Re: [PATCH v2 02/12] can: rx-offload: Add support for timestamp based irq offloading

[Andri Yngvason <andri.yngvason@marel.com>]

On Tue, Jul 05, 2016 at 12:30:32AM +0200, Marc Kleine-Budde wrote:
> On 07/04/2016 10:59 PM, Marc Kleine-Budde wrote:
> > Actually I want to have the difference between two timestamps in order
> > to sort . The timestamps are monotonically increasing and will
> > eventually roll over. As the flexcan core only has a 16 bit timer, I
> > shift the timestamp by 16 bit (cb = reg_ts << 16), to fill the whole 32
> > bit. This way I don't have to take care about the overflow, right?
> > 
Hmmm, yes, that is quite clever. If I understand correctly, this means that if
you have three timestamps on the roll-over boundary: 0xfffe0000, 0xffff0000 and
0x0001000, the sorting algorithm will sort them in this particular order because
	(int)(0xfffe0000 - 0xffff0000) < 0
and
	(int)(0xffff0000 - 0x00010000) < 0

Perhaps there should be a comment explaining this?

> >> main: a=0xfffffff0 b=0xfffffffc diff= 12
> >> main: a=0xfffffff0 b=0xfffffffd diff= 13
> >> main: a=0xfffffff0 b=0xfffffffe diff= 14
> >> main: a=0xfffffff0 b=0xffffffff diff= 15
> >> main: a=0xfffffff0 b=0x00000000 diff= 16
> >> main: a=0xfffffff0 b=0x00000001 diff= 17
> >> main: a=0xfffffff0 b=0x00000002 diff= 18
> >> main: a=0xfffffff0 b=0x00000003 diff= 19
> >> main: a=0xfffffff0 b=0x00000004 diff= 20
> 
> It's also working when the difference is negative:
> 
> > main: a=0xfffffffc b=0xfffffff0 diff=-12
> > main: a=0xfffffffd b=0xfffffff0 diff=-13
> > main: a=0xfffffffe b=0xfffffff0 diff=-14
> > main: a=0xffffffff b=0xfffffff0 diff=-15
> > main: a=0x00000000 b=0xfffffff0 diff=-16
> > main: a=0x00000001 b=0xfffffff0 diff=-17
> > main: a=0x00000002 b=0xfffffff0 diff=-18
> > main: a=0x00000003 b=0xfffffff0 diff=-19
> 
> It will get "problematic" if the difference between the two u32 is
> around 2^31. But then it's hard to tell which of the mailboxes holds the
> older timestamp.
> 
Yes, it's also safe to assume that if you ever get this much of a difference
between timestamps in the mailbox, something else must be wrong before that
happens.

I've run into bugs involving this kind of a comparison function where the
difference actually can be large enough to cause problems, so this was a red
flag for me, but it turns out to be OK.

Best regards,
Andri