From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759366Ab3BYLSx (ORCPT ); Mon, 25 Feb 2013 06:18:53 -0500 Received: from cam-admin0.cambridge.arm.com ([217.140.96.50]:39206 "EHLO cam-admin0.cambridge.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752030Ab3BYLSw (ORCPT ); Mon, 25 Feb 2013 06:18:52 -0500 Date: Mon, 25 Feb 2013 11:18:35 +0000 From: Will Deacon To: Stephen Boyd Cc: "linux-arm-kernel@lists.infradead.org" , "linux-arm-msm@vger.kernel.org" , Steve Muckle , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH] ARM: vfp: fix fpsid register subarchitecture field mask width Message-ID: <20130225111835.GA24254@mudshark.cambridge.arm.com> References: <1361520485-30269-1-git-send-email-sboyd@codeaurora.org> <20130222182705.GC19527@mudshark.cambridge.arm.com> <5128034A.4000008@codeaurora.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5128034A.4000008@codeaurora.org> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Feb 22, 2013 at 11:46:18PM +0000, Stephen Boyd wrote: > 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 > >> > >> 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. 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). > > > > 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. Cheers, Will