From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S936184AbdKPQvV (ORCPT <rfc822;w@1wt.eu>);
        Thu, 16 Nov 2017 11:51:21 -0500
Received: from us-smtp-delivery-107.mimecast.com ([216.205.24.107]:32770 "EHLO
        us-smtp-delivery-107.mimecast.com" rhost-flags-OK-OK-OK-OK)
        by vger.kernel.org with ESMTP id S936205AbdKPQvN (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Thu, 16 Nov 2017 11:51:13 -0500
X-MC-Unique: Db1GZLvMPWa0Tv1zmEMwFQ-1
Subject: Re: [RFC] Improving udelay/ndelay on platforms where that is possible
To: Nicolas Pitre <nicolas.pitre@linaro.org>
CC: Russell King - ARM Linux <linux@armlinux.org.uk>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Alan Cox <gnomes@lxorguk.ukuu.org.uk>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Peter Zijlstra" <peterz@infradead.org>,
        John Stultz <john.stultz@linaro.org>,
        Douglas Anderson <dianders@chromium.org>,
        Mark Rutland <mark.rutland@arm.com>, Will Deacon <will.deacon@arm.com>,
        Jonathan Austin <jonathan.austin@arm.com>,
        Arnd Bergmann <arnd@arndb.de>, Kevin Hilman <khilman@kernel.org>,
        Michael Turquette <mturquette@baylibre.com>,
        "Stephen Boyd" <sboyd@codeaurora.org>,
        Boris Brezillon <boris.brezillon@free-electrons.com>,
        Thibaud Cornic <thibaud_cornic@sigmadesigns.com>,
        Mason <slash.tmp@free.fr>
References: <d3f42f35-8175-58f6-78b7-3331efe64da2@sigmadesigns.com>
 <20171101175325.2557ce85@alans-desktop>
 <4b707ce0-6067-ab36-e167-1acf348d26bf@free.fr>
 <CA+55aFxxH0HE9SfUoLVqsvDKO0PfeM+vSoDmbXQv85yJcPZoPQ@mail.gmail.com>
 <e03436d1-b140-3f29-59a6-3b1eb6bf9da4@free.fr>
 <11393e07-b042-180c-3bcd-484bf51eada6@sigmadesigns.com>
 <20171115131351.GE31757@n2100.armlinux.org.uk>
 <1fa81694-7bd2-564b-e5b9-ae53b9ea6620@sigmadesigns.com>
 <20171116153625.GJ31757@n2100.armlinux.org.uk>
 <9a4cfa9d-3940-b7f2-5a4d-59e89af85bb7@sigmadesigns.com>
 <nycvar.YSQ.7.76.1711161106240.20817@knanqh.ubzr>
 <48c38055-20f7-e565-aa56-74f360e6e3d9@sigmadesigns.com>
 <nycvar.YSQ.7.76.1711161140030.20817@knanqh.ubzr>
From: Marc Gonzalez <marc_gonzalez@sigmadesigns.com>
Message-ID: <870aeb80-bbc5-52ed-cca2-3e762e27bfe2@sigmadesigns.com>
Date: Thu, 16 Nov 2017 17:51:05 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Firefox/52.0 SeaMonkey/2.49.1
MIME-Version: 1.0
In-Reply-To: <nycvar.YSQ.7.76.1711161140030.20817@knanqh.ubzr>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 7bit
X-Originating-IP: [172.27.0.114]
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On 16/11/2017 17:47, Nicolas Pitre wrote:

> Look at cpufreq_callback() in arch/arm/kernel/smp.c.

Are you pointing at the scaling of loops_per_jiffy done in that function?

As I wrote earlier:

If I'm reading arch/arm/kernel/smp.c correctly, loops_per_jiffy is scaled
when the frequency changes.

But arch/arm/lib/delay-loop.S starts by loading the current value of
loops_per_jiffy, computes the number of times to loop, and then loops.
If the frequency increases when the core is in __loop_delay, the
delay will be much shorter than requested.