[U-Boot-Users] [Patch 02/17] U-Boot-V2:Common:Clock Handle case ofclockrollover for get_time_ns

[Sascha Hauer]

On Tue, Jun 03, 2008 at 09:39:51AM -0500, Menon, Nishanth wrote:
> Sascha,
> > -----Original Message-----
> > From: Sascha Hauer [mailto:s.hauer at pengutronix.de]
> > Sent: Tuesday, June 03, 2008 9:15 AM
> > To: Menon, Nishanth
> > Cc: u-boot-users at lists.sourceforge.net; Laurent Desnogues; dirk.behme at googlemail.com;
> > philip.balister at gmail.com; Gopinath, Thara; Kamat, Nishant; Syed Mohammed, Khasim
> > Subject: Re: [Patch 02/17] U-Boot-V2:Common:Clock Handle case ofclockrollover for get_time_ns
> > 
> > > Assumptions made:
> > > A) The bits masked out by cs->mask will remain constant. This may not be true.
> > 
> > Eh? That's why they are masked out.
> For the sake of discussion:
> unsigned int now = 0x120;
> unsigned int last = 0x128;
> unsigned int mask = 0xFFF0;
> unsigned int delta1 = now - last;
> unsigned int delta2 = (now & mask) - (last & mask);
> printf ("delta1=0x%08X maskdelta1=0x%08X delta2=0x%08X\n",delta1, delta1 & mask, delta2);
> 
> Output will be:
> delta1=0xFFFFFFF8 maskdelta1=0x0000FFF0 delta2=0x00000000
> 
> What we will get now is maskdelta1, while delta2 is the right value.

It's not that I thought of the code by myself, I just looked into the
kernel and it's exactly like this in the kernel code. So you either just
found a kernel bug or we both understand something wrong.

> 
> > 
> > > B) Roll over assume the min is 0 and max is cs->mask. This need not be the case.
> > > It would be good to be explicit.
> > 
> > Do you know any counter that does not start counting from zero? If you
> I do not. I am just being a paranoid idiot ;)..
> 
> > have, noone prevents you from substracting the value in your clocksource
> > read function.
> Max however is assumed to be cs->mask then. We could have timers which can be configured for ticking till a configured max. probably my paranoia kicking in again?

If that's the only possibility to program this timer then go out and
kill the chip designer ;)

Sascha


> 
> Regards,
> Nishanth Menon
> 

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