From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751306AbbJIXyy (ORCPT ); Fri, 9 Oct 2015 19:54:54 -0400 Received: from pandora.arm.linux.org.uk ([78.32.30.218]:52408 "EHLO pandora.arm.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750753AbbJIXyu (ORCPT ); Fri, 9 Oct 2015 19:54:50 -0400 Date: Sat, 10 Oct 2015 00:54:41 +0100 From: Russell King - ARM Linux To: =?iso-8859-1?Q?M=E5ns_Rullg=E5rd?= Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-omap@vger.kernel.org, linux-cris-kernel@axis.com, linux-mips@linux-mips.org, linux-xtensa@linux-xtensa.org, kernel@stlinux.com, linux-rpi-kernel@lists.infradead.org, linux-samsung-soc@vger.kernel.org, linux-tegra@vger.kernel.org Subject: Re: [PATCH] sched_clock: add data pointer argument to read callback Message-ID: <20151009235441.GD32536@n2100.arm.linux.org.uk> References: <1444427858-576-1-git-send-email-mans@mansr.com> <20151009232015.GC32536@n2100.arm.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Oct 10, 2015 at 12:48:22AM +0100, Måns Rullgård wrote: > Russell King - ARM Linux writes: > > > On Fri, Oct 09, 2015 at 10:57:35PM +0100, Mans Rullgard wrote: > >> This passes a data pointer specified in the sched_clock_register() > >> call to the read callback allowing simpler implementations thereof. > >> > >> In this patch, existing uses of this interface are simply updated > >> with a null pointer. > > > > This is a bad description. It tells us what the patch is doing, > > (which we can see by reading the patch) but not _why_. Please include > > information on why the change is necessary - describe what you are > > trying to achieve. > > Currently most of the callbacks use a global variable to store the > address of a counter register. This has several downsides: > > - Loading the address of a global variable can be more expensive than > keeping a pointer next to the function pointer. > > - It makes it impossible to have multiple instances of a driver call > sched_clock_register() since the caller can't know which clock will > win in the end. > > - Many of the existing callbacks are practically identical and could be > replaced with a common generic function if it had a pointer argument. > > If I've missed something that makes this a stupid idea, please tell. So my next question is whether you intend to pass an iomem pointer through this, or a some kind of structure, or both. It matters, because iomem pointers have a __iomem attribute to keep sparse happy. Having to force that attribute on and off pointers is frowned upon, as it defeats the purpose of the sparse static checker. -- FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up according to speedtest.net.