From mboxrd@z Thu Jan 1 00:00:00 1970 From: Arnd Bergmann Subject: Re: [RFC/NOT FOR MERGING] HACK: add global/private timers for A9 Date: Wed, 03 Jun 2015 22:55:27 +0200 Message-ID: <2466648.WjMxjIpAUG@wuerfel> References: <1433363565-17725-1-git-send-email-balbi@ti.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7Bit Return-path: Received: from mout.kundenserver.de ([212.227.17.10]:63869 "EHLO mout.kundenserver.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753977AbbFCU4M (ORCPT ); Wed, 3 Jun 2015 16:56:12 -0400 In-Reply-To: <1433363565-17725-1-git-send-email-balbi@ti.com> Sender: linux-omap-owner@vger.kernel.org List-Id: linux-omap@vger.kernel.org To: linux-arm-kernel@lists.infradead.org Cc: Felipe Balbi , Tony Lindgren , Russell King , Linux OMAP Mailing List On Wednesday 03 June 2015 15:32:45 Felipe Balbi wrote: > > Hi Tony and Russell, > > AM43xx, even though it's a single processor A9, it still has TWD and global > timer. I was doing some profiling with RT v4.0 and latency is 3.5x lower just > by switching from gptimer to twd/global. > > The only problem is that currently, is_smp() check prevents me from using twd > with AM43xx (that's why it's commented below, for testing purposes). > > In the hopes that we can start a, hopefully, small thread around the subject, > I'm sending this HACK which I used to get TWD and global timer enabled so I > could measure latencies with cyclictest. > > Is it so that TWD shouldn't be available on UP integrations of ARM's Cortex-A > processors ? > > I wondered about this recently when looking at something unrelated and noticed that the check had been introduced as part of 904464b91eca8 ("ARM: 7655/1: smp_twd: make twd_local_timer_of_register() no-op for nosmp"). I suspect this was just the wrong fix at the time, and that the real culprit is either alloc_percpu() or request_percpu_irq() getting called too early on a machine without SMP support. Possibly the problem is already resolved independently, if you didn't run into it. Arnd