From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759820AbdKPQrc (ORCPT ); Thu, 16 Nov 2017 11:47:32 -0500 Received: from mail-qt0-f196.google.com ([209.85.216.196]:55810 "EHLO mail-qt0-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S936037AbdKPQrX (ORCPT ); Thu, 16 Nov 2017 11:47:23 -0500 X-Google-Smtp-Source: AGs4zMaHlT6+HGQbmQ0DYoekcMMJH9FXcNYz/L+v1665cOFwm55aHLOZu1MGwEs3mlT3zouaeWNcZQ== Date: Thu, 16 Nov 2017 11:47:20 -0500 (EST) From: Nicolas Pitre To: Marc Gonzalez cc: Russell King - ARM Linux , Linus Torvalds , Alan Cox , LKML , Linux ARM , Steven Rostedt , Ingo Molnar , Thomas Gleixner , Peter Zijlstra , John Stultz , Douglas Anderson , Mark Rutland , Will Deacon , Jonathan Austin , Arnd Bergmann , Kevin Hilman , Michael Turquette , Stephen Boyd , Boris Brezillon , Thibaud Cornic , Mason Subject: Re: [RFC] Improving udelay/ndelay on platforms where that is possible In-Reply-To: <48c38055-20f7-e565-aa56-74f360e6e3d9@sigmadesigns.com> Message-ID: References: <20171101175325.2557ce85@alans-desktop> <4b707ce0-6067-ab36-e167-1acf348d26bf@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> <48c38055-20f7-e565-aa56-74f360e6e3d9@sigmadesigns.com> User-Agent: Alpine 2.21 (LFD 202 2017-01-01) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 16 Nov 2017, Marc Gonzalez wrote: > On 16/11/2017 17:08, Nicolas Pitre wrote: > > > On Thu, 16 Nov 2017, Marc Gonzalez wrote: > > > >> On 16/11/2017 16:36, Russell King - ARM Linux wrote: > >>> On Thu, Nov 16, 2017 at 04:26:51PM +0100, Marc Gonzalez wrote: > >>>> On 15/11/2017 14:13, Russell King - ARM Linux wrote: > >>>> > >>>>> udelay() needs to offer a consistent interface so that drivers know > >>>>> what to expect no matter what the implementation is. Making one > >>>>> implementation conform to your ideas while leaving the other > >>>>> implementations with other expectations is a recipe for bugs. > >>>>> > >>>>> If you really want to do this, fix the loops_per_jiffy implementation > >>>>> as well so that the consistency is maintained. > >>>> > >>>> Hello Russell, > >>>> > >>>> It seems to me that, when using DFS, there's a serious issue with loop-based > >>>> delays. (IIRC, it was you who pointed this out a few years ago.) > >>>> > >>>> 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. > >>>> > >>>> Is this a correct assessment of the situation? > >>> > >>> Absolutely correct, and it's something that people are aware of, and > >>> have already catered for while writing their drivers. > >> > >> In their cpufreq driver? > >> In "real" device drivers that happen to use delays? > >> > >> On my system, the CPU frequency may ramp up from 120 MHz to 1.2 GHz. > >> If the frequency increases at the beginning of __loop_delay, udelay(100) > >> would spin only 10 microseconds. This is likely to cause issues in > >> any driver using udelay. > >> > >> How does one cater for that? > > > > You make sure your delays are based on a stable hardware timer. > > Most platforms nowadays should have a suitable timer source. > > So you propose fixing loop-based delays by using clock-based delays, > is that correct? (That is indeed what I did on my platform.) > > Russell stated that there are platforms using loop-based delays with > cpufreq enabled. I'm asking how they manage the brokenness. Look at cpufreq_callback() in arch/arm/kernel/smp.c. Nicolas