Messages in this thread | | | From | Arnd Bergmann <> | Subject | Re: [PATCH v2 0/3] let Marvell Berlin SoCs make use of the best delay timer | Date | Wed, 04 Nov 2015 11:30:43 +0100 |
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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.
Arnd
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