Re: [PATCH] USB: usbmon: Remove timeval usage for timestamp

[Arnd Bergmann <arnd@arndb.de>]

On Tuesday 05 May 2015 09:19:37 Pete Zaitcev wrote:
> On Tue, 05 May 2015 11:24:16 +0200
> Arnd Bergmann <arnd@arndb.de> wrote:
> 
> > On Tuesday 05 May 2015 11:44:33 Tina Ruchandani wrote:
> 
> > >  static inline unsigned int mon_get_timestamp(void)
> > >  {
> > > -	struct timeval tval;
> > > +	struct timespec64 now;
> > >  	unsigned int stamp;
> > >  
> > > -	do_gettimeofday(&tval);
> > > -	stamp = tval.tv_sec & 0xFFF;	/* 2^32 = 4294967296. Limit to 4096s. */
> > > -	stamp = stamp * 1000000 + tval.tv_usec;
> > > +	getnstimeofday64(&now);
> > > +	stamp = now.tv_sec & 0xFFF;  /* now.tv_sec is 64-bit. Limit to 4096s */
> > > +	stamp = stamp * USEC_PER_SEC + now.tv_nsec / NSEC_PER_USEC;
> > >  	return stamp;
> > >  }
> > 
> > Your conversion looks entirely correct, but the original code is a bit
> > odd here as it does not use the entire range of the 32-bit microsecond
> > value, and counts from 0 to 4096000000us instead of the more intuitive
> > 0 to 4294967296 us range before wrapping around.
> 
> The intent was to create a rolling timestamp that is not too large.
> Remember that the text format is intended to be eyeballed. The wrap point
> could be anything. No consideration was given to what's intuitive.

Ok

> > it might be more obvious what is going on, but it would slightly change
> > the output in the debugfs file to use the full range. Do we know what
> > behavior is expected by normal user space here?
> > [...]
> > I also wonder if we should make the output use monotonic time instead
> 
> The only guarantee we give is that the time corresponds to microseconds.
> This is sometimes necessary to debug things in microntrollers in USB
> devices.
> 
> A monotonic time is fine.
> 
> One thing though, I object to Tina's new comment. It does not matter what
> now.tv_sec is. The comment is about the destination, that is the stamp.
> So, please don't change it, unless you change the type of the ep->tstamp.

The point of the patch that I thought she had made clear enough is to
eliminate the use of 'struct timeval' from one of the 236 files that
currently use it, to make it clearer what the remaining problems
are. 

> > static inline unsigned int mon_get_timestamp(void)
> > {
> > 	return ktime_to_us(ktime_get_real());
> > }
> > 
> > it might be more obvious what is going on, but it would slightly change
> 
> Now you made the truncation explicit, so if anyhing it's less obvious.
> The code is fine, but at least add a comment to that effect, if you don't
> want to tack %4096000000 or %4000000000.

As Alan Stern commented, we should probably leave the wrap point alone,
and adding "% 4096000000" (actually do_div() or ktime_divns()) on a 64-bit
value is more expensive than the current code, so it's probably best if
Tina leaves the existing logic and just changes it to using ktime_get_ts64()
for the monotonic time, as well as trying to be more explicit in the
changelog about the intention.

	Arnd