Re: [PATCH] joydev: fix HZ->millisecond transformation

[george anzinger <george@mvista.com>]

Vojtech Pavlik wrote:
> 
> On Fri, Dec 20, 2002 at 10:16:27AM -0800, george anzinger wrote:
> > Vojtech Pavlik wrote:
> > >
> > > On Fri, Dec 20, 2002 at 09:24:58AM -0800, george anzinger wrote:
> > > > Vojtech Pavlik wrote:
> > > > >
> > > > > On Fri, Dec 20, 2002 at 02:41:50AM -0800, george anzinger wrote:
> > > > > > Bjorn Helgaas wrote:
> > > > > > >
> > > > > > > * fix a problem with HZ->millisecond transformation on
> > > > > > >   non-x86 archs (from 2.5 change by vojtech@suse.cz)
> > > > > > >
> > > > > > > Applies to 2.4.20.
> > > > > > >
> > > > > > > diff -Nru a/drivers/input/joydev.c b/drivers/input/joydev.c
> > > > > > > --- a/drivers/input/joydev.c    Mon Dec 16 12:16:32 2002
> > > > > > > +++ b/drivers/input/joydev.c    Mon Dec 16 12:16:32 2002
> > > > > > > @@ -50,6 +50,8 @@
> > > > > > >  #define JOYDEV_MINORS          32
> > > > > > >  #define JOYDEV_BUFFER_SIZE     64
> > > > > > >
> > > > > > > +#define MSECS(t)       (1000 * ((t) / HZ) + 1000 * ((t) % HZ) / HZ)
> > > > > > Uh...
> > > > > > ^^^^^^^^^^^^^^^^
> > > > > > by definition this is zero, is it not?
> > > > >
> > > > > No, both parts of the equaition can be nonzero.
> > > >
> > > > I don't think so.  s%HZ has to be less than HZ.  Then
> > > > dividing that by HZ should result in zero.  Where is my
> > > > thinking flawed?
> > >
> > > You first multiply it by 1000.
> >
> > But then it should read: (1000 * (t)) % HZ) / HZ
> 
> The expression is evaluated from left to right. No need for parentheses
> here.
> 
> > But this is still zero.  There is no way that ((X % HZ) /
> > HZ) is other than zero, me thinks.
> 
> Example:
> 
> HZ = 100
> t = 70
> 
> 1000 * (t % HZ) / HZ = 1000 * 70 / 100 = 700 != 0
> 
> Anyway (x % HZ) / HZ must be zero, but that's something different.

OK, I miss read the "("s.  :(
> 
> > > > > Though it might be easier to say (1000 * t) / HZ, now that I think about
> > > > > it.
> > > >
> > > > That overflows...  As does the other if HZ is less than 1000....
> > >
> > > You're right, t can be all 32 bits.
> >
> > What, exactly, is this code trying to do?  It looks like the
> > number is being passed to user space...
> 
> It's trying to convert jiffies to microseconds from computer start (or
> any other arbitrary point in time), thus making the value independent of
> HZ.
> 
This is possible, but in general it can not be kept in one
32-bit word.  I don't know what it is used for or if the
format can be changed, but the struct timeval comes to mind.
-- 
George Anzinger   george@mvista.com
High-res-timers: 
http://sourceforge.net/projects/high-res-timers/
Preemption patch:
http://www.kernel.org/pub/linux/kernel/people/rml