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Date: Thu, 5 Nov 2015 10:36:28 +0800 From: Jisheng Zhang <jszhang@...vell.com> To: Arnd Bergmann <arnd@...db.de> CC: Daniel Lezcano <daniel.lezcano@...aro.org>, <tglx@...utronix.de>, <linux@....linux.org.uk>, <linux-arm-kernel@...ts.infradead.org>, <linux-kernel@...r.kernel.org> Subject: Re: [PATCH v2 0/3] let Marvell Berlin SoCs make use of the best delay timer Dear Arnd and Daniel, On Wed, 4 Nov 2015 13:19:53 +0100 Arnd Bergmann wrote: > On Wednesday 04 November 2015 12:19:57 Daniel Lezcano wrote: > > 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 ? > > How about simply hardcoding the fact that we prefer the arch timer > over any other one for delay as I suggested earlier? > > Another idea I just had is to do nothing: According to Jisheng's > description for this series, the reason for preferring the arch > timer is that it is faster to access. However, we could argue > that this actually doesn't matter at all, because the entire > point of the ndelay()/udelay()/mdelay() functions is to waste > CPU cycles doing not much at all, so we can just as well waste > them reading the timer register than spinning on the CPU reading > the arch timer more often. > I like this "Another idea", indeed, the delay timer speed doesn't matter at all. So I just cooked v3 to simply register dw apb based delay timer. Thanks a lot for the inspiration, Jisheng -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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