From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mason Subject: Re: Delays, clocks, timers, hrtimers, etc Date: Mon, 09 Feb 2015 15:50:12 -0800 Message-ID: <54D947B4.4080401@free.fr> References: <54C8E125.3070905@free.fr> <54D52F84.9050600@free.fr> <266c7b1ff2d1a8ba0ae4866f4fb4eca5@agner.ch> <54D576B1.4000001@free.fr> <54D903F6.3050608@codeaurora.org> <54D93556.9050008@free.fr> <54D93FFA.3090506@codeaurora.org> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <54D93FFA.3090506@codeaurora.org> Sender: cpufreq-owner@vger.kernel.org To: Stephen Boyd Cc: Linux ARM , cpufreq , Linux PM , Mark Rutland , Thomas Gleixner , Peter Zijlstra , John Stultz , Stefan Agner List-Id: linux-pm@vger.kernel.org Stephen Boyd wrote: > Mason wrote: > >> My platform provides a 32-bit counter, ticking at a constant 27 MHz. >> Reading this counter has a latency of roughly 70 ns (it has to go >> over the system memory bus). I think this is good enough for both >> the clock source and sched_clock, is it not? >> >> So the plan would be: >> - clocksource and sched_clock : 27 MHz, 32-bit counter, platform >> - clockevents : TWD, standard > > Yep, that sounds like a good plan. If your platform has the ARM global > timer (drivers/clocksource/arm_global_timer.c) then you don't need > anything besides that timer because it provides both the clocksource, > sched_clock, and clockevents. Sounds like you don't have that timer I'm using Cortex A9, so I do have the global timer (AFAIU). However, I don't think I can safely use it as a clock source because I'm also using cpufreq, and I don't think the arm_global_timer driver handles CPU frequency updates, while the TWD driver does (?) /* * Updates clockevent frequency when the cpu frequency changes. * Called on the cpu that is changing frequency with interrupts disabled. */ "The Interrupt Controller, global timer, private timers, and watchdogs are clocked with PERIPHCLK." And PERIPHCLK is tied to CLK, which is modified by cpufreq. I didn't see any code in the arm_global_timer driver to deal with with that. Did I overlook something fundamental? > though, so you have to write a driver for your custom platform timer and > at least hook up clocksource and sched_clock to it. If you have > interrupts with your platform timer you can skip out on TWD and also > register a clockevent in your platform timer driver. Registering a clock source or a sched_clock seems straight-forward. All I need to provide is a function to read the platform counter. However, why would I skip out on TWD? (I'm trying to minimize code needed for the port.) Regards.