Re: Too much error in __const_udelay() ?

[john stultz <johnstul@us.ibm.com>]

On Sat, 2004-06-05 at 08:23, Dominik Brodowski wrote:
> Hi,
> 
> > However I've started to see some problems w/ 2.6 and USB on x440/x445s,
> > both of which use the 100Mhz cyclone time source. Further digging has
> > pointed to the fact that certain important udelay()s in the USB
> > subsystem aren't actually waiting long enough. 
> 
> Certain? AFAICS _no_ call to a delay routine actually passed a big enough
> argument. Or am I missing something? Also, __ndelay seems to be affected 
> as well: it returns zero for 550 nsec even for the TSC variant in your 
> test.c.

Indeed its likely. 

> > So I'm no math wiz. What's the proper fix here? 
> 
> Below are three changes I'd like to discuss. I'll build a fresh kernel with
> all three changes enabled + PM_TIMER soon.

Ah, your test output is a bit confusing (changes to __const_udealy
affect the output of my_udelay), but I think I understand it. Forgive me
if I miss-interpret. 

> Change 1:
> 
> Move the multiplication with HZ up into the mull instruction:
> 
> unsigned long  __const_udelay(unsigned long xloops)
> {
> 	int d0;
> 	__asm__("mull %0"
> 		:"=d" (xloops), "=&a" (d0)
> 		:"1" (xloops),"0" (LPJ * HZ));
>         return __delay(xloops);
> }

This does make a good bit of difference! Good catch!


> Change 2:
> 
> Round up in __udelay. While it can be argued that some time is also
> spent in the delay functions, it's better to spend _at least_ the specified
> time sleeping, in my humble opinion. 
> 
> 
> -	return __const_udelay2(usecs * 0x000010c6);  /* 2**32 / 1000000 */
> +	return __const_udelay2(usecs * 0x000010c7);  /* 2**32 / 1000000 (rounded up)*/
> 

This change looks right to me. 


> Change 3:
> 
> Asserting at least 1 loop is spent: in really small ndelay() calls to
> low-mhz timers, this might be better.
> 
>         return __delay(xloops ? xloops : 1);

I agree w/ Pavel that rounding up sounds better, but I can't get the
math to work, so this may be the best solution. 

I'm also spinning up a patch w/ these changes to test, let me know how
your testing went and I'll do the same.
-john