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

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Date:	Fri, 22 Feb 2013 15:46:18 -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 2/22/2013 10:27 AM, Will Deacon wrote:
> Hi guys,
>
> On Fri, Feb 22, 2013 at 08:08:05AM +0000, Stephen Boyd wrote:
>> From: Steve Muckle <smuckle@...eaurora.org>
>>
>> The subarchitecture field in the fpsid register is 7 bits wide.
>> The topmost bit is used to designate that the subarchitecture
>> designer is not ARM. We use this field to determine which VFP
>> version is supported by the CPU. Since the topmost bit is masked
>> off we detect non-ARM subarchitectures as supporting only
>> HWCAP_VFP and not HWCAP_VFPv3 as it should be for Qualcomm's
>> processors.
> I'm struggling to see why this has anything to do with the hwcaps being set
> incorrectly. 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.

>
> 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.

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

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