From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932603AbeARWug (ORCPT ); Thu, 18 Jan 2018 17:50:36 -0500 Received: from smtprelay.synopsys.com ([198.182.60.111]:39184 "EHLO smtprelay.synopsys.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932131AbeARWu1 (ORCPT ); Thu, 18 Jan 2018 17:50:27 -0500 Subject: Re: [PATCH] ARC: Add a knob to control usage of dual-issue To: Alexey Brodkin CC: "linux-kernel@vger.kernel.org" , "linux-snps-arc@lists.infradead.org" References: <20180118134831.41489-1-abrodkin@synopsys.com> <7d97edc0-d0ee-326b-bcd2-c6731aa953b2@synopsys.com> <1516302661.8324.32.camel@synopsys.com> From: Vineet Gupta Message-ID: <67318494-971f-0e3f-31e6-d19c59950cd5@synopsys.com> Date: Thu, 18 Jan 2018 14:50:22 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.0 MIME-Version: 1.0 In-Reply-To: <1516302661.8324.32.camel@synopsys.com> Content-Type: text/plain; charset="utf-8"; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US X-Originating-IP: [10.10.161.79] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/18/2018 11:11 AM, Alexey Brodkin wrote: > Hi Vineet, > > On Thu, 2018-01-18 at 10:54 -0800, Vineet Gupta wrote: >> On 01/18/2018 05:48 AM, Alexey Brodkin wrote: >>> HS48 core starts with dual-issue enabled but in some cases like >>> debugging as well as benchmarking it might be useful to disable >>> dual-issue for a particular run. >>> >>> Note: >>> 1. To disable dual-issue user has to change a value of a global variable >>> in target's memory right before start of Linu kernel execution >>> (most probably via JTAG) >>> >>> 2. Disabling happens very early on boot and to get it back enabled it's >>> required to restart Linux kernel. I.e. with this change we don't allow >>> toggling dual-issue state in random moments of run-time >> But we need access to a debugger anyways to change this global variable. >> If you already have that won't it be better to change the aux register itself from >> the debugger itself. >> I don't really see how the global variable way of toggle adds any value here ? > I think there's some sense in a global variable. > > 1. Not sure if OpenOCD/GDB pair for ARC allows to read/write random AUX regs. > Probably allows but I guess we'll need a new .xml description which lists EXEC_CTRL > AUX reg. Correct me if I'm wrong here. Not sure of this a strong argument ... > > 2. If somebody wants to disable dual-issue persistently (here I mean building an image > which had dual-issue disabled and there's no need to alter anything after image loading) Ok this is more reasonable ! But can we do this in early boot code then. There's a fair bit of code leading up to where we are doing now so I'd prefer we do it just as we do for disabling L1 caches. Please note that SLC / IOC are special - as much as I liked to do them super early - we can't as there's fair bit of code bloat / mem size etc which we don't knwo that early. > then it's possible just to change a simple and very obvious line in arch/arc/kernel/setup.c > other than trying to guess what could be good place in sources to stick that AUX reg write. > Note we don't explicitly do reads of AUX_EXEC_CTRL instead we just use READ_BCR macro so > for newcomers it might not be immediately obvious what to do and where. > Add the fact that logic of the flag is reversed... > > -Alexey