From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932129AbbKCHEA (ORCPT ); Tue, 3 Nov 2015 02:04:00 -0500 Received: from mx0a-0016f401.pphosted.com ([67.231.148.174]:58066 "EHLO mx0a-0016f401.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754127AbbKCHD6 (ORCPT ); Tue, 3 Nov 2015 02:03:58 -0500 Date: Tue, 3 Nov 2015 14:59:40 +0800 From: Jisheng Zhang To: Arnd Bergmann , Daniel Lezcano CC: , , Subject: Re: [PATCH] clocksource: dw_apb_timer_of: support timer-based delay Message-ID: <20151103145940.18ab648f@xhacker> In-Reply-To: <5909853.Bs72yAP0HH@wuerfel> References: <1446193659-1698-1-git-send-email-jszhang@marvell.com> <3950525.pKSJGoymu4@wuerfel> <20151102110334.049b5439@xhacker> <5909853.Bs72yAP0HH@wuerfel> X-Mailer: Claws Mail 3.13.0 (GTK+ 2.24.28; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2015-11-03_06:,, signatures=0 X-Proofpoint-Spam-Details: rule=inbound_notspam policy=inbound score=0 spamscore=0 suspectscore=0 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1507310000 definitions=main-1511030129 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Dear Arnd, On Mon, 2 Nov 2015 22:56:02 +0100 Arnd Bergmann wrote: > On Monday 02 November 2015 11:03:34 Jisheng Zhang wrote: > > On Fri, 30 Oct 2015 13:42:01 +0100 Arnd Bergmann wrote: > > > > > > This is not ideal from an overall maintenance perspective. We want to > > > be able to have a kernel with all drivers enabled that gives us the > > > best behavior on all platforms. > > > > > > The current behavior appears to be that we override all previous > > > registrations as long as the new one is higher resolution. Is that > > > the case here? I.e. does the arch timer have a lower resultion than > > > the dw-apb timer but have some other advantages? > > > > Take one Marvell Berlin platform for example, the arch timer freq is 25MHZ, > > whose resolution is lower than the dw apb timer at 100MHZ. But dw apb timer > > is on the APB bus while arch timer sits in CPU, so I guess the cost of > > accessing the apb timer is higher than arch timer. > > Ok, I see. > > > I have a solution for this case: in platforms with arch timer, I can mark > > the dw apb timer as "disabled" in the dts even though the timer sits there. > > Then I could make DW_APB_TIMER_BASED_DELAY non-optional but selected by the > > the ARCH_XYZ. Is this acceptable? > > That would do the right thing, but doesn't look ideal: The DW_APB timer > on those platforms is fully functional, and a future Linux version or > another OS might decide to use both timers for one reason or another. > > I'd be happier with a solution that keeps the DT describing the hardware > and not the way we expect Linux to use it, and instead has some heuristic > in the selection of the delay timer. At the moment, we purely base this > on the frequency, which as you say is suboptimal. > > One possible way to improve this would be to add an optional 'latency' > property to the DT nodes (or the driver), and use a combination of latency > and resolution to make the decision. Got it. Thanks for the suggestions. The 'latency' here seems a 'rating' similar as the one in clocksource. I will cook a series for review: patch 1 to make register_current_timer_delay() aware of 'rating' patch 2 to set rating of arch timer as 400 patch 3 to add timer based delay support to dw_apb_timer whose rating is 300 Thanks a lot, Jisheng > A simpler way would be to always prefer the arch timer on ARM if that > is present, even if another timer has a higher resolution. This should > be only a few additional lines in register_current_timer_delay(), or > possibly an additional function argument. >