Messages in this thread | | | Date | Thu, 16 Nov 2017 11:47:20 -0500 (EST) | From | Nicolas Pitre <> | Subject | Re: [RFC] Improving udelay/ndelay on platforms where that is possible |
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On Thu, 16 Nov 2017, Marc Gonzalez wrote:
> On 16/11/2017 17:08, Nicolas Pitre wrote: > > > On Thu, 16 Nov 2017, Marc Gonzalez wrote: > > > >> On 16/11/2017 16:36, Russell King - ARM Linux wrote: > >>> On Thu, Nov 16, 2017 at 04:26:51PM +0100, Marc Gonzalez wrote: > >>>> On 15/11/2017 14:13, Russell King - ARM Linux wrote: > >>>> > >>>>> udelay() needs to offer a consistent interface so that drivers know > >>>>> what to expect no matter what the implementation is. Making one > >>>>> implementation conform to your ideas while leaving the other > >>>>> implementations with other expectations is a recipe for bugs. > >>>>> > >>>>> If you really want to do this, fix the loops_per_jiffy implementation > >>>>> as well so that the consistency is maintained. > >>>> > >>>> Hello Russell, > >>>> > >>>> It seems to me that, when using DFS, there's a serious issue with loop-based > >>>> delays. (IIRC, it was you who pointed this out a few years ago.) > >>>> > >>>> If I'm reading arch/arm/kernel/smp.c correctly, loops_per_jiffy is scaled > >>>> when the frequency changes. > >>>> > >>>> But arch/arm/lib/delay-loop.S starts by loading the current value of > >>>> loops_per_jiffy, computes the number of times to loop, and then loops. > >>>> If the frequency increases when the core is in __loop_delay, the > >>>> delay will be much shorter than requested. > >>>> > >>>> Is this a correct assessment of the situation? > >>> > >>> Absolutely correct, and it's something that people are aware of, and > >>> have already catered for while writing their drivers. > >> > >> In their cpufreq driver? > >> In "real" device drivers that happen to use delays? > >> > >> On my system, the CPU frequency may ramp up from 120 MHz to 1.2 GHz. > >> If the frequency increases at the beginning of __loop_delay, udelay(100) > >> would spin only 10 microseconds. This is likely to cause issues in > >> any driver using udelay. > >> > >> How does one cater for that? > > > > You make sure your delays are based on a stable hardware timer. > > Most platforms nowadays should have a suitable timer source. > > So you propose fixing loop-based delays by using clock-based delays, > is that correct? (That is indeed what I did on my platform.) > > Russell stated that there are platforms using loop-based delays with > cpufreq enabled. I'm asking how they manage the brokenness.
Look at cpufreq_callback() in arch/arm/kernel/smp.c.
Nicolas
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