About the accuracy of delay loop

[Jamie Huang]

On Wed, 10 Aug 2016 15:34:29 +0800
Jisheng Zhang <jszhang at marvell.com> wrote:

> Hi,
> 
> On Wed, 10 Aug 2016 15:24:56 +0800 Jamie Huang wrote:
> 
> > Hi, all.
> > 	I have a question about the commit
> > 	d0a533b18235d36206b9b422efadb7cee444dfdb ARM: 7452/1: delay:
> > allow timer-based delay implementation to be selected.
> > 
> > 	Recently, I found the BogoMIPS info of kernel start-up log
> > is not correct.The correct info is as follows:
> > 
> > 	Calibrating delay loop... 719.25 BogoMIPS (lpj=3596288)
> > 
> > 	But, after the patch above, the info changed as follows:
> > 
> > 	Calibrating delay loop... 478.41 BogoMIPS (lpj=2392064)
> > 
> > 	I find the difference from assembly code.
> > 	Before the patch:
> > 	  __delay(loops_per_jiffy);
> >              c002bcf0:       e50b3034   str r3, [fp, #-52]  ; 0x34
> >              c002bcf4:       e50b2030   str r2, [fp, #-48]  ; 0x30
> >              c002bcf8:       eb06ddd8   bl c01e3460 <__delay>
> > 
> > 	After the patch:
> >           __delay(loops_per_jiffy);
> >              c04add08:       e5932000   ldr r2, [r3]
> >              c04add0c:       e50b105c   str r1, [fp, #-92]  ; 0x5c
> >              c04add10:       e50b3060   str r3, [fp, #-96]  ; 0x60
> >              c04add14:       e50bc064   str ip, [fp, #-100] ; 0x64
> >              c04add18:       e12fff32   blx r2
> > 
> >  	There are two additional instructions, and these will
> >  	influence calibration of delay loop. I also worry that this
> >  	new implementation will influence the accuracy of udelay.
> > Does  
> 
> Per my understanding, udelay is never ensured "accurate", it only
> ensures that it at least delays as required.
> 
> If udelay is timer-based, then it could gives more accuracy.
> 
> Thanks,
> Jisheng
> 
> >  	any one know this problem?
> > 
> > 
> > _______________________________________________
> > linux-arm-kernel mailing list
> > linux-arm-kernel at lists.infradead.org
> > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel  
> 

Thank you for reply. But the users may be confused because of the
different results. You mean there is no need to worry about it?