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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_MUTT autolearn=ham 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 7CA63C31E5B for ; Tue, 18 Jun 2019 22:48:24 +0000 (UTC) Received: from mail.linuxfoundation.org (mail.linuxfoundation.org [140.211.169.12]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 58C8A20B1F for ; Tue, 18 Jun 2019 22:48:24 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 58C8A20B1F Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.intel.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=iommu-bounces@lists.linux-foundation.org Received: from mail.linux-foundation.org (localhost [127.0.0.1]) by mail.linuxfoundation.org (Postfix) with ESMTP id 311CAE19; Tue, 18 Jun 2019 22:48:24 +0000 (UTC) Received: from smtp1.linuxfoundation.org (smtp1.linux-foundation.org [172.17.192.35]) by mail.linuxfoundation.org (Postfix) with ESMTPS id 15A17DB2 for ; Tue, 18 Jun 2019 22:48:23 +0000 (UTC) X-Greylist: domain auto-whitelisted by SQLgrey-1.7.6 Received: from mga12.intel.com (mga12.intel.com [192.55.52.136]) by smtp1.linuxfoundation.org (Postfix) with ESMTPS id B0DCF180 for ; Tue, 18 Jun 2019 22:48:22 +0000 (UTC) X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from fmsmga008.fm.intel.com ([10.253.24.58]) by fmsmga106.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 18 Jun 2019 15:48:22 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.63,390,1557212400"; d="scan'208";a="160196241" Received: from ranerica-svr.sc.intel.com ([172.25.110.23]) by fmsmga008.fm.intel.com with ESMTP; 18 Jun 2019 15:48:22 -0700 Date: Tue, 18 Jun 2019 15:48:00 -0700 From: Ricardo Neri To: Thomas Gleixner Subject: Re: [RFC PATCH v4 03/21] x86/hpet: Calculate ticks-per-second in a separate function Message-ID: <20190618224800.GD30488@ranerica-svr.sc.intel.com> References: <1558660583-28561-1-git-send-email-ricardo.neri-calderon@linux.intel.com> <1558660583-28561-4-git-send-email-ricardo.neri-calderon@linux.intel.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.4 (2018-02-28) Cc: Kate Stewart , "Ravi V. Shankar" , x86@kernel.org, Ashok Raj , Arnd Bergmann , Peter Zijlstra , Philippe Ombredanne , Randy Dunlap , Clemens Ladisch , linux-kernel@vger.kernel.org, Stephane Eranian , Ricardo Neri , "Rafael J. Wysocki" , iommu@lists.linux-foundation.org, Tony Luck , "H. Peter Anvin" , Andi Kleen , Borislav Petkov , Ingo Molnar X-BeenThere: iommu@lists.linux-foundation.org X-Mailman-Version: 2.1.12 Precedence: list List-Id: Development issues for Linux IOMMU support List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: iommu-bounces@lists.linux-foundation.org Errors-To: iommu-bounces@lists.linux-foundation.org On Fri, Jun 14, 2019 at 05:54:05PM +0200, Thomas Gleixner wrote: > On Thu, 23 May 2019, Ricardo Neri wrote: > > > > +u64 hpet_get_ticks_per_sec(u64 hpet_caps) > > +{ > > + u64 ticks_per_sec, period; > > + > > + period = (hpet_caps & HPET_COUNTER_CLK_PERIOD_MASK) >> > > + HPET_COUNTER_CLK_PERIOD_SHIFT; /* fs, 10^-15 */ > > + > > + /* > > + * The frequency is the reciprocal of the period. The period is given > > + * in femtoseconds per second. Thus, prepare a dividend to obtain the > > + * frequency in ticks per second. > > + */ > > + > > + /* 10^15 femtoseconds per second */ > > + ticks_per_sec = 1000000000000000ULL; > > + ticks_per_sec += period >> 1; /* round */ > > + > > + /* The quotient is put in the dividend. We drop the remainder. */ > > + do_div(ticks_per_sec, period); > > + > > + return ticks_per_sec; > > +} > > + > > int hpet_alloc(struct hpet_data *hdp) > > { > > u64 cap, mcfg; > > @@ -844,7 +867,6 @@ int hpet_alloc(struct hpet_data *hdp) > > struct hpets *hpetp; > > struct hpet __iomem *hpet; > > static struct hpets *last; > > - unsigned long period; > > unsigned long long temp; > > u32 remainder; > > > > @@ -894,12 +916,7 @@ int hpet_alloc(struct hpet_data *hdp) > > > > last = hpetp; > > > > - period = (cap & HPET_COUNTER_CLK_PERIOD_MASK) >> > > - HPET_COUNTER_CLK_PERIOD_SHIFT; /* fs, 10^-15 */ > > - temp = 1000000000000000uLL; /* 10^15 femtoseconds per second */ > > - temp += period >> 1; /* round */ > > - do_div(temp, period); > > - hpetp->hp_tick_freq = temp; /* ticks per second */ > > + hpetp->hp_tick_freq = hpet_get_ticks_per_sec(cap); > > Why are we actually computing this over and over? > > In hpet_enable() which is the first function invoked we have: > > /* > * The period is a femto seconds value. Convert it to a > * frequency. > */ > freq = FSEC_PER_SEC; > do_div(freq, hpet_period); > hpet_freq = freq; > > So we already have ticks per second, aka frequency, right? So why do we > need yet another function instead of using the value which is computed > once? The frequency of the HPET channels has to be identical no matter > what. If it's not HPET is broken beyond repair. I don't think it needs to be recomputed again. I missed the fact that the frequency was already computed here. Also, the hpet char driver has its own frequency computation. Perhaps it could also obtain it from here, right? Thanks and BR, Ricardo > > Thanks, > > tglx > > _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu