Messages in this thread | | | Subject | Re: [RFC] Improving udelay/ndelay on platforms where that is possible | From | Marc Gonzalez <> | Date | Thu, 16 Nov 2017 17:26:32 +0100 |
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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.
Regards.
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