From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757732Ab3A1SJe (ORCPT ); Mon, 28 Jan 2013 13:09:34 -0500 Received: from mail-qe0-f52.google.com ([209.85.128.52]:47076 "EHLO mail-qe0-f52.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751626Ab3A1SJc (ORCPT ); Mon, 28 Jan 2013 13:09:32 -0500 X-Greylist: delayed 305 seconds by postgrey-1.27 at vger.kernel.org; Mon, 28 Jan 2013 13:09:32 EST Date: Mon, 28 Jan 2013 13:04:24 -0500 (EST) From: Nicolas Pitre To: Will Deacon cc: Joonsoo Kim , Russell King , "js1304@gmail.com" , Catalin Marinas , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" Subject: Re: [PATCH v3 0/3] introduce static_vm for ARM-specific static mapped area In-Reply-To: <20130128175106.GI23470@mudshark.cambridge.arm.com> Message-ID: References: <1358990934-4893-1-git-send-email-iamjoonsoo.kim@lge.com> <20130128175106.GI23470@mudshark.cambridge.arm.com> User-Agent: Alpine 2.02 (LFD 1266 2009-07-14) 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 Mon, 28 Jan 2013, Will Deacon wrote: > Hello, > > On Thu, Jan 24, 2013 at 01:28:51AM +0000, Joonsoo Kim wrote: > > In current implementation, we used ARM-specific flag, that is, > > VM_ARM_STATIC_MAPPING, for distinguishing ARM specific static mapped area. > > The purpose of static mapped area is to re-use static mapped area when > > entire physical address range of the ioremap request can be covered > > by this area. > > > > This implementation causes needless overhead for some cases. > > For example, assume that there is only one static mapped area and > > vmlist has 300 areas. Every time we call ioremap, we check 300 areas for > > deciding whether it is matched or not. Moreover, even if there is > > no static mapped area and vmlist has 300 areas, every time we call > > ioremap, we check 300 areas in now. > > > > If we construct a extra list for static mapped area, we can eliminate > > above mentioned overhead. > > With a extra list, if there is one static mapped area, > > we just check only one area and proceed next operation quickly. > > > > In fact, it is not a critical problem, because ioremap is not frequently > > used. But reducing overhead is better idea. > > > > Another reason for doing this work is for removing vm_struct list management, > > entirely. For more information, look at the following link. > > http://lkml.org/lkml/2012/12/6/184 > > First patch looks good (removing the unused vmregion stuff) but I'm not so > sure about the rest of it. If you really care about ioremap performance, > perhaps it would be better to have a container struct around the vm_struct > for static mappings and then stick them in an augmented rbtree so you can > efficiently find the mapping encompassing a particular physical address? How can ioremap performance be a problem is the question I had since the beginning. Firstly, ioremap is _not_ meant to be used in performance critical paths. Secondly, there shouldn't be _that_ many entries on the vmlist such as 300. That sounds a bit excessive. So please, can we discuss the reasons that motivated those patches in the first place? Maybe that's where the actual problem is. Nicolas