From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 463DAC282DD for ; Thu, 9 Jan 2020 14:06:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1E8D8206ED for ; Thu, 9 Jan 2020 14:06:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731486AbgAIOGN (ORCPT ); Thu, 9 Jan 2020 09:06:13 -0500 Received: from Galois.linutronix.de ([193.142.43.55]:54422 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728280AbgAIOGM (ORCPT ); Thu, 9 Jan 2020 09:06:12 -0500 Received: from [5.158.153.52] (helo=nanos.tec.linutronix.de) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1ipYRV-0003WO-4H; Thu, 09 Jan 2020 15:05:45 +0100 Received: by nanos.tec.linutronix.de (Postfix, from userid 1000) id A3E5F1060CF; Thu, 9 Jan 2020 15:05:44 +0100 (CET) From: Thomas Gleixner To: Christophe Leroy Cc: Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , vincenzo.frascino@arm.com, luto@kernel.org, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org Subject: Re: [RFC PATCH] powerpc/32: Switch VDSO to C implementation. In-Reply-To: <207cef10-3da8-6a52-139c-0620b21b64af@c-s.fr> References: <8ce3582f7f7da9ff0286ced857e5aa2e5ae6746e.1571662378.git.christophe.leroy@c-s.fr> <95bd2367-8edc-29db-faa3-7729661e05f2@c-s.fr> <439bce37-9c2c-2afe-9c9e-2f500472f9f8@c-s.fr> <207cef10-3da8-6a52-139c-0620b21b64af@c-s.fr> Date: Thu, 09 Jan 2020 15:05:44 +0100 Message-ID: <87d0bslo7b.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Christophe! Christophe Leroy writes: > In do_hres(), I see: > > cycles = __arch_get_hw_counter(vd->clock_mode); > ns = vdso_ts->nsec; > last = vd->cycle_last; > if (unlikely((s64)cycles < 0)) > return -1; > > __arch_get_hw_counter() returns a u64 values. On the PPC, this is read > from the timebase which is a 64 bits counter. > > Why returning -1 if (s64)cycles < 0 ? Does it means we have to mask out > the most significant bit when reading the HW counter ? Only if you expect the HW counter to reach a value which has bit 63 set. That'd require: uptime counter frequency ~292 years 1GHz ~ 58 years 5GHz assumed that the HW counter starts at 0 when the box is powered on. The reason why this is implemented in this way is that __arch_get_hw_counter() needs a way to express that the clocksource of the moment is not suitable for VDSO so that the syscall fallback gets invoked. Sure we could have used a pointer for the value and a return value indicating the validity, but given the required uptime the resulting code overhead seemed to be not worth it. At least not for me as I'm not planning to be around 58 years from now :) Thanks, tglx