From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752521Ab3KLCeR (ORCPT ); Mon, 11 Nov 2013 21:34:17 -0500 Received: from smtp.codeaurora.org ([198.145.11.231]:51706 "EHLO smtp.codeaurora.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750964Ab3KLCeK (ORCPT ); Mon, 11 Nov 2013 21:34:10 -0500 Message-ID: <528193A0.7050505@codeaurora.org> Date: Mon, 11 Nov 2013 18:34:08 -0800 From: Stephen Boyd User-Agent: Mozilla/5.0 (X11; Linux i686 on x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.1.0 MIME-Version: 1.0 To: Nicolas Pitre CC: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Jean-Christophe PLAGNIOL-VILLARD , Christopher Covington , Russell King - ARM Linux , =?ISO-8859-1?Q?M=E5ns_Rullg=E5rd?= , Rob Herring Subject: Re: [PATCH v2] ARM: Use udiv/sdiv for __aeabi_{u}idiv library functions References: <1383951632-6090-1-git-send-email-sboyd@codeaurora.org> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/09/13 21:03, Nicolas Pitre wrote: > Bah..... NAK. We are doing runtime patching of the kernel for many > many things already. So why not do the same here? static keys are a form of runtime patching, albeit not as extreme as you're suggesting. > > The obvious strategy is to simply overwrite the start of the existing > __aeabi_idiv code with the "sdiv r0, r0, r1" and "bx lr" opcodes. > > Similarly for the unsigned case. I was thinking the same thing when I wrote this, but I didn't know how to tell the compiler to either inline this function or to let me inilne an assembly stub with some section magic. > > That let you test the hardware capability only once during boot instead > of everytime a divide operation is performed. The test for hardware capability really isn't done more than once during boot. The assembly is like so at compile time 00000000 <__aeabi_idiv>: 0: nop {0} 4: b 0 <___aeabi_idiv> 8: sdiv r0, r0, r1 c: bx lr and after we test and find support for the instruction it will be replaced with 00000000 <__aeabi_idiv>: 0: b 8 4: b 0 <___aeabi_idiv> 8: sdiv r0, r0, r1 c: bx lr Unfortunately we still have to jump to this function. It would be great if we could inline this function at the call site but as I already said I don't know how to do that. -- Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by The Linux Foundation