Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752633AbdLNUh3 (ORCPT ); Thu, 14 Dec 2017 15:37:29 -0500 Received: from mail.free-electrons.com ([62.4.15.54]:45922 "EHLO mail.free-electrons.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752160AbdLNUh1 (ORCPT ); Thu, 14 Dec 2017 15:37:27 -0500 Date: Thu, 14 Dec 2017 21:37:15 +0100 From: Boris Brezillon To: Rob Herring Cc: Alexandre Belloni , Mark Rutland , "devicetree@vger.kernel.org" , Daniel Lezcano , "linux-kernel@vger.kernel.org" , Thomas Gleixner , "linux-arm-kernel@lists.infradead.org" Subject: Re: [PATCH 1/3] dt-bindings: chosen: Add clocksource and clockevent selection Message-ID: <20171214213715.4e0bdb44@bbrezillon> In-Reply-To: <20171214210120.6b436e0d@bbrezillon> References: <20171213185313.20017-1-alexandre.belloni@free-electrons.com> <20171213185313.20017-2-alexandre.belloni@free-electrons.com> <20171214210120.6b436e0d@bbrezillon> X-Mailer: Claws Mail 3.14.1 (GTK+ 2.24.31; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4115 Lines: 84 On Thu, 14 Dec 2017 21:01:20 +0100 Boris Brezillon wrote: > Hi Rob, > > On Wed, 13 Dec 2017 16:57:50 -0600 > Rob Herring wrote: > > > On Wed, Dec 13, 2017 at 12:53 PM, Alexandre Belloni > > wrote: > > > The clocksource and clockevent timer are probed early in the boot process. > > > At that time it is difficult for linux to know whether a particular timer > > > can be used as the clocksource or the clockevent or by another driver, > > > especially when they are all identical or have similar features. > > > > If all identical, then it shouldn't matter. "similar" means some > > difference. Describe those differences. > > We had this discussion already. Those timers might be connected to > external pins and may serve the role of PWM generators or capture > devices. We can also chain timers and provide a clocksource with a > better resolution or one that wraps less often. In the end it's all > about how the user plans to use its system, and this has to be > described somehow. Assuming that the software can decide by itself > which timer should be used or how many of them should be chained is > just an utopia. > > > > > > Until now, multiple strategies have been used to solve that: > > > - use Kconfig option as MXC_USE_EPIT or ATMEL_TCB_CLKSRC_BLOCK > > > > Compile time probably means only one option is really used. > > Compile time selection prevents using the same kernel for different > boards (in other words, no multi-v7), so not really an option here. > > > > > > - use a kernel parameter as the "clocksource" early_param in mach-omap2 > > > > Yeah, OMAP was one of the previous times this came up and also > > attempted something like this. This parameter predates selecting > > timers based on features described in DT. Look at commit > > 2eb03937df3ebc (ARM: OMAP3: Update clocksource timer selection). > > Then, would you accept to have a property saying that a specific timer > is a free-running timer (suitable for clocksource) or a programmable > timer (suitable for clkevent)? Or are you saying that you don't like the > way timers are differentiated on omap? > > > > > > - registering the first seen timer as a clockevent and the second one as > > > a clocksource as in rk_timer_init or dw_apb_timer_init > > > > > > Add a linux,clocksource and a linux,clockevent node in chosen with a timer > > > property pointing to the timer to use. Other properties, like the targeted > > > precision may be added later. > > > > Open ended expansion of this does not help convince me it is needed. > > It's not an open ended expansion, we're just trying to find a way to > describe which timer blocks should be used as free running timers > (clksource) and which one should be used as programmable timers > (clkevent). Automatically selecting timer blocks to assign to the > clkevent or clocksource is not so easy (as has been explained earlier) > because at the time the system registers its clksource/clkevent devices > we might not have all the necessary information to know which timer > blocks will be reserved for other usage later on. The use case I have > in mind is DT overlays, where one of the overlay is using a timer as a > PWM generator. If the clkevent or clksource has already claimed the > timer connected to the pins the overlay is using, then we're screwed, > and there's no way the system can know that ahead of time except by > pre-assigning a timer to the clksource or clkevent feature. > > So really, we need a way to assign a specific timer to a specific > feature. You've refused the different proposals we made so far, so > what's your alternative? I mean a real alternative that solve the "an > auto-selected timer might be claimed by another driver at some point" > problem. Okay, it seems I was wrong, you already agreed on a generic atmel,tcb-timer compatible [1], so it seems the only thing we're missing is a way to assign a timer to a clocksource or a clkevent. [1]https://patchwork.kernel.org/patch/9755341/