From mboxrd@z Thu Jan 1 00:00:00 1970 From: daniel.lezcano@linaro.org (Daniel Lezcano) Date: Wed, 4 Nov 2015 12:19:57 +0100 Subject: [PATCH v2 0/3] let Marvell Berlin SoCs make use of the best delay timer In-Reply-To: <4359736.Q4L68M2aRW@wuerfel> References: <1446560917-6318-1-git-send-email-jszhang@marvell.com> <5639D409.1030302@linaro.org> <4359736.Q4L68M2aRW@wuerfel> Message-ID: <5639E9DD.1080309@linaro.org> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On 11/04/2015 11:30 AM, Arnd Bergmann wrote: > On Wednesday 04 November 2015 10:46:49 Daniel Lezcano wrote: >> On 11/03/2015 03:28 PM, Jisheng Zhang wrote: >>> In case there are several possible delay timers, we purely base the >>> selection on the frequency, which is suboptimal in some cases. Take >>> one Marvell Berlin platform for example: we have arch timer and dw-apb >>> timer. The arch timer freq is 25MHZ while the dw-apb timer freq is >>> 100MHZ, current selection would choose the dw-apb timer. But the dw >>> apb timer is on the APB bus while arch timer sits in CPU, the cost >>> of accessing the apb timer is higher than the arch timer. >>> >>> This series firstly modifies register_current_timer_delay() to choose >>> the highest rating delay timer: use the rating as a primary indication >>> and fall back to comparing the frequency if the rating is not set or >>> the same. Then we set the arch_delay_timer rating as 400, finally >>> Implement ARM delay timer for the dw_apb_timer and set its rating as 300. >> >> Hi Jisheng, Arnd, >> >> I don't feel comfortable with the rating / freq think. I am afraid this >> approach based on heuristic will bring a lot of complexity and >> workarounds in the code for a small benefit. >> >> Why don't we define a DT entry for the delay timer ? So we delegate the >> choice to the platform DT definition. > > That would be wrong, because the fact that Linux uses a timer to > optimize its udelay() function is not a feature of the hardware. True. Any ideas / suggestions for an alternative ? -- Linaro.org ? Open source software for ARM SoCs Follow Linaro: Facebook | Twitter | Blog From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965172AbbKDLUD (ORCPT ); Wed, 4 Nov 2015 06:20:03 -0500 Received: from mail-wi0-f173.google.com ([209.85.212.173]:34896 "EHLO mail-wi0-f173.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755288AbbKDLUA (ORCPT ); Wed, 4 Nov 2015 06:20:00 -0500 Subject: Re: [PATCH v2 0/3] let Marvell Berlin SoCs make use of the best delay timer To: Arnd Bergmann References: <1446560917-6318-1-git-send-email-jszhang@marvell.com> <5639D409.1030302@linaro.org> <4359736.Q4L68M2aRW@wuerfel> Cc: Jisheng Zhang , tglx@linutronix.de, linux@arm.linux.org.uk, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org From: Daniel Lezcano Message-ID: <5639E9DD.1080309@linaro.org> Date: Wed, 4 Nov 2015 12:19:57 +0100 User-Agent: Mozilla/5.0 (X11; Linux i686; rv:38.0) Gecko/20100101 Thunderbird/38.3.0 MIME-Version: 1.0 In-Reply-To: <4359736.Q4L68M2aRW@wuerfel> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/04/2015 11:30 AM, Arnd Bergmann wrote: > On Wednesday 04 November 2015 10:46:49 Daniel Lezcano wrote: >> On 11/03/2015 03:28 PM, Jisheng Zhang wrote: >>> In case there are several possible delay timers, we purely base the >>> selection on the frequency, which is suboptimal in some cases. Take >>> one Marvell Berlin platform for example: we have arch timer and dw-apb >>> timer. The arch timer freq is 25MHZ while the dw-apb timer freq is >>> 100MHZ, current selection would choose the dw-apb timer. But the dw >>> apb timer is on the APB bus while arch timer sits in CPU, the cost >>> of accessing the apb timer is higher than the arch timer. >>> >>> This series firstly modifies register_current_timer_delay() to choose >>> the highest rating delay timer: use the rating as a primary indication >>> and fall back to comparing the frequency if the rating is not set or >>> the same. Then we set the arch_delay_timer rating as 400, finally >>> Implement ARM delay timer for the dw_apb_timer and set its rating as 300. >> >> Hi Jisheng, Arnd, >> >> I don't feel comfortable with the rating / freq think. I am afraid this >> approach based on heuristic will bring a lot of complexity and >> workarounds in the code for a small benefit. >> >> Why don't we define a DT entry for the delay timer ? So we delegate the >> choice to the platform DT definition. > > That would be wrong, because the fact that Linux uses a timer to > optimize its udelay() function is not a feature of the hardware. True. Any ideas / suggestions for an alternative ? -- Linaro.org │ Open source software for ARM SoCs Follow Linaro: Facebook | Twitter | Blog