch = (vfpsid & FPSID_ARCH_MASK) >> FPSID_ARCH_BIT;
>>
>> causes VFP_arch to be equal to 0 because 0x40 & 0xf == 0.
>>
>> and then a little bit later we have
>>
>>                if (VFP_arch >= 2) {
>>                         elf_hwcap |= HWCAP_VFPv3;
>>
>>
>> The branch is not taken so we never set VFPv3.
> Ah, that's what I feared: the low bits are zero yet you are compatible with
> VFPv3. That's fine, but the proposed fix feels like a kludge; the only reason
> we'd choose on VFPv3 is because the implementor is not ARM, which may not hold
> true for other vendors. I think it would be better if we translated
> vendor-specific subarchitectures that are compatible with VFPvN into the
> corresponding architecture number instead. This would also allow us to add
> extra hwcaps for extensions other than VFP.

Ok. We should be able to make VFP_arch into 0x4 if the implementer is
0x51 and the subarch bits are 0x40.

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
hosted by The Linux Foundation

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