Messages in this thread | | | Date | Thu, 16 Nov 2017 11:08:11 -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 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 nowdays should have a suitable timer source.
Nicolas
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